''Galileo'' was an American robotic space program that studied the planet
Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...
and
its moons, as well as several other
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...
bodies. Named after the Italian astronomer
Galileo Galilei
Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642), commonly referred to as Galileo Galilei ( , , ) or mononymously as Galileo, was an Italian astronomer, physicist and engineer, sometimes described as a poly ...
, the
''Galileo'' spacecraft consisted of an orbiter and an
atmospheric entry
Atmospheric entry (sometimes listed as Vimpact or Ventry) is the movement of an object from outer space into and through the gases of an atmosphere of a planet, dwarf planet, or natural satellite. Atmospheric entry may be ''uncontrolled entr ...
probe. It was delivered into Earth orbit on October 18, 1989, by on the
STS-34
STS-34 was a NASA Space Shuttle mission using ''Atlantis''. It was the 31st shuttle mission overall, and the fifth flight for ''Atlantis''. STS-34 launched from Kennedy Space Center, Florida, on October 18, 1989, and landed at Edwards Air For ...
mission, and arrived at Jupiter on December 7, 1995, after
gravity assist
A gravity assist, gravity assist maneuver, swing-by, or generally a gravitational slingshot in orbital mechanics, is a type of spaceflight flyby (spaceflight), flyby which makes use of the relative movement (e.g. orbit around the Sun) and gra ...
flybys of
Venus
Venus is the second planet from the Sun. It is often called Earth's "twin" or "sister" planet for having almost the same size and mass, and the closest orbit to Earth's. While both are rocky planets, Venus has an atmosphere much thicker ...
and
Earth
Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
, and became the first spacecraft to orbit Jupiter. The spacecraft then launched the first probe to directly measure its
atmosphere
An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...
. Despite suffering major antenna problems, ''Galileo'' achieved the first
asteroid
An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...
flyby, of
951 Gaspra
951 Gaspra is an S-type asteroid, S-type asteroid that orbits very close to the inner edge of the asteroid belt. Gaspra was discovered by Russian astronomer G. N. Neujmin in 1916. Neujmin named it after Gaspra, a Black Sea retreat that was visite ...
, and discovered the first
asteroid moon
A minor-planet moon is an astronomical object that orbits a minor planet as its natural satellite. , there are 457 minor planets known or suspected to have moons. Discoveries of minor-planet moons (and binary objects, in general) are important ...
,
Dactyl, around
243 Ida
243 Ida is an asteroid in the Koronis family of the asteroid belt. It was discovered on 29 September 1884 by Austrian astronomer Johann Palisa at Vienna Observatory and named after Ida (nurse of Zeus), a nymph from Greek mythology. Later telesc ...
. In 1994, ''Galileo'' observed
Comet Shoemaker–Levy 9
Comet Shoemaker–Levy 9 ( formally designated D/1993 F2) was a comet that broke apart in July 1992 and collided with Jupiter in July 1994, providing the first direct observation of an extraterrestrial collision of Solar System objects. ...
's collision with Jupiter.
Jupiter's atmospheric composition and
ammonia
Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...
clouds were recorded, as were the
volcanism
Volcanism, vulcanism, volcanicity, or volcanic activity is the phenomenon where solids, liquids, gases, and their mixtures erupt to the surface of a solid-surface astronomical body such as a planet or a moon. It is caused by the presence of a he ...
and
plasma interactions on
Io with Jupiter's atmosphere. The data ''Galileo'' collected supported
the theory of a liquid ocean under the icy surface of
Europa
Europa may refer to:
Places
* Europa (Roman province), a province within the Diocese of Thrace
* Europa (Seville Metro), Seville, Spain; a station on the Seville Metro
* Europa City, Paris, France; a planned development
* Europa Cliffs, Alexan ...
, and there were indications of similar liquid-
saltwater
Saline water (more commonly known as salt water) is water that contains a high concentration of dissolved salts (mainly sodium chloride). On the United States Geological Survey (USGS) salinity scale, saline water is saltier than brackish wat ...
layers under the surfaces of
Ganymede and
Callisto
CALLISTO (''Cooperative Action Leading to Launcher Innovation in Stage Toss-back Operations'') is a reusable VTVL Prototype, demonstrator propelled by a small 40 kN Japanese LOX-LH2 rocket engine. It is being developed jointly by the CNES, French ...
. Ganymede was shown to possess a
magnetic field
A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...
and the spacecraft found new evidence for
exosphere
The exosphere is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the density is so low that the molecules are essentially collision-less. In the case of ...
s around Europa, Ganymede, and Callisto. ''Galileo'' also discovered that Jupiter's faint
ring system
A ring system is a disc or torus orbiting an astronomical object that is composed of solid material such as dust, meteoroids, planetoids, moonlets, or stellar objects.
Ring systems are best known as planetary rings, common components of sate ...
consists of dust from
impact event
An impact event is a collision between astronomical objects causing measurable effects. Impact events have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or meteoroids and have minimal effe ...
s on the four small inner moons. The extent and structure of Jupiter's
magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior Dynamo ...
was also mapped.
The primary mission concluded on December 7, 1997, but the ''Galileo'' orbiter commenced an extended mission known as the ''Galileo'' Europa Mission (GEM), which ran until December 31, 1999. By the time GEM ended, most of the spacecraft was operating well beyond its original design specifications, having absorbed three times the radiation exposure that it had been built to withstand. Many of the instruments were no longer operating at peak performance, but were still functional, so a second extension, the ''Galileo'' Millennium Mission (GMM) was authorized. On September 20, 2003, after 14 years in space and 8 years in the Jovian system, ''Galileo'' mission was terminated by sending the orbiter into Jupiter's atmosphere at a speed of over to eliminate the possibility of
contaminating the moons with bacteria.
Background
Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...
is the largest planet in the
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...
, with more than twice the mass of all the other planets combined. Consideration of sending a probe to Jupiter began as early as 1959, when the
National Aeronautics and Space Administration
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the United States's civil space program, aeronautics research and space research. Established in 1958, it su ...
(NASA)
Jet Propulsion Laboratory
The Jet Propulsion Laboratory (JPL) is a Federally funded research and development centers, federally funded research and development center (FFRDC) in La Cañada Flintridge, California, Crescenta Valley, United States. Founded in 1936 by Cali ...
(JPL) developed four mission concepts:
* Deep space flights would fly through interplanetary space;
*
Planetary flyby
A planetary flyby is the act of sending a space probe past a planet or a dwarf planet close enough to record scientific data. This is a subset of the overall concept of a flyby in spaceflight.
The first flyby of another planet with a functionin ...
missions would fly past planets close enough to collect scientific data and could visit multiple planets on a single mission;
* Orbiter missions would place a spacecraft in orbit around a planet for prolonged and detailed study;
*
Atmospheric entry
Atmospheric entry (sometimes listed as Vimpact or Ventry) is the movement of an object from outer space into and through the gases of an atmosphere of a planet, dwarf planet, or natural satellite. Atmospheric entry may be ''uncontrolled entr ...
and
lander missions would explore a planet's atmosphere and surface.
Two missions to Jupiter, ''
Pioneer 10
''Pioneer 10'' (originally designated Pioneer F) is a NASA space probe launched in 1972 that completed the first mission to the planet Jupiter. ''Pioneer 10'' became the first of five artificial objects to achieve the escape velocity needed ...
'' and ''
Pioneer 11
''Pioneer 11'' (also known as ''Pioneer G'') is a NASA robotic space probe launched on April 5, 1973, to study the asteroid belt, the environment around Jupiter and Saturn, the solar wind, and cosmic rays. It was the first probe to Exploration ...
'', were approved in 1969, with NASA's
Ames Research Center
The Ames Research Center (ARC), also known as NASA Ames, is a major NASA research center at Moffett Federal Airfield in California's Silicon Valley. It was founded in 1939 as the second National Advisory Committee for Aeronautics (NACA) laborat ...
given responsibility for planning the missions. ''Pioneer 10'' was launched in March 1972 and passed within of Jupiter in December 1973. It was followed by ''Pioneer 11'', which was launched in April 1973, and passed within of Jupiter in December 1974, before heading on to an
encounter with
Saturn
Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...
. They were followed by the more advanced ''
Voyager 1
''Voyager 1'' is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and the interstellar medium, interstellar space beyond the Sun's heliosphere. It was launched 16 days afte ...
'' and ''
Voyager 2
''Voyager 2'' is a space probe launched by NASA on August 20, 1977, as a part of the Voyager program. It was launched on a trajectory towards the gas giants (Jupiter and Saturn) and enabled further encounters with the ice giants (Uranus and ...
'' spacecraft, which were launched on 5 September and 20 August 1977 respectively, and reached Jupiter in March and July 1979.
Planning
Initiation
Following the approval of the ''
Voyager
Voyager may refer to:
Computing and communications
* LG Voyager, a mobile phone model manufactured by LG Electronics
* NCR Voyager, a computer platform produced by NCR Corporation
* Voyager (computer worm), a computer worm affecting Oracle ...
'' missions, NASA's Scientific Advisory Group for Outer Solar System Missions considered the requirements for Jupiter orbiters and atmospheric probes. It noted that the technology to build a
heat shield
In engineering, a heat shield is a component designed to protect an object or a human operator from being burnt or overheated by dissipating, reflecting, and/or absorbing heat. The term is most often used in reference to exhaust heat management a ...
for an atmospheric probe did not yet exist, and indeed facilities to test one under the conditions found on Jupiter would not be available until 1980. There was also concern about the effects of radiation on spacecraft components, which would be better understood after ''Pioneer 10'' and ''Pioneer 11'' had conducted their flybys. ''Pioneer 10''s flyby in December 1973 indicated that the effects were not as severe as had been feared. NASA management designated JPL as the lead center for the Jupiter Orbiter Probe (JOP) Project.
John R. Casani, who had headed the ''
Mariner
A sailor, seaman, mariner, or seafarer is a person who works aboard a watercraft as part of its crew, and may work in any one of a number of different fields that are related to the operation and maintenance of a ship. While the term ''sailor' ...
'' and ''Voyager'' projects, became the first project manager. The JOP would be the fifth spacecraft to visit Jupiter, but the first to orbit it, and the probe the first to enter its atmosphere.
Ames and JPL decided to use a ''Mariner'' spacecraft for the Jupiter orbiter like the ones used for ''Voyager'' rather than a ''Pioneer'' spacecraft. ''Pioneer'' was stabilized by spinning the spacecraft at 60
rpm
Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or r⋅min−1) is a unit of rotational speed (or rotational frequency) for rotating machines.
One revolution per minute is equivalent to hertz.
Standards
ISO 80000-3:2019 def ...
, which gave a 360-degree view of the surroundings, and did not require an
attitude control system
Spacecraft attitude control is the process of controlling the orientation of a spacecraft (vehicle or satellite) with respect to an inertial frame of reference or another entity such as the celestial sphere, certain fields, and nearby objects, ...
. By contrast, ''Mariner'' had an attitude control system with three
gyroscopes
A gyroscope (from Ancient Greek γῦρος ''gŷros'', "round" and σκοπέω ''skopéō'', "to look") is a device used for measuring or maintaining Orientation (geometry), orientation and angular velocity. It is a spinning wheel or disc in ...
and two sets of six
nitrogen
Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
jet thrusters. Attitude was determined with reference to the Sun and
Canopus
Canopus is the brightest star in the southern constellation of Carina (constellation), Carina and the list of brightest stars, second-brightest star in the night sky. It is also Bayer designation, designated α Carinae, which is Rom ...
, which were monitored with two primary and four secondary
star tracker
A star tracker is an optical device that measures the positions of stars using photocells or a camera.
As the positions of many stars have been measured by astronomers to a high degree of accuracy, a star tracker on a satellite or spacecraft may ...
sensors. There was also an
inertial reference unit
An inertial reference unit (IRU) is a type of inertial sensor which uses gyroscopes (electromechanical, ring laser gyro or MEMS) and accelerometers (electromechanical or MEMS) to determine a moving aircraft’s or spacecraft’s change in ...
and an
accelerometer
An accelerometer is a device that measures the proper acceleration of an object. Proper acceleration is the acceleration (the rate of change (mathematics), rate of change of velocity) of the object relative to an observer who is in free fall (tha ...
. The attitude control system allowed the spacecraft to take high-resolution images, but the functionality came at the cost of increased weight: a ''Mariner'' weighed compared to just for a ''Pioneer''.
The increase in weight had implications. The Voyager spacecraft had been launched by
Titan IIIE
The Titan IIIE or Titan 3E, also known as the Titan III-Centaur, was an American expendable launch system. Launched seven times between 1974 and 1977, it enabled several high-profile NASA missions, including the Voyager and Viking planetary pro ...
rockets with a ''
Centaur
A centaur ( ; ; ), occasionally hippocentaur, also called Ixionidae (), is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse that was said to live in the mountains of Thessaly. In one version o ...
'' upper stage, but Titan was retired afterwards. In the late 1970s, NASA was focused on the development of the reusable
Space Shuttle
The Space Shuttle is a retired, partially reusable launch system, reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. ...
, which was expected to make expendable rockets obsolete. In late 1975, NASA decreed that all future planetary missions would be launched by the Space Shuttle. The JOP would be the first to do so. The Space Shuttle was supposed to have the services of a
space tug
''Space Tug'' is a young adult fiction, young adult science fiction novel by author Murray Leinster. It was published in 1953 in literature, 1953 by Shasta Publishers in an edition of 5,000 copies. It is the second novel in the author's Joe K ...
to launch payloads requiring something more than a
low Earth orbit
A low Earth orbit (LEO) is an geocentric orbit, orbit around Earth with a orbital period, period of 128 minutes or less (making at least 11.25 orbits per day) and an orbital eccentricity, eccentricity less than 0.25. Most of the artificial object ...
, but this was never approved. The
United States Air Force
The United States Air Force (USAF) is the Air force, air service branch of the United States Department of Defense. It is one of the six United States Armed Forces and one of the eight uniformed services of the United States. Tracing its ori ...
(USAF) instead developed the
solid-fueled
Solid fuel refers to various forms of solid material that can be burnt to release energy, providing heat and light through the process of combustion. Solid fuels can be contrasted with liquid fuels and gaseous fuels. Common examples of solid fue ...
Interim Upper Stage (IUS), later renamed the
Inertial Upper Stage
The Inertial Upper Stage (IUS), originally designated the Interim Upper Stage, was a Multistage rocket, two-stage, Solid-propellant rocket, solid-fueled space launch system developed by Boeing for the United States Air Force beginning in 1976 for ...
(with the same acronym), for the purpose.
The IUS was constructed in a modular fashion, with two stages, a large one with of propellant, and a smaller one with . This was sufficient for most satellites. It could also be configured with two large stages to launch multiple satellites. A configuration with three stages, two large and one small, would be enough for a planetary mission, so NASA contracted with
Boeing
The Boeing Company, or simply Boeing (), is an American multinational corporation that designs, manufactures, and sells airplanes, rotorcraft, rockets, satellites, and missiles worldwide. The company also provides leasing and product support s ...
for the development of a three-stage IUS. A two-stage IUS was not powerful enough to launch a payload to Jupiter without resorting to using a series of
gravity-assist maneuvers around planets to garner additional speed. Most engineers regarded this solution as inelegant and planetary scientists at JPL disliked it because it meant that the mission would take months or even years longer to reach Jupiter. Longer travel times meant that the spacecraft's components would age and possibly fail, and the onboard power supply and propellant would be depleted. Some of the gravity assist options also involved flying closer to the Sun, which would induce thermal stresses that also might cause failures.
It was estimated that the JOP would cost $634 million (equivalent to $ billion in ), and it had to compete for
fiscal year
A fiscal year (also known as a financial year, or sometimes budget year) is used in government accounting, which varies between countries, and for budget purposes. It is also used for financial reporting by businesses and other organizations. La ...
1978 funding with the Space Shuttle and the
Hubble Space Telescope
The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...
. A successful lobbying campaign secured funding for both JOP and Hubble over the objections of
Senator
A senate is a deliberative assembly, often the upper house or Legislative chamber, chamber of a bicameral legislature. The name comes from the Ancient Rome, ancient Roman Senate (Latin: ''Senatus''), so-called as an assembly of the senior ...
William Proxmire
Edward William Proxmire (November 11, 1915 – December 15, 2005) was an American politician. A member of the Democratic Party (United States), Democratic Party, he served as a United States Senate, United States Senator from Wisconsin from 1957 ...
, the chairman of the Independent Agencies Appropriations Subcommittee. The
United States Congress
The United States Congress is the legislature, legislative branch of the federal government of the United States. It is a Bicameralism, bicameral legislature, including a Lower house, lower body, the United States House of Representatives, ...
approved funding for the Jupiter Orbiter Probe on July 19, 1977, and JOP officially commenced on October 1, 1977, the start of the fiscal year. Project manager Casani solicited suggestions for a more inspirational name for the project from people associated with it. The most votes went to "Galileo", after
Galileo Galilei
Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642), commonly referred to as Galileo Galilei ( , , ) or mononymously as Galileo, was an Italian astronomer, physicist and engineer, sometimes described as a poly ...
, the first person to view Jupiter through a telescope, and the discoverer of what are now known as the
Galilean moons
The Galilean moons (), or Galilean satellites, are the four largest moons of Jupiter. They are, in descending-size order, Ganymede (moon), Ganymede, Callisto (moon), Callisto, Io (moon), Io, and Europa (moon), Europa. They are the most apparent m ...
in 1610. It was noted at the time that the name was also that of a
spacecraft
A spacecraft is a vehicle that is designed spaceflight, to fly and operate in outer space. Spacecraft are used for a variety of purposes, including Telecommunications, communications, Earth observation satellite, Earth observation, Weather s ...
in the ''
Star Trek
''Star Trek'' is an American science fiction media franchise created by Gene Roddenberry, which began with the Star Trek: The Original Series, series of the same name and became a worldwide Popular culture, pop-culture Cultural influence of ...
'' television show. In February 1978, Casani officially announced the choice of the name "Galileo".
Preparation
To enhance reliability and reduce costs, the project engineers decided to switch from a pressurized
atmospheric probe to a vented one, so the pressure inside the probe would be the same as that outside, thus extending its lifetime in Jupiter's atmosphere, but this added to its weight. Another was added in structural changes to improve reliability. This required additional fuel in the IUS, but the three-stage IUS was itself overweight with respect to its design specifications, by about .
Lifting ''Galileo'' and the three-stage IUS required a special lightweight version of the
Space Shuttle external tank
The Space Shuttle external tank (ET) was the component of the Space Shuttle launch vehicle that contained the liquid hydrogen fuel and liquid oxygen oxidizer. During lift-off and ascent it supplied the fuel and oxidizer under pressure to the ...
, the
Space Shuttle orbiter
The Space Shuttle orbiter is the spaceplane component of the Space Shuttle, a partially reusable launch system, reusable orbital spaceflight, orbital spacecraft system that was part of the discontinued Space Shuttle program. Operated from 1981 ...
stripped of all non-essential equipment, and the
Space Shuttle main engine
The RS-25, also known as the Space Shuttle Main Engine (SSME), is a liquid-fuel cryogenic rocket engine that was used on NASA's Space Shuttle and is used on the Space Launch System.
Designed and manufactured in the United States by Rocketd ...
s (SSME) running at full power level—109 percent of their rated power level. Running at this power level necessitated the development of a more elaborate engine cooling system. Concerns were raised over whether the engines could be run at 109 percent by the launch date, so a gravity-assist maneuver using Mars was substituted for a direct flight.

Plans called for the to launch ''Galileo'' on the
STS-23 mission, tentatively scheduled for sometime between January 2 and 12, 1982, this being the launch window when Earth, Mars and Jupiter were aligned to permit Mars to be used for the gravity-assist maneuver. By 1980, delays in the Space Shuttle program pushed the launch date for ''Galileo'' back to 1984. While a Mars slingshot was still possible in 1984, it would no longer be sufficient.
NASA decided to launch ''Galileo'' on two separate missions, launching the orbiter in February 1984 with the probe following a month later. The orbiter would be in orbit around Jupiter when the probe arrived, allowing the orbiter to perform its role as a relay. This configuration required a second Space Shuttle mission and a second carrier spacecraft to be built for the probe to take it to Jupiter, and was estimated to cost an additional $50 million (equivalent to $ million in ), but NASA hoped to be able to recoup some of this through competitive bidding. The problem was that while the atmospheric probe was light enough to launch with the two-stage IUS, the Jupiter orbiter was too heavy to do so, even with a gravity assist from Mars, so the three-stage IUS was still required.
By late 1980, the price tag for the IUS had risen to $506 million (equivalent to $ billion in ). The USAF could absorb this cost overrun on the development of the two-stage IUS (and indeed anticipated that it might cost far more), but NASA was faced with a quote of $179 million (equivalent to $ million in ) for the development of the three-stage version, which was $100 million (equivalent to $ million in ) more than it had budgeted for. At a press conference on January 15, 1981,
Robert A. Frosch, the
NASA Administrator
The administrator of the National Aeronautics and Space Administration is the highest-ranking official of NASA, the national List of space agencies, space agency of the United States. The administrator is NASA's chief decision maker, responsible ...
, announced that NASA was withdrawing support for the three-stage IUS, and going with a
Centaur G Prime upper stage because "no other alternative upper stage is available on a reasonable schedule or with comparable costs."
Centaur provided many advantages over the IUS. The main one was that it was far more powerful. The probe and orbiter could be recombined, and the probe could be delivered directly to Jupiter in two years' flight time. The second was that, despite this, it was gentler than the IUS, because it had lower thrust. This reduced the chance of damage to the payload. Thirdly, unlike solid-fuel rockets which burned to completion once ignited, a Centaur could be switched off and on again. This gave it flexibility, which increased the chances of a successful mission, and permitted options like asteroid flybys. Centaur was proven and reliable, whereas the IUS had not yet flown. The only concern was about safety; solid-fuel rockets were considered safer than liquid-fuel ones, especially ones containing
liquid hydrogen
Liquid hydrogen () is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecule, molecular H2 form.
To exist as a liquid, H2 must be cooled below its critical point (thermodynamics), critical point of 33 Kelvins, ...
. NASA engineers estimated that additional safety features might take up to five years to develop and cost up to $100 million (equivalent to $ million in ).
In February 1981, JPL learned that the
Office of Management and Budget
The Office of Management and Budget (OMB) is the largest office within the Executive Office of the President of the United States (EOP). The office's most prominent function is to produce the president's budget, while it also examines agency pro ...
(OMB) was planning major cuts to NASA's budget, and was considering cancelling ''Galileo''. The USAF intervened to save ''Galileo'' from cancellation. JPL had considerable experience with autonomous spacecraft that could make their own decisions. This was a necessity for deep space probes, since a signal from Earth takes from 35 to 52 minutes to reach Jupiter, depending on the relative position of the planets in their orbits. The USAF was interested in providing this capability for its satellites, so that they would be able to determine their
attitude
Attitude or Attitude may refer to:
Philosophy and psychology
* Attitude (psychology), a disposition or state of mind
** Attitude change
* Propositional attitude, a mental state held towards a proposition
Science and technology
* Orientation ...
using onboard systems rather than relying on
ground station
A ground station, Earth station, or Earth terminal is a terrestrial radio station designed for extraplanetary telecommunication with spacecraft (constituting part of the ground segment of the spacecraft system), or reception of radio waves fr ...
s, which were not "hardened" against
nuclear weapons
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either nuclear fission, fission (fission or atomic bomb) or a combination of fission and nuclear fusion, fusion reactions (thermonuclear weap ...
, and could take independent evasive action against
anti-satellite weapon
Anti-satellite weapons (ASAT) are space weapons designed to incapacitate or destroy satellites for Military strategy, strategic or Military tactics, tactical purposes. Although no ASAT system has been utilized in warfare, a few countries (China, ...
s. It was also interested in the manner in which JPL was designing ''Galileo'' to withstand the intense radiation of the
magnetosphere of Jupiter
The magnetosphere of Jupiter is the cavity created in the solar wind by Jupiter's magnetic field. Extending up to seven million kilometers in the Sun's direction and almost to the orbit of Saturn in the opposite direction, Jupiter's magnetospher ...
, as this could be used to harden satellites against the
electromagnetic pulse
An electromagnetic pulse (EMP), also referred to as a transient electromagnetic disturbance (TED), is a brief burst of electromagnetic energy. The origin of an EMP can be natural or artificial, and can occur as an electromagnetic field, as an ...
of nuclear explosions. On February 6, 1981
Strom Thurmond
James Strom Thurmond Sr. (December 5, 1902 – June 26, 2003) was an American politician who represented South Carolina in the United States Senate from 1954 to 2003. Before his 49 years as a senator, he served as the 103rd governor of South ...
, the
President pro tem of the Senate, wrote directly to
David Stockman
David Alan Stockman (born November 10, 1946) is an American politician and former businessman who was a Republican U.S. Representative from the state of Michigan (1977–1981) and the Director of the Office of Management and Budget (1981–1985 ...
, the director of the OMB, arguing that ''Galileo'' was vital to the nation's defense.

In December 1984, Casani proposed adding a flyby of asteroid
29 Amphitrite
29 Amphitrite is one of the largest S-type asteroids, approximately in diameter, and probably fifth largest after 15 Eunomia, Eunomia, 3 Juno, Juno, 7 Iris, Iris and 532 Herculina, Herculina.
Discovery
Amphitrite was discovered by Albert Mart ...
to the ''Galileo'' mission. In plotting a course to Jupiter, the engineers wanted to avoid asteroids. Little was known about them at the time, and it was suspected that they could be surrounded by dust particles. Flying through a dust cloud could damage the spacecraft's optics and possibly other parts of the spacecraft as well. To be safe, JPL wanted to avoid asteroids by at least . Most of the asteroids in the vicinity of the flight path like
1219 Britta and
1972 Yi Xing were only a few kilometers in diameter and promised little scientific value when observed from a safe distance, but 29 Amphitrite was one of the largest, and a flyby at even could have great value. The flyby would delay the spacecraft's arrival in Jupiter orbit from August 29 to December 10, 1988, and the expenditure of propellant would reduce the number of orbits of Jupiter from eleven to ten. This was expected to add $20 to $25 million (equivalent to $ to $ million in ) to the cost of the ''Galileo'' project. The 29 Amphitrite flyby was approved by NASA Administrator
James M. Beggs on December 6, 1984.
During testing, contamination was discovered in the system of metal
slip ring
A slip ring is an electromechanical device that allows the transmission of Electric power, power and electrical Signal, signals from a stationary to a rotating structure. A slip ring can be used in any Electromechanics, electromechanical system ...
s and brushes used to transmit electrical signals around the spacecraft, and they were returned to be refabricated. The problem was traced back to a
chlorofluorocarbon
Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are fully or partly Halogenation, halogenated hydrocarbons that contain carbon (C), hydrogen (H), chlorine (Cl), and fluorine (F). They are produced as volatility (chemistry), volat ...
used to clean parts after soldering. It had been absorbed, and was then released in a vacuum environment. It mixed with debris generated as the brushes wore down, and caused intermittent problems with electrical signal transmission. Problems were also detected in the performance of memory devices in an electromagnetic radiation environment. The components were replaced, but then a
read disturb problem arose, in which reads from one memory location disturbed the contents of adjacent locations. This was found to have been caused by the changes made to make the components less sensitive to electromagnetic radiation. Each component had to be removed, retested, and replaced. All of the spacecraft components and spare parts received a minimum of 2,000 hours of testing. The spacecraft was expected to last for at least five years—long enough to reach Jupiter and perform its mission. On December 19, 1985, it departed JPL in
Pasadena, California
Pasadena ( ) is a city in Los Angeles County, California, United States, northeast of downtown Los Angeles. It is the most populous city and the primary cultural center of the San Gabriel Valley. Old Pasadena is the city's original commerci ...
, on the first leg of its journey, a road trip to the
Kennedy Space Center
The John F. Kennedy Space Center (KSC, originally known as the NASA Launch Operations Center), located on Merritt Island, Florida, is one of the NASA, National Aeronautics and Space Administration's (NASA) ten NASA facilities#List of field c ...
in
Florida
Florida ( ; ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders the Gulf of Mexico to the west, Alabama to the northwest, Georgia (U.S. state), Georgia to the north, the Atlantic ...
. The ''Galileo'' mission was scheduled for
STS-61-G on May 20, 1986, using .
Spacecraft
JPL built the Galileo spacecraft and managed the Galileo program for NASA, but West Germany's
Messerschmitt-Bölkow-Blohm
Messerschmitt-Bölkow-Blohm (MBB) was a West Germany, West German aerospace manufacturer. It was formed during the late 1960s as the result of efforts to consolidate the West German aerospace industry; aircraft manufacturer Messerschmitt AG merged ...
supplied the propulsion module, and Ames managed the atmospheric probe, which was built by the
Hughes Aircraft Company
The Hughes Aircraft Company was a major American aerospace and defense contractor founded on February 14, 1934 by Howard Hughes in Glendale, California, as a division of the Hughes Tool Company. The company produced the Hughes H-4 Hercules air ...
. At launch, the orbiter and probe together had a mass of and stood tall. There were twelve experiments on the orbiter and seven on the atmospheric probe. The orbiter was powered by a pair of
general-purpose heat source radioisotope thermoelectric generators (GPHS-RTGs) fueled by
plutonium-238
Plutonium-238 ( or Pu-238) is a radioactive isotope of plutonium that has a half-life of 87.7 years.
Plutonium-238 is a very powerful alpha emitter; as alpha particles are easily blocked, this makes the plutonium-238 isotope suitable for usage ...
that generated 570 watts at launch. The atmospheric probe had a
lithium–sulfur battery
The lithium–sulfur battery (Li–S battery) is a type of rechargeable battery. It is notable for its high specific energy. The low atomic weight of lithium and moderate atomic weight of sulfur means that Li–S batteries are relatively light (a ...
rated at 730 watt-hours.
Probe instruments included sensors for measuring atmospheric temperature and pressure. There was a
mass spectrometer
Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a '' mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is us ...
and a
helium
Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...
-abundance detector to study atmospheric composition, and a
whistler detector for measurements of lightning activity and Jupiter's radiation belt. There were magnetometer sensors, a plasma-wave detector, a
high-energy particle
Particle physics or high-energy physics is the study of fundamental particles and forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the scale of protons and neutrons, while the study ...
detector, a
cosmic and Jovian dust detector, and a
heavy ion
High-energy nuclear physics studies the behavior of nuclear matter in energy regimes typical of high-energy physics. The primary focus of this field is the study of heavy-ion collisions, as compared to lighter atoms in other particle accelerator ...
counter. There was a
near-infrared mapping spectrometer
''Galileo'' was an American robotic space probe that studied the planet Jupiter and its moons, as well as the asteroids Gaspra and Ida. Named after the Italian astronomer Galileo Galilei, it consisted of an orbiter and an entry probe. It was ...
for
multispectral images for atmospheric and moon surface chemical analysis, and an
ultraviolet spectrometer to study gases.
Reconsideration
On January 28, 1986, lifted off on the
STS-51-L
STS-51-L was the disastrous 25th mission of NASA's Space Shuttle program and the final flight of Space Shuttle ''Challenger''.
It was planned as the first Teacher in Space Project flight in addition to observing Halley's Comet for six day ...
mission. A failure of the solid rocket booster 73 seconds into flight tore the spacecraft apart, resulting in the deaths of all seven crew members. The
Space Shuttle ''Challenger'' disaster was America's worst space disaster up to that time. The immediate impact on the ''Galileo'' project was that the May launch date could not be met because the Space Shuttles were grounded while the cause of the disaster was investigated. When they did fly again, ''Galileo'' would have to compete with high-priority
Department of Defense
The United States Department of Defense (DoD, USDOD, or DOD) is an executive department of the U.S. federal government charged with coordinating and supervising the six U.S. armed services: the Army, Navy, Marines, Air Force, Space Force, ...
launches, the
tracking and data relay satellite system, and the Hubble Space Telescope. By April 1986, it was expected that the Space Shuttles would not fly again before July 1987 at the earliest, and ''Galileo'' could not be launched before December 1987.

The
Rogers Commission
The Rogers Commission Report was written by a Presidential Commission charged with investigating the Space Shuttle ''Challenger'' disaster during its 10th mission, STS-51-L. The report, released and submitted to President Ronald Reagan on June ...
into the ''Challenger'' disaster handed down its report on June 6, 1986. It was critical of NASA's safety protocols and risk management. In particular, it noted the hazards of a Centaur-G stage. On June 19, 1986, NASA Administrator
James C. Fletcher
James Chipman Fletcher (June 5, 1919 – December 22, 1991) served as the 4th and 7th Administrator of NASA, first from April 27, 1971, to May 1, 1977, under Presidents Richard Nixon, Gerald Ford and Jimmy Carter, and again from May 12, 1986, to ...
canceled the Shuttle-Centaur project. This was only partly due to the NASA management's increased aversion to risk in the wake of the ''Challenger'' disaster; NASA management also considered the money and manpower required to get the Space Shuttle flying again, and decided that there were insufficient resources to resolve lingering issues with Shuttle-Centaur as well. The changes to the Space Shuttle proved more extensive than anticipated, and in April 1987, JPL was informed that ''Galileo'' could not be launched before October 1989. The ''Galileo'' spacecraft was shipped back to JPL.
Without Centaur, it looked like there was no means of getting ''Galileo'' to Jupiter. For a time, ''
Los Angeles Times
The ''Los Angeles Times'' is an American Newspaper#Daily, daily newspaper that began publishing in Los Angeles, California, in 1881. Based in the Greater Los Angeles city of El Segundo, California, El Segundo since 2018, it is the List of new ...
'' science reporter
Usha Lee McFarling
Usha Lee McFarling is an American science reporter who is an Artist In Residence at the University of Washington Department of Communication. She won a 2007 Pulitzer Prize for Explanatory Reporting.[Smithsonian Institution
The Smithsonian Institution ( ), or simply the Smithsonian, is a group of museums, Education center, education and Research institute, research centers, created by the Federal government of the United States, U.S. government "for the increase a ...]
."
The cost of keeping it ready to fly in space was reckoned at $40 to $50 million per year (equivalent to $ to $ million in ), and the estimated cost of the whole project had blown out to $1.4 billion (equivalent to $ billion in ).
At JPL, the ''Galileo'' Mission Design Manager and Navigation Team Chief, Robert Mitchell, assembled a team that consisted of Dennis Byrnes, Louis D'Amario, Roger Diehl and himself, to see if they could find a trajectory that would get ''Galileo'' to Jupiter using only a two-stage IUS. Roger Diehl came up with the idea of using a series of gravity assists to provide the additional velocity required to reach Jupiter. This would require ''Galileo'' to fly past Venus, and then past Earth twice. This was referred to as the Venus-Earth-Earth Gravity Assist (VEEGA) trajectory.
The reason no one had considered the VEEGA trajectory before was that the second encounter with Earth would not give the spacecraft any extra energy. Diehl realised that this was not necessary; the second encounter would merely change its direction to put it on a course for Jupiter. In addition to increasing the flight time, the VEEGA trajectory had another drawback from the point of view of
NASA Deep Space Network
The NASA Deep Space Network (DSN) is a worldwide Telecommunications network, network of spacecraft communication ground segment facilities, located in the United States (California), Spain (Madrid), and Australia (Canberra), that supports NASA' ...
(DSN): ''Galileo'' would arrive at Jupiter when it was at the maximum range from Earth, and maximum range meant minimum signal strength. It would have a
declination
In astronomy, declination (abbreviated dec; symbol ''δ'') is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. The declination angle is measured north (positive) or ...
of 23 degrees south instead of 18 degrees north, so the tracking station would be the
Canberra Deep Space Communication Complex
The Canberra Deep Space Communication Complex (CDSCC) is a satellite communication station, part of the Deep Space Network of NASA's Jet Propulsion Laboratory (JPL), located at Tidbinbilla in the Australian Capital Territory. Opened in 196 ...
in Australia, with its two 34-meter and one 70-meter antennae. A northerly declination could have been supported by two sites, at
Goldstone and
Madrid
Madrid ( ; ) is the capital and List of largest cities in Spain, most populous municipality of Spain. It has almost 3.5 million inhabitants and a Madrid metropolitan area, metropolitan area population of approximately 7 million. It i ...
. The Canberra antennae were supplemented by the 64-meter antenna at the
Parkes Observatory
Parkes Observatory is a radio astronomy observatory, located north of the town of Parkes, New South Wales, Australia. It hosts Murriyang, the 64 m CSIRO Parkes Radio Telescope also known as "The Dish", along with two smaller radio telescopes. T ...
.
Initially it was thought that the VEEGA trajectory demanded a November launch, but D'Amario and Byrnes calculated that a mid-course correction between Venus and Earth would permit an October launch as well. Taking such a roundabout route meant that ''Galileo'' would require sixty months to reach Jupiter instead of just thirty, but it would get there.
Consideration was given to using the USAF's
Titan IV
Titan IV was a family of heavy-lift space launch vehicles developed by Martin Marietta and operated by the United States Air Force from 1989 to 2005. Launches were conducted from Cape Canaveral Air Force Station, Florida and Vandenberg Air Forc ...
launch system with its Centaur G Prime upper stage. This was retained as a backup for a time, but in November 1988 the USAF informed NASA that it could not provide a Titan IV in time for the May 1991 launch opportunity, owing to the backlog of high priority Department of Defense missions. However, the USAF supplied IUS-19, which had originally been earmarked for a Department of Defense mission, for use by the ''Galileo'' mission.
Nuclear concerns
As the launch date of ''Galileo'' neared,
anti-nuclear groups
Anti-nuclear organizations may oppose uranium mining, nuclear power, and/or nuclear weapons. Anti-nuclear groups have undertaken public protests and acts of civil disobedience which have included occupations of nuclear plant sites. Some of the mo ...
, concerned over what they perceived as an unacceptable risk to the public's safety from the
plutonium
Plutonium is a chemical element; it has symbol Pu and atomic number 94. It is a silvery-gray actinide metal that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibits six allotropes and four ...
in ''Galileo''s GPHS-RTG modules, sought a court injunction prohibiting ''Galileo'' launch.
RTGs were necessary for deep space probes because they had to fly distances from the Sun that made the use of solar energy impractical.
They had been used for years in planetary exploration without mishap: the Department of Defense's
Lincoln Experimental Satellite
The Lincoln Experimental Satellite series was designed and built by Lincoln Laboratory at Massachusetts Institute of Technology between 1965 and 1976, under USAF sponsorship, for testing devices and techniques for satellite communication.
Develo ...
s 8/9 had 7 percent more plutonium on board than ''Galileo'', and the two
''Voyager'' spacecraft each carried 80 percent of ''Galileo'' load of plutonium.
By 1989, plutonium had been used in 22 spacecraft.
Activists remembered the crash of the
Soviet Union
The Union of Soviet Socialist Republics. (USSR), commonly known as the Soviet Union, was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 until Dissolution of the Soviet ...
's nuclear-powered
Kosmos 954
Kosmos 954 () was a reconnaissance satellite launched by the Soviet Union in 1977. A malfunction prevented safe separation of its onboard Nuclear reactor technology, nuclear reactor; when the satellite Atmospheric reentry, reentered the Earth's ...
satellite in Canada in 1978, and the ''Challenger'' disaster, while it did not involve nuclear fuel, raised public awareness about spacecraft failures. No RTGs had ever done a non-orbital swing past the Earth at close range and high speed, as ''Galileo'' VEEGA trajectory required it to do. This created the possibility of a mission failure in which Galileo struck Earth's atmosphere and dispersed plutonium.
Planetary scientist
Planetary science (or more rarely, planetology) is the scientific study of planets (including Earth), celestial bodies (such as moons, asteroids, comets) and planetary systems (in particular those of the Solar System) and the processes of their ...
Carl Sagan
Carl Edward Sagan (; ; November 9, 1934December 20, 1996) was an American astronomer, planetary scientist and science communicator. His best known scientific contribution is his research on the possibility of extraterrestrial life, including e ...
, a strong supporter of the ''Galileo'' mission, wrote that "there is nothing absurd about either side of this argument."
Before the ''Challenger'' disaster, JPL had conducted shock tests on the RTGs that indicated that they could withstand a pressure of without a failure, which would have been sufficient to withstand an explosion on the launch pad. The possibility of adding additional shielding was considered but rejected, mainly because it would add an unacceptable amount of extra weight. After the ''Challenger'' disaster, NASA commissioned a study on the possible effects if such an event occurred with ''Galileo'' on board. Angus McRonald, a JPL engineer, concluded that what would happen would depend on the altitude at which the Space Shuttle broke up. If the ''Galileo''/IUS combination fell free from the orbiter at , the RTGs would fall to Earth without melting, and drop into the Atlantic Ocean about from the Florida coast. On the other hand, if the orbiter broke up at an altitude of it would be traveling at and the RTG cases and GPHS modules would melt before falling into the Atlantic off the Florida coast.
NASA concluded that the chance of a disaster was 1 in 2,500, although anti-nuclear groups thought it might be as high as 1 in 430.
NASA assessed the risk to an individual at 1 in 100 million, about two orders of magnitude less than the danger of being killed by lightning. The prospect of an inadvertent re-entry into the atmosphere during the VEEGA maneuvers was reckoned at less than 1 in 2 million,
but an accident might have released a maximum of . This could result in up to 9 fatalities from cancer per 10 million exposed people.
Launch
STS-34
STS-34 was a NASA Space Shuttle mission using ''Atlantis''. It was the 31st shuttle mission overall, and the fifth flight for ''Atlantis''. STS-34 launched from Kennedy Space Center, Florida, on October 18, 1989, and landed at Edwards Air For ...
was the mission designated to launch ''Galileo'', scheduled for October 12, 1989, in the Space Shuttle ''Atlantis''. The spacecraft was delivered to the Kennedy Space Center by a high-speed truck convoy that departed JPL in the middle of the night. There were fears that the trucks might be hijacked by anti-nuclear activists or terrorists after the plutonium, so the route was kept secret from the drivers beforehand, and they drove through the night and the following day and only stopped for food and fuel.
Last-minute efforts by three environmental groups (the
Christic Institute
The Christic Institute was a public interest law firm founded in 1980 by Daniel Sheehan, his wife Sara Nelson, and their partner, William J. Davis, a Jesuit priest, after the successful conclusion of their work on the ''Silkwood'' case. Based on ...
, the Florida Coalition for Peace and Justice and the
Foundation on Economic Trends
Jeremy Rifkin (born January 26, 1945) is an American economic and social theorist, writer, public speaker, political advisor, and activist. Rifkin is the author of 23 books about the influence of scientific and technological changes on the econo ...
) to halt the launch were rejected by the
District of Columbia Circuit on technical grounds rather than the merits of the case, but in a concurring opinion, Chief Justice
Patricia Wald
Patricia Ann McGowan Wald (; September 16, 1928 – January 12, 2019) was an American lawyer and jurist who served as the chief judge of the United States Court of Appeals for the District of Columbia Circuit from 1986 until 1991. She was the Cou ...
wrote that while the legal challenge was not
frivolous, there was no evidence of the plaintiffs' claim that NASA had acted improperly in compiling the mission's environmental assessment. On October 16, eight protesters were arrested for trespassing at the Kennedy Space Center; three were jailed and the remaining five released.
Federal judge
Oliver Gasch
Oliver Gasch (May 4, 1906 – July 8, 1999) was a United States district judge of the United States District Court for the District of Columbia.
Education and career
Born in Washington, D.C., Gasch received an Artium Baccalaureus degree from P ...
ruled on October 21 that the launch was in the public interest, as canceling it would cost the public $164 million and increased knowledge of the Solar system.
The launch was twice delayed; first by a faulty main engine controller that forced a postponement to October 17, and then by inclement weather, which necessitated a postponement to the following day,
but this was not a concern since the launch window extended until November 21.
''Atlantis'' finally lifted off at 16:53:40
UTC
Coordinated Universal Time (UTC) is the primary time standard globally used to regulate clocks and time. It establishes a reference for the current time, forming the basis for civil time and time zones. UTC facilitates international communica ...
on October 18, and went into a orbit.
''Galileo'' was successfully deployed at 00:15 UTC on October 19. Following the IUS burn, the ''Galileo'' spacecraft adopted its configuration for solo flight, and separated from the IUS at 01:06:53 UTC on October 19.
The launch was perfect, and ''Galileo'' was soon headed towards Venus at over . ''Atlantis'' returned to Earth safely on October 23.
Venus encounter
The encounter with
Venus
Venus is the second planet from the Sun. It is often called Earth's "twin" or "sister" planet for having almost the same size and mass, and the closest orbit to Earth's. While both are rocky planets, Venus has an atmosphere much thicker ...
on February 9 was in view of the DSN's Canberra and
Madrid Deep Space Communications Complex
The Madrid Deep Space Communications Complex (MDSCC; in Spanish and officially ''Complejo de Comunicaciones de Espacio Profundo de Madrid'') is a satellite ground station located in Robledo de Chavela, Spain, and operated by the Instituto Nac ...
es. ''Galileo''s closest approach to Venus came at 05:58:48 UTC on February 10, 1990, at a range of .
Due to the
Doppler effect
The Doppler effect (also Doppler shift) is the change in the frequency of a wave in relation to an observer who is moving relative to the source of the wave. The ''Doppler effect'' is named after the physicist Christian Doppler, who described ...
, the spacecraft's velocity relative to Earth could be computed by measuring the change in carrier frequency of the spacecraft's transmission compared to the nominal frequency. Doppler data collected by the DSN allowed JPL to verify that the gravity-assist maneuver had been successful, and the spacecraft had obtained the expected increase in speed. Unfortunately, three hours into the flyby, the tracking station at Goldstone had to be shut down due to high winds, and Doppler data was lost.

Because Venus was much closer to the Sun than the spacecraft had been designed to operate, great care was taken to avoid thermal damage. In particular, the
X-band
The X band is the designation for a band of frequency, frequencies in the microwave radio region of the electromagnetic spectrum. In some cases, such as in communication engineering, the frequency range of the X band is set at approximately 7.0� ...
high gain antenna
A directional antenna or beam antenna is an antenna that radiates or receives greater radio wave power in specific directions. Directional antennas can radiate radio wave
Radio waves (formerly called Hertzian waves) are a type of electroma ...
(HGA) was not deployed, but was kept folded up like an umbrella and pointed away from the Sun to keep it shaded and cool. This meant that the two small
S-band
The S band is a designation by the Institute of Electrical and Electronics Engineers (IEEE) for a part of the microwave band of the electromagnetic spectrum covering frequencies from 2 to 4 gigahertz (GHz). Thus it crosses the convention ...
low-gain antennae (LGAs) had to be used instead. They had a maximum bandwidth of 1,200
bits per second
In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time.
The bit rate is expressed in the unit bit per second (symbol: bit/s), often in conjunction ...
(bit/s) compared to the 134,000 bit/s expected from the HGA. As the spacecraft moved further from Earth, reception necessitated the use of the DSN's 70-meter dishes, to the detriment of other users, who had lower priority than ''Galileo''. Even so, the
downlink
In a telecommunications network, a link is a communication channel that connects two or more devices for the purpose of data transmission. The link may be a dedicated physical link or a virtual circuit that uses one or more physical links or shar ...
telemetry
Telemetry is the in situ collection of measurements or other data at remote points and their automatic transmission to receiving equipment (telecommunication) for monitoring. The word is derived from the Greek roots ''tele'', 'far off', an ...
rate fell to 40 bit/s within a few days of the Venus flyby, and by March it was down to just 10 bit/s.
Venus had been the focus of many automated flybys, probes, balloons and landers, most recently the 1989 ''
Magellan
Ferdinand Magellan ( – 27 April 1521) was a Portuguese people, Portuguese explorer best known for having planned and led the Magellan expedition, 1519–22 Spanish expedition to the East Indies. During this expedition, he also discovered t ...
'' spacecraft, and ''Galileo'' had not been designed with Venus in mind. Nonetheless, there were useful observations that it could make, as it carried some instruments that had never flown on spacecraft to Venus, such as the near-infrared mapping spectrometer (NIMS). Telescopic observations of Venus had revealed that there were certain parts of the infrared spectrum that the
greenhouse gas
Greenhouse gases (GHGs) are the gases in the atmosphere that raise the surface temperature of planets such as the Earth. Unlike other gases, greenhouse gases absorb the radiations that a planet emits, resulting in the greenhouse effect. T ...
es in the Venusian atmosphere did not block, making them transparent on these wavelengths. This permitted the NIMS to both view the clouds and obtain maps of the equatorial- and mid-latitudes of the night side of Venus with three to six times the resolution of Earth-based telescopes. The ultraviolet spectrometer (UVS) was also deployed to observe the Venusian clouds and their motions.
Another set of observations was conducted using Galileo's energetic-particles detector (EPD) when ''Galileo'' moved through the
bow shock
In astrophysics, bow shocks are shock waves in regions where the conditions of density and pressure change dramatically due to blowing stellar wind. Bow shock occurs when the magnetosphere of an astrophysical object interacts with the nearby fl ...
caused by Venus's interaction with the
solar wind
The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...
.
Earth's magnetic field
Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from structure of Earth, Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from ...
causes the bow shock to occur at around from its center, but Venus's weak magnetic field causes it to occur nearly on the surface, so the solar wind interacts with the atmosphere. A search for
lightning
Lightning is a natural phenomenon consisting of electrostatic discharges occurring through the atmosphere between two electrically charged regions. One or both regions are within the atmosphere, with the second region sometimes occurring on ...
on Venus was conducted using the
plasma-wave detector, which noted nine bursts likely to have been caused by lightning, but efforts to capture an image of lightning with the solid-state imaging system (SSI) were unsuccessful.
Earth encounters
Flybys
''Galileo'' made two course corrections on April 9 to 12 and May 11 to 12, 1990, to alter its velocity by . The spacecraft flew by Earth twice; the first time at a range of at 20:34:34 UTC on December 8, 1990.
This was higher than predicted, and the time of the closest approach was within a second of the prediction. It was the first time that a deep space probe had returned to Earth from interplanetary space. A second flyby of Earth was at at 15:09:25 UTC on December 8, 1992.
This time the spacecraft passed within a kilometer of its aiming point over the South Atlantic. This was so accurate that a scheduled course correction was cancelled, thereby saving of propellant.
Earth's bow shock and the solar wind

The Earth encounters provided an opportunity for a series of experiments. A study of Earth's bow shock was conducted as ''Galileo'' passed by Earth's day side. The solar wind travels at and is deflected by
Earth's magnetic field
Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from structure of Earth, Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from ...
, creating a
magnetic tail on Earth's dark side over a thousand times the radius of the planet. Observations were made by ''Galileo'' when it passed through the magnetic tail on Earth's dark side at a distance of from the planet. The magnetosphere was quite active at the time, and ''Galileo'' detected magnetic storms and
whistlers
Whistler may refer to:
* Someone who whistles
Places
Canada
* Whistler, British Columbia, a resort town
** Whistler railway station
** Whistler Secondary School
* Whistler Blackcomb, a ski resort in British Columbia
* Whistler Mountain, Bri ...
caused by lightning strikes.
The NIMS was employed to look for
mesospheric clouds, which were thought to be caused by
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...
released by industrial processes. The water vapor in the clouds breaks down the
ozone
Ozone () (or trioxygen) is an Inorganic compound, inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , break ...
in the upper atmosphere. Normally the clouds are only seen in September or October, but ''Galileo'' was able to detect them in December, an indication of possible damage to Earth's ozone layer.
Remote detection of life on Earth
Carl Sagan, pondering the question of whether
life on Earth could be easily detected from space, devised a set of experiments in the late 1980s using ''Galileo'' remote sensing instruments during the mission's first Earth flyby in December 1990. After data acquisition and processing, Sagan published a paper in ''
Nature
Nature is an inherent character or constitution, particularly of the Ecosphere (planetary), ecosphere or the universe as a whole. In this general sense nature refers to the Scientific law, laws, elements and phenomenon, phenomena of the physic ...
'' in 1993 detailing the results of the experiment. ''Galileo'' had indeed found what are now referred to as the "Sagan criteria for life". These included strong absorption of light at the red end of the visible spectrum (especially over
continents
A continent is any of several large geographical regions. Continents are generally identified by convention rather than any strict criteria. A continent could be a single large landmass, a part of a very large landmass, as in the case of As ...
) by chlorophyll in photosynthesizing plants; absorption bands of molecular oxygen as a result of plant activity; infrared bands caused by the approximately 1 micromole per
mole
Mole (or Molé) may refer to:
Animals
* Mole (animal) or "true mole"
* Golden mole, southern African mammals
* Marsupial mole
Marsupial moles, the Notoryctidae family, are two species of highly specialized marsupial mammals that are found i ...
of methane (a gas which must be replenished by volcanic or biological activity) in the atmosphere; and modulated narrowband radio wave transmissions uncharacteristic of any known natural source. ''Galileo'' experiments were thus the first
scientific control
A scientific control is an experiment or observation designed to minimize the effects of variables other than the independent variable (i.e. confounding variables). This increases the reliability of the results, often through a comparison betwe ...
s in the newborn science of
astrobiological remote sensing.
Lunar observations
File:Moon-galileo-color.jpg, Mare Orientale
Mare Orientale (Latin ''orientāle'', the "eastern sea") is a lunar mare. It is located on the western border of the near side and far side of the Moon, and is difficult to see from an Earthbound perspective. Images from spacecraft have revea ...
on the Moon
The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
, alt=The maria are large areas with less cratering
File:The Moon from Galileo - GPN-2000-000473.jpg, ''Galileo'' shot of the lunar north pole
The lunar north pole is the point in the Northern Hemisphere of the Moon where the lunar axis of rotation meets its surface.
The lunar North Pole is the northernmost point on the Moon, lying diametrically opposite the lunar south pole. It defines ...
, alt=The far side is cratered; maria on the near side
File:Moon Crescent - False Color Mosaic.jpg, False-color
False colors and pseudo colors respectively refers to a group of color rendering methods used to display images in colors which were recorded in the visible or non-visible parts of the electromagnetic spectrum. A false-color image is an im ...
mosaic by ''Galileo'' showing compositional variations of the Moon's surface , alt=refer to caption
En route to ''Galileo'' second gravity-assist flyby of Earth, the spacecraft flew over the
lunar north pole
The lunar north pole is the point in the Northern Hemisphere of the Moon where the lunar axis of rotation meets its surface.
The lunar North Pole is the northernmost point on the Moon, lying diametrically opposite the lunar south pole. It defines ...
on December 8, 1992, at an altitude of . The north pole had been photographed before, by ''
Mariner 10
''Mariner 10'' was an American Robotic spacecraft, robotic space probe launched by NASA on 3 November 1973, to fly by the planets Mercury (planet), Mercury and Venus. It was the first spacecraft to perform flybys of multiple planets.
''Marin ...
'' in 1973, but ''Galileo'' cameras, with their per
pixel
In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a Raster graphics, raster image, or the smallest addressable element in a dot matrix display device. In most digital display devices, p ...
imagery, provided new information about a region that still held some scientific mysteries. The infrared spectromer surveyed the surface minerals and revealed that the region was more minerallogically diverse than expected. There was evidence that the
Moon
The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
had been volcanically active earlier than originally thought, and the spectrometer clearly distinguished different lava flows on the
Mare Serenitatis
Mare Serenitatis (Latin ''serēnitātis'', the "Sea of Serenity") is a lunar mare located to the east of Mare Imbrium on the Moon. Its diameter is .
Geology
Mare Serenitatis is located within the Serenitatis basin, which is of the Nectarian ...
. Areas where titanium-rich material had been blasted from vents, like the one sampled by
Apollo 17
Apollo 17 (December 7–19, 1972) was the eleventh and final mission of NASA's Apollo program, the sixth and most recent time humans have set foot on the Moon. Commander Gene Cernan and Lunar Module Pilot Harrison Schmitt walked on the Moon, ...
, showed up clearly.
''Galileo'' Optical Experiment
During the second Earth flyby, another experiment was performed. Optical communications in space were assessed by detecting light pulses from powerful lasers with ''Galileo'' CCD. The experiment, dubbed ''Galileo'' Optical Experiment or GOPEX,
used two separate sites to beam laser pulses to the spacecraft, one at
Table Mountain Observatory
Table Mountain Observatory (TMO) is an Observatory, astronomical observation facility operated by NASA's Jet Propulsion Laboratory (California Institute of Technology). It is located in Big Pines, California, in the Angeles National Forest near Wr ...
in California and the other at the
Starfire Optical Range in
New Mexico
New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...
. The Table Mountain site used a
Nd:YAG laser operating at a
frequency-doubled wavelength of 532 nm, with a repetition rate of 15 to 30 Hertz and a pulse power
full width at half maximum
In a distribution, full width at half maximum (FWHM) is the difference between the two values of the independent variable at which the dependent variable is equal to half of its maximum value. In other words, it is the width of a spectrum curve ...
(FWHM) in the tens of megawatts range, which was coupled to a
Cassegrain reflector
The Cassegrain reflector is a combination of a primary concave mirror and a secondary convex mirror, often used in optical telescopes and Antenna (radio), radio antennas, the main characteristic being that the optical path folds back onto itself, ...
telescope for transmission to ''Galileo''. The Starfire range site used a similar setup with a larger transmitting telescope. Long-exposure (~0.1 to 0.8 s) images using ''Galileo'' 560 nm centered green filter produced images of Earth clearly showing the laser pulses even at distances of up to .
Adverse weather conditions, restrictions placed on laser transmissions by the U.S.
Space Defense Operations Center (
SPADOC) and a pointing error caused by the scan platform on the spacecraft not being able to change direction and speed as quickly as expected (which prevented laser detection on all frames with less than 400 ms exposure times) contributed to a reduction in the number of successful detections of the laser transmission to 48 of the total 159 frames taken.
Nonetheless, the experiment was considered a resounding success and the data acquired were used to design laser downlinks to send large volumes of data very quickly from spacecraft to Earth. The scheme was studied in 2004 for a data link to a future Mars-orbiting spacecraft.
On December 5, 2023, NASA's
Deep Space Optical Communications
Deep Space Optical Communications (DSOC) is a laser space communication system in operation that improved communications performance 10 to 100 times over radio frequency technology without incurring increases in mass, volume or power.[Psyche
Psyche (''Psyché'' in French) is the Greek term for "soul" ( ψυχή).
Psyche or La Psyché may also refer to:
Psychology
* Psyche (psychology), the totality of the human mind, conscious and unconscious
* ''Psyche'', an 1846 book about the unc ...]
'' spacecraft used infrared lasers for two-way communication between Earth and the spacecraft.
High-gain antenna problem
After the Venus flyby and ''Galileo'' passing beyond Earth’s orbit, it was no longer risky to employ the , so on April 11, 1991, ''Galileo'' was ordered to unfurl it. This was done using two small dual drive actuator (DDA) motors to drive a
worm gear
A worm drive is a gear train, gear arrangement in which a worm (which is a gear in the form of a Screw thread, screw) meshes with a worm wheel (which is similar in appearance to a spur gear). Its main purpose is to translate the motion of two p ...
, and was expected to take 165 seconds, or 330 seconds if one actuator failed. The antenna had 18
graphite-epoxy ribs; when the driver motor started and put pressure on the ribs, they were supposed to pop out of the cup their tips were held in, and the antenna would unfold like an umbrella. When it reached the fully deployed configuration, redundant
microswitch
Comparison of some different switches
A miniature snap-action switch, also trademarked and frequently known as a micro switch or microswitch, is an electric switch that is actuated by very little physical force, through the use of a tipping-poi ...
es would shut down the motors. Otherwise they would run for eight minutes before being automatically shut down to prevent them from overheating.
Through telemetry from ''Galileo'', investigators determined that the electric motors had stalled at 56 seconds. The spacecraft's spin rate had decreased due to an increase in its
moment of inertia
The moment of inertia, otherwise known as the mass moment of inertia, angular/rotational mass, second moment of mass, or most accurately, rotational inertia, of a rigid body is defined relatively to a rotational axis. It is the ratio between ...
and its wobble increased, indicative of an asymmetric unfolding. Only 15 ribs had popped out, leaving the antenna looking like a lop-sided, half-open umbrella. It was not possible to re-fold the antenna and try the opening sequence again; although the motors were capable of running in reverse, the antenna was not designed for this, and human assistance was required when it was done on Earth to ensure that the wire mesh did not snag.
The first thing the ''Galileo'' team tried was to rotate the spacecraft away from the Sun and back again on the assumption that the problem was with friction holding the pins in their sockets. If so, then heating and cooling the ribs might cause them to pop out of their sockets. This was done seven times, but with no result. They then tried swinging LGA-2 (which faced in the opposite direction to the HGA and LGA-1) 145 degrees to a hard stop, thereby shaking the spacecraft. This was done six times with no effect. Finally, they tried shaking the antenna by pulsing the DDA motors at 1.25 and 1.875 Hertz. This increased the torque by up to 40 percent. The motors were pulsed 13,000 times over a three-week period in December 1992 and January 1993, but only managed to move the ballscrew by one and a half revolutions beyond the stall point.

Investigators concluded that during the 4.5 years that ''Galileo'' spent in storage after the ''Challenger'' disaster, the
lubricant
A lubricant (sometimes shortened to lube) is a substance that helps to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, ...
s between the tips of the ribs and the cup were eroded. They were then worn down by
vibration
Vibration () is a mechanical phenomenon whereby oscillations occur about an equilibrium point. Vibration may be deterministic if the oscillations can be characterised precisely (e.g. the periodic motion of a pendulum), or random if the os ...
during the three cross-country journeys by truck between California and Florida for the spacecraft. The failed ribs were those closest to the flat-bed trailers carrying ''Galileo'' on these trips. The use of land transport was partly to save costs—air transport would have cost an additional $65,000 () or so per trip—but also to reduce the amount of handling required in loading and unloading the aircraft, which was considered a major risk of damage. The spacecraft was also subjected to severe vibration in a vacuum environment by the IUS. Experiments on Earth with the test HGA showed that having a set of stuck ribs all on one side reduced the DDA torque produced by up to 40 percent.
The antenna lubricants were applied only once, nearly a decade before launch. Furthermore, the HGA was not subjected to the usual rigorous testing, because there was no backup unit that could be installed in ''Galileo'' in case of damage. The flight-ready HGA was never given a thermal evaluation test, and was unfurled only a half dozen or so times before the mission. Testing might not have revealed the problem in any case; the
Lewis Research Center
NASA John H. Glenn Research Center at Lewis Field is a NASA center within the cities of Brook Park and Cleveland between Cleveland Hopkins International Airport and the Rocky River Reservation of Cleveland Metroparks, with a subsidiary facil ...
was never able to replicate the problem on Earth, and it was assumed to be the combination of loss of lubricant during transportation, vibration during launch by the IUS, and a prolonged period of time in the vacuum of space where bare metal touching could undergo
cold welding
Cold welding or contact welding is a solid-state welding process in which joining takes place without fusion or heating at the interface of the two parts to be welded. Unlike in fusion welding, no liquid or molten phase is present in the join ...
. Whatever the cause, the HGA was rendered useless.
The two LGAs were capable of transmitting information back to Earth, but since it transmitted its signal over a cone with a 120-degree
half-angle
In geometry, a cone is a three-dimensional figure that tapers smoothly from a flat base (typically a circle) to a point not contained in the base, called the ''apex'' or '' vertex''.
A cone is formed by a set of line segments, half-lines, ...
, allowing it to communicate even when not pointed at Earth, its
bandwidth
Bandwidth commonly refers to:
* Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range
* Bandwidth (computing), the rate of data transfer, bit rate or thr ...
was significantly less than that of the HGA would have been, as the HGA transmitted over a half-angle of one-sixth of a degree. The HGA was to have transmitted at 134
kilobit
The bit is the most basic unit of information in computing and digital communication. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represented as ...
s per second, whereas LGA-1 was only intended to transmit at about 8 to 16 bits per second. LGA-1 transmitted with a power of about 15 to 20 watts, which by the time it reached Earth and had been collected by one of the large aperture 70-meter DSN antennas, had a total power of about 10
20 watts. The change to mission plan required a series of software changes to be uploaded.
Image data collected was buffered and collected in ''Galileo''s Command and Data Subsystem (CDS) memory. This represented 192 kilobytes of the 384 kilobyte CDS storage, and had been added late, out of concern that the 6504 Complementary metal–oxide–semiconductor (
CMOS
Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss
", , ) is a type of MOSFET, metal–oxide–semiconductor field-effect transistor (MOSFET) semiconductor device fabrication, fabrication process that uses complementary an ...
) memory devices might not be reliable during a mission. As it happened, they gave no trouble, but the CDS memory could store up to 31 minutes of data from the Radio Relay Hardware (RRH) channels. To conserve bandwidth,
data-compression software was implemented. Image compression used an integer approximation of the
discrete cosine transform
A discrete cosine transform (DCT) expresses a finite sequence of data points in terms of a sum of cosine functions oscillating at different frequency, frequencies. The DCT, first proposed by Nasir Ahmed (engineer), Nasir Ahmed in 1972, is a widely ...
, while other data were compressed with variant of the
Lempel–Ziv–Welch
Lempel–Ziv–Welch (LZW) is a universal lossless data compression algorithm created by Abraham Lempel, Jacob Ziv, and Terry Welch. It was published by Welch in 1984 as an improved implementation of the LZ78 algorithm published by Lem ...
algorithm. Using compression, the arraying of several Deep Space Network antennas, and sensitivity upgrades to the receivers used to listen to ''Galileo'' signal, data throughput was increased to a maximum of 160 bits per second.
By further using data compression, the effective bandwidth could be raised to 1,000 bits per second.
The data collected on Jupiter and its moons were stored in the spacecraft's onboard tape recorder, and transmitted back to Earth during the long
apoapsis
An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values.
Apsides perta ...
portion of the probe's orbit using the low-gain antenna. At the same time, measurements were made of Jupiter's magnetosphere and transmitted back to Earth. The reduction in available bandwidth reduced the total amount of data transmitted throughout the mission,
but William J. O'Neil, ''Galileo'' project manager from 1992 to 1997, expressed confidence that 70 percent of ''Galileo'' science goals could still be met. The decision to use magnetic tape for storage was a conservative one, taken in the late 1970s when the use of tape was common. Conservatism was not restricted to engineers; a 1980 suggestion that the results of ''Galileo'' could be distributed electronically instead of on paper was regarded as ridiculous by geologists, on the grounds that storage would be prohibitively expensive; some of them thought that taking measurements on a computer involved putting a wooden ruler up to the screen.
Asteroid encounters
951 Gaspra

Two months after entering the
asteroid belt
The asteroid belt is a torus-shaped region in the Solar System, centered on the Sun and roughly spanning the space between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies called asteroids ...
, ''Galileo'' performed the first asteroid encounter by a spacecraft. ''Galileo'' passed
951 Gaspra
951 Gaspra is an S-type asteroid, S-type asteroid that orbits very close to the inner edge of the asteroid belt. Gaspra was discovered by Russian astronomer G. N. Neujmin in 1916. Neujmin named it after Gaspra, a Black Sea retreat that was visite ...
, an
S-type asteroid
S-type (stony-type or silicaceous-type) asteroids are asteroids with a spectral type that is indicative of a siliceous (i.e. stony) mineralogical composition, hence the name. They have relatively high density. Approximately 17% of asteroids are ...
, at a distance of at 22:37 UTC on October 29, 1991, at a relative speed of about .
Fifty-seven images of Gaspra were taken with the SSI, covering about 80 percent of the asteroid. Without the HGA, the bit rate was only about 40 bit/s, so an image took up to 60 hours to transmit back to Earth. The ''Galileo'' project was able to secure 80 hours of Canberra's 70-meter dish time between 7 and 14 November 1991, but most of images taken, including low-resolution images of more of the surface, were not transmitted to Earth until November 1992.
The imagery revealed a cratered and irregular body, measuring about . Its shape was not remarkable for an asteroid of its size. Measurements were taken using the NIMS to indicate the asteroid's composition and physical properties. While Gaspra has plenty of small craters—over 600 of them ranging in size from —it lacks large ones, hinting at a relatively recent origin, although it is possible that some of the depressions were eroded craters. Several relatively flat planar areas were found, suggesting that Gaspra was formed from another body by a collision. Measurements of the solar wind in the vicinity of the asteroid showed it changing direction a few hundred kilometers from Gaspra, which hinted that Gaspra might have a magnetic field, but this was not certain.
243 Ida and Dactyl
Following the second Earth encounter, ''Galileo'' performed close observations of another asteroid,
243 Ida
243 Ida is an asteroid in the Koronis family of the asteroid belt. It was discovered on 29 September 1884 by Austrian astronomer Johann Palisa at Vienna Observatory and named after Ida (nurse of Zeus), a nymph from Greek mythology. Later telesc ...
. A slight trajectory correction was made to enable this on August 26, 1993. With four hours to go before the encounter with Ida, ''Galileo'' spontaneously abandoned the observation configuration and resumed its cruise configuration. Engineers were able to correct the problem and have the instruments ready by 16:52:04 UTC on August 28, 1993, when ''Galileo'' flew past Ida at a range of . High-resolution images were taken to create a color mosaic of one side of the asteroid, with the highest resolution image taken at a range of . Measurements were taken using SSI and NIMS.

Transmission was still limited to the 40 bit/s data rate available during the Gaspra flyby. At that rate, it took thirty hours to send each of the five frames. In September, the line of sight between ''Galileo'' and Earth was close to the Sun, so there was only time to send one mosaic before it was blocked by the Sun on September 29, 1993; the rest of the mosaics were transmitted in February and March, after Earth had come around the Sun. ''Galileo'' tape recorder was used to store the images, but tape space was also required for the primary Jupiter mission. A technique was developed whereby only image fragments of two or three lines out of every 330 were initially sent. A determination could then be made as to whether the image was of 243 Ida or of empty space. Ultimately, only about 16 percent of the SSI data recorded could be sent back to Earth.
When astronomer Ann Harch examined the images on February 17, 1994, she found that Ida had a small moon measuring around in diameter, which appeared in 47 images. A competition was held among ''Galileo'' project members to select a name for the moon, which was ultimately dubbed
Dactyl after the legendary
Dactyls, mythical beings which lived on
Mount Ida
In Greek mythology, two sacred mountains are called Mount Ida, the "Mountain of the Goddess": Mount Ida in Crete, and Mount Ida in the ancient Troad region of western Anatolia (in modern-day Turkey), which was also known as the '' Phrygian Ida' ...
, the geographical feature on
Crete
Crete ( ; , Modern Greek, Modern: , Ancient Greek, Ancient: ) is the largest and most populous of the Greek islands, the List of islands by area, 88th largest island in the world and the List of islands in the Mediterranean#By area, fifth la ...
the asteroid was named for. Craters on Dactyl were named after individual dactyloi. Regions on 243 Ida were named after cities where
Johann Palisa
Johann Palisa (6 December 1848 – 2 May 1925) was an Austrian astronomer, born in Troppau, Austrian Silesia, now Czech Republic. He was a prolific discoverer of asteroids, discovering 122 in all, from 136 Austria in 1874 to 1073 G ...
, the discover of 243 Ida, made his observations, while ridges on 243 Ida were named in honor of deceased ''Galileo'' team members.
Dactyl was the first
asteroid moon
A minor-planet moon is an astronomical object that orbits a minor planet as its natural satellite. , there are 457 minor planets known or suspected to have moons. Discoveries of minor-planet moons (and binary objects, in general) are important ...
to be discovered. Moons of asteroids had been assumed to be rare, but the discovery of Dactyl hinted that they might in fact be quite common. From subsequent analysis of this data, Dactyl appeared to be an
S-type asteroid
S-type (stony-type or silicaceous-type) asteroids are asteroids with a spectral type that is indicative of a siliceous (i.e. stony) mineralogical composition, hence the name. They have relatively high density. Approximately 17% of asteroids are ...
, and spectrally different from 243 Ida, although Ida is also an S-type asteroid. It was hypothesized that both may have been produced by the breakup of a
Koronis parent body.
Voyage to Jupiter
Comet Shoemaker–Levy 9

''Galileo'' prime mission was a two-year study of the Jovian system, but on March 26, 1993, while it was en route, astronomers
Carolyn S. Shoemaker,
Eugene M. Shoemaker and
David H. Levy discovered fragments of a comet orbiting Jupiter, the remains of a comet that had passed within Jupiter's
Roche limit
In celestial mechanics, the Roche limit, also called Roche radius, is the distance from a celestial body within which a second celestial body, held together only by its own force of gravity, will disintegrate because the first body's tidal force ...
and had been torn apart by
tidal force
The tidal force or tide-generating force is the difference in gravitational attraction between different points in a gravitational field, causing bodies to be pulled unevenly and as a result are being stretched towards the attraction. It is the ...
s. It was named
Comet Shoemaker–Levy 9
Comet Shoemaker–Levy 9 ( formally designated D/1993 F2) was a comet that broke apart in July 1992 and collided with Jupiter in July 1994, providing the first direct observation of an extraterrestrial collision of Solar System objects. ...
. Calculations indicated that it would crash into the planet sometime between July 16 and 24, 1994. Although ''Galileo'' was still away, Jupiter was 66 pixels wide in its camera, and it was perfectly positioned to observe this event. Terrestrial telescopes had to wait to see the
impact event
An impact event is a collision between astronomical objects causing measurable effects. Impact events have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or meteoroids and have minimal effe ...
sites as they rotated into view because it would occur on Jupiter's night side.
Instead of burning up in Jupiter's atmosphere as expected, the first of the 21 comet fragments struck the planet at around and exploded with a fireball high, easily discernible to Earth-based telescopes even though it was on the night side of the planet. The impact left a series of dark scars on the planet, some two or three times as large as the Earth, that persisted for weeks. When ''Galileo'' observed an impact in ultraviolet light, the fireballs lasted for about ten seconds, but in the infrared they persisted for 90 seconds or more. When a fragment hit the planet, it increased Jupiter's overall brightness by about 20 percent. The NIMS observed one fragment create a fireball in diameter that burned with a temperature of , which was hotter than the surface of the Sun.
Probe deployment
The ''Galileo'' probe separated from the orbiter at 03:07 UTC on July 13, 1995,
five months before its rendezvous with the planet on December 7. At this point, the spacecraft was from Jupiter, but from Earth, and telemetry from the spacecraft, transmitted at the
speed of light
The speed of light in vacuum, commonly denoted , is a universal physical constant exactly equal to ). It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time i ...
, took 37 minutes to reach JPL. A tiny frequency change in the radio signal indicated that the separation had been accomplished. The ''Galileo'' orbiter was still on a collision course with Jupiter. Previously, course corrections had been made using the twelve thrusters, but with the probe on its way, the ''Galileo'' orbiter could now fire its
Messerschmitt-Bölkow-Blohm
Messerschmitt-Bölkow-Blohm (MBB) was a West Germany, West German aerospace manufacturer. It was formed during the late 1960s as the result of efforts to consolidate the West German aerospace industry; aircraft manufacturer Messerschmitt AG merged ...
main engine which had been covered by the probe until then. At 07:38 UTC on July 27, it was fired for the first time to place the ''Galileo'' orbiter on course to enter orbit around Jupiter, whence it would act as a communications relay for the ''Galileo'' probe. The ''Galileo'' probe's project manager, Marcie Smith at the
Ames Research Center
The Ames Research Center (ARC), also known as NASA Ames, is a major NASA research center at Moffett Federal Airfield in California's Silicon Valley. It was founded in 1939 as the second National Advisory Committee for Aeronautics (NACA) laborat ...
, was confident that the LGAs could be used as relays. The burn lasted for five minutes and eight seconds, and changed the velocity of the ''Galileo'' orbiter by .
Dust storms
In August 1995, the ''Galileo'' orbiter encountered a severe dust storm from Jupiter that took several months to traverse. Normally the spacecraft's dust detector picked up a dust particle every three days; now it detected up to 20,000 particles a day. Interplanetary dust storms had previously been encountered by the ''
Ulysses
Ulysses is the Latin name for Odysseus, a legendary Greek hero recognized for his intelligence and cunning. He is famous for his long, adventurous journey home to Ithaca after the Trojan War, as narrated in Homer's Odyssey.
Ulysses may also refer ...
'' probe, which had passed by Jupiter three years before on its mission to study the Sun's polar regions, but those encountered by ''Galileo'' were more intense. The dust particles were 5 to 10 nm in size, about the same as those in cigarette smoke, and had speeds ranging from depending on their size. The existence of the dust storms had come as a complete surprise to scientists when ''Ulysses'' encountered them. While data from both ''Ulysses'' and ''Galileo'' hinted that they originated somewhere in the Jovian system, it was a mystery how they had been created and how they had escaped from Jupiter's strong
gravitational
In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force be ...
and
electromagnetic field
An electromagnetic field (also EM field) is a physical field, varying in space and time, that represents the electric and magnetic influences generated by and acting upon electric charges. The field at any point in space and time can be regarde ...
s.
Tape recorder anomaly
The failure of ''Galileo'' high-gain antenna meant that data storage to the tape recorder for later compression and playback was crucial in order to obtain any substantial information from the flybys of Jupiter and its moons. The four-track, 114-
megabyte
The megabyte is a multiple of the unit byte for digital information. Its recommended unit symbol is MB. The unit prefix ''mega'' is a multiplier of (106) in the International System of Units (SI). Therefore, one megabyte is one million bytes ...
digital tape recorder was manufactured by
Odetics Corporation. On October 11, it was stuck in rewind mode for 15 hours before engineers learned what had happened and were able to send commands to shut it off. Although the recorder itself was still in working order, the malfunction had possibly damaged a length of tape at the end of the reel. This section of tape was declared "off limits" to any future data recording, and was covered with 25 more turns of tape to secure the section and reduce any further stresses, which could tear it. Because it happened only weeks before ''Galileo'' entered orbit around Jupiter, the anomaly prompted engineers to sacrifice data acquisition of almost all of the
Io and
Europa
Europa may refer to:
Places
* Europa (Roman province), a province within the Diocese of Thrace
* Europa (Seville Metro), Seville, Spain; a station on the Seville Metro
* Europa City, Paris, France; a planned development
* Europa Cliffs, Alexan ...
observations during the orbit insertion phase in order to focus on recording data sent from the atmospheric probe during its descent.
Jupiter
Arrival
The ''Galileo'' orbiter's magnetometers reported that the spacecraft had encountered the bow shock of Jupiter's magnetosphere on November 16, 1995, when it was from Jupiter. The bow shock moved to and fro in response to solar wind gusts, and was therefore crossed multiple times between 16 and 26 November, by which time ''Galileo'' was from Jupiter.
On December 7, 1995, the orbiter arrived in the Jovian system. That day it made a flyby of Europa at 11:09 UTC, and then an flyby of Io at 15:46 UTC, using Io's gravity to reduce its speed, and thereby conserve propellant for use later in the mission. At 19:54 it made its closest approach to Jupiter. The orbiter's electronics had been heavily shielded against radiation, but the radiation surpassed expectations, and nearly exceeded the spacecraft's design limits. One of the navigational systems failed, but the backup took over. Most robotic spacecraft respond to failures by entering
safe mode
Safe mode is a diagnosis, diagnostic mode of a computer operating system (OS). It can also refer to a mode of operation by application software. ''Safe mode'' is intended to help fix most, if not all, problems within an operating system. It is a ...
and awaiting further instructions from Earth, but this was not possible for ''Galileo'' during the arrival sequence due to the great distance and consequent long turnaround time.
Atmospheric probe

The descent probe awoke in response to an alarm at 16:00 UTC and began powering up its instruments. It passed through the
rings of Jupiter
The rings of Jupiter are a system of faint planetary rings. The Jovian rings were the third ring system to be discovered in the Solar System, after those of Saturn and Uranus. The main ring was discovered in 1979 by the ''Voyager 1'' space probe ...
and encountered a previously undiscovered
radiation belt
The Van Allen radiation belt is a zone of energetic charged particles, most of which originate from the solar wind, that are captured by and held around a planet by that planet's magnetosphere. Earth has two such belts, and sometimes others ma ...
ten times as strong as Earth's
Van Allen radiation belt
The Van Allen radiation belt is a zone of energetic charged particles, most of which originate from the solar wind, that are captured by and held around a planet by that planet's magnetosphere. Earth has two such belts, and sometimes others ma ...
above Jupiter's cloud tops. It had been predicted that the probe would pass through three layers of clouds; an upper one consisting of
ammonia
Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...
-ice particles at a pressure of ; a middle one of
ammonium hydrosulfide
Ammonium hydrosulfide is the chemical compound with the formula .
Composition
It is the salt derived from the ammonium cation and the hydrosulfide anion. The salt exists as colourless, water-soluble, micaceous crystals. On Earth the compound ...
ice particles at a pressure of ; and one of water vapor at . The atmosphere through which the probe descended was much denser and hotter than expected. Jupiter was also found to have only half the amount of helium expected and the data did not support the three-layered cloud structure theory: only one significant cloud layer was measured by the probe, at a pressure of around but with many indications of smaller areas of increased particle densities along the whole length of its trajectory.
The descent probe entered
Jupiter's atmosphere
The atmosphere of Jupiter is the largest planetary atmosphere in the Solar System. It is mostly made of molecular hydrogen and helium in roughly solar proportions; other chemical compounds are present only in small amounts and include methane, am ...
, defined for the purpose as being above the pressure level, without any braking at 22:04 UTC on December 7, 1995. At this point it was moving at relative to Jupiter.
This was by far the most difficult
atmospheric entry
Atmospheric entry (sometimes listed as Vimpact or Ventry) is the movement of an object from outer space into and through the gases of an atmosphere of a planet, dwarf planet, or natural satellite. Atmospheric entry may be ''uncontrolled entr ...
yet attempted by any spacecraft; the probe had to withstand a peak
deceleration
In mechanics, acceleration is the rate of change of the velocity of an object with respect to time. Acceleration is one of several components of kinematics, the study of motion. Accelerations are vector quantities (in that they have magnit ...
of . The rapid flight through the atmosphere produced a plasma with a temperature of about , and the probe's
carbon phenolic heat shield lost more than half of its mass, , during the descent. As the probe passed through Jupiter's cloud tops, it started transmitting data to the orbiter, above. The data was not immediately relayed to Earth, but a single bit was transmitted from the orbiter as a notification that the signal from the probe was being received and recorded, which would then take days to be transmitted using the LGA.
The atmospheric probe deployed its
parachute
A parachute is a device designed to slow an object's descent through an atmosphere by creating Drag (physics), drag or aerodynamic Lift (force), lift. It is primarily used to safely support people exiting aircraft at height, but also serves va ...
fifty-three seconds later than anticipated, resulting in a small loss of upper-atmospheric readings. This was attributed to wiring problems with an accelerometer that determined when to begin the parachute deployment sequence. The probe then dropped its heat shield, which fell into Jupiter's interior.
The parachute reduced the probe's speed to . The signal from the probe was no longer detected by the orbiter after 61.4 minutes, at an elevation of below the cloud tops and a pressure of . It was believed that the probe continued to fall at
terminal velocity
Terminal velocity is the maximum speed attainable by an object as it falls through a fluid (air is the most common example). It is reached when the sum of the drag force (''Fd'') and the buoyancy is equal to the downward force of gravity (''FG ...
, as the temperature increased to and the pressure to , destroying it.
File:Galileo Probe - AC81-0174.jpg, Artist's impression of the probe's entry into Jupiter's atmosphere
The atmosphere of Jupiter is the largest planetary atmosphere in the Solar System. It is mostly made of molecular hydrogen and helium in roughly solar proportions; other chemical compounds are present only in small amounts and include methane, am ...
, alt=refer to caption
Image:Galileo atmospheric probe.jpg, Timeline of the probe's atmospheric entry , alt=Probe enters, deploys parachute, transmission ends 61.4 minutes after entry where the pressure is ~
File:Jupiter's clouds.jpg, Jupiter's clouds – expected and actual results of ''Galileo''s atmospheric probe mission , alt=The clouds of ammonia
Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...
and ammonium sulfide were much thinner than expected, and clouds of water vapor were not detected.
The probe detected less lightning, less water, but stronger winds than expected. Scientists had expected to find wind speeds of up to , but winds of up to were detected. The implication was that the winds are not produced by heat generated by sunlight (as Jupiter gets less sunlight than Earth) or the condensation of water vapor (the main causes on Earth), but are due to an internal heat source. It was already well known that the atmosphere of Jupiter was mainly composed of hydrogen, but the clouds of
ammonia
Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...
and
ammonium sulfide were much thinner than expected, and clouds of water vapor were not detected. This was the first observation of ammonia clouds in another planet's atmosphere. The atmosphere creates ammonia-ice particles from material coming up from lower depths.
The atmosphere was more turbulent than expected. Wind speeds in the outermost layers were , in agreement with previous measurements from afar, but those wind speeds increased dramatically at pressure levels of , then remaining consistently high at around . The abundance of
nitrogen
Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
,
carbon
Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
and
sulfur
Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...
was three times that of the Sun, raising the possibility that they had been acquired from other bodies in the Solar system,
[ but the low abundance of water cast doubt on theories that Earth's water had been acquired from comets.
There was far less lightning activity than expected, only about a tenth of the level of activity on Earth, but this was consistent with the lack of water vapor. More surprising was the high abundance of ]noble gas
The noble gases (historically the inert gases, sometimes referred to as aerogens) are the members of Group (periodic table), group 18 of the periodic table: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn) and, in some ...
es (argon
Argon is a chemical element; it has symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...
, krypton
Krypton (from 'the hidden one') is a chemical element; it has symbol (chemistry), symbol Kr and atomic number 36. It is a colorless, odorless noble gas that occurs in trace element, trace amounts in the Earth's atmosphere, atmosphere and is of ...
and xenon
Xenon is a chemical element; it has symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
), with abundances up to three times that found in the Sun. For Jupiter to trap these gases, it would have had to be much colder than today, around , which suggested that either Jupiter had once been much further from the Sun, or that the interstellar debris that the Solar system had formed from was much colder than had been thought.
Orbiter
With the probe data collected, the ''Galileo'' orbiter's next task was to slow down in order to avoid heading off into the outer solar system. A burn sequence commencing at 00:27 UTC on December 8 and lasting 49 minutes reduced the spacecraft's speed by and it entered a parking orbit
A parking orbit is a temporary orbit used during the launch of a spacecraft. A launch vehicle follows a trajectory to the parking orbit, then coasts for a while, then engines fire again to enter the final desired trajectory.
An alternative trajec ...
with an orbital period
The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...
of 198 days. The ''Galileo'' orbiter thus became the first artificial satellite of Jupiter. Most of its initial orbit was occupied transmitting the data from the probe back to Earth. When the orbiter reached its apojove on March 26, 1996, the main engine was fired again to increase the orbit from four times the radius of Jupiter to ten times. By this time the orbiter had received half the radiation allowed for in the mission plan, and the higher orbit was to conserve the instruments for as long as possible by limiting the radiation exposure.
The spacecraft traveled around Jupiter in elongated ellipse
In mathematics, an ellipse is a plane curve surrounding two focus (geometry), focal points, such that for all points on the curve, the sum of the two distances to the focal points is a constant. It generalizes a circle, which is the special ty ...
s, each orbit lasting about two months. The differing distances from Jupiter afforded by these orbits allowed ''Galileo'' to sample different parts of the planet's extensive magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior Dynamo ...
. The orbits were designed for close-up flybys of Jupiter's largest moons. A naming scheme was devised for the orbits: a code with the first letter of the moon being encountered on that orbit (or "J" if none was encountered) plus the orbit number.
Mission extension
After the primary mission concluded on December 7, 1997, most of the mission staff departed, including O'Neil, but about a fifth of them remained. The ''Galileo'' orbiter commenced an extended mission known as the ''Galileo'' Europa Mission (GEM), which ran until December 31, 1999. This was a low-cost mission, with a budget of $30 million (equivalent to $ million in ). The reason for calling it as the "Europa" mission rather than the "Extended" mission was political; although it was wasteful to scrap a spacecraft that was still functional and capable of performing a continuing mission, Congress took a dim view of requests for more money for projects that had already been fully funded. This was avoided through rebranding.
The smaller GEM team did not have the resources to deal with problems, but when they arose it was able to temporarily recall former team members for intensive efforts to solve them. The spacecraft performed several flybys of Europa
Europa may refer to:
Places
* Europa (Roman province), a province within the Diocese of Thrace
* Europa (Seville Metro), Seville, Spain; a station on the Seville Metro
* Europa City, Paris, France; a planned development
* Europa Cliffs, Alexan ...
, Callisto
CALLISTO (''Cooperative Action Leading to Launcher Innovation in Stage Toss-back Operations'') is a reusable VTVL Prototype, demonstrator propelled by a small 40 kN Japanese LOX-LH2 rocket engine. It is being developed jointly by the CNES, French ...
and Io. On each one the spacecraft collected only two days' worth of data instead of the seven it had collected during the prime mission. The radiation
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. This includes:
* ''electromagnetic radiation'' consisting of photons, such as radio waves, microwaves, infr ...
environment near Io, which ''Galileo'' approached to within on November 26, 1999, on orbit I25, was very unhealthy for ''Galileo'' systems, and so these flybys were saved for the extended mission when loss of the spacecraft would be more acceptable.
By the time GEM ended, most of the spacecraft was operating well beyond its original design specifications, having absorbed more than 600 kilorads in between 1995 and 2002, three times the radiation exposure that it had been built to withstand. Many of the instruments were no longer operating at peak performance, but were still functional, so a second extension, the ''Galileo'' Millennium Mission (GMM) was authorized. This was intended to run until March 2001, but it was subsequently extended until January 2003. GMM included return visits to Europa, Io, Ganymede and Callisto, and for the first time to Amalthea. The total cost of the original ''Galileo'' mission was about (equivalent to $ billion in ). Of this, (equivalent to $ million in ) was spent on spacecraft development. Another $110 million (equivalent to $ million in ) was contributed by international agencies.
Io
The innermost of the four Galilean moons, Io is roughly the same size as Earth's moon, with a mean radius
In applied sciences, the equivalent radius (or mean radius) is the radius of a circle or sphere with the same perimeter, area, or volume of a non-circular or non-spherical object. The equivalent diameter (or mean diameter) (D) is twice the equiva ...
of . It is in orbital resonance
In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relation ...
with Ganymede and Europa, and tidally locked
Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked ...
with Jupiter, so just as the Earth's Moon always has the same side facing Earth, Io always has the same side facing Jupiter. It has a faster orbit though, with a rotation period of 1.769 days. As a result, the rotational and tidal forces on Io are 220 times as great as those on Earth's moon. These frictional forces are sufficient to melt rock, creating volcanoes and lava flows. Although only a third of the size of Earth, Io generates twice as much heat. While geological events occur on Earth over periods of thousands or even millions of years, cataclysmic events are common on Io. Visible changes occurred between orbits of ''Galileo''. The colorful surface is a mixture of red, white and yellow sulfur compounds.
''Galileo'' flew past Io, but in the interest of protecting the tape recorder, O'Neil decided to forego collecting images. To use the SSI camera meant operating the tape recorder at high speed, with sudden stops and starts, whereas the fields and particles instruments only required the tape recorder to run continuously at slow speeds, and it was believed that it could handle this. This was a crushing blow to scientists, some of whom had waited years for the opportunity. No other Io encounters were scheduled during the prime mission because it was feared that the high radiation levels close to Jupiter would damage the spacecraft. However, valuable information was still obtained; Doppler data used to measure Io's gravitational field revealed that Io had a core of molten iron
Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...
and iron sulfide
Iron sulfide or iron sulphide can refer to range of chemical compounds composed of iron and sulfur.
Minerals
By increasing order of stability:
* Iron(II) sulfide, FeS
* Greigite, Fe3S4 (cubic)
* Pyrrhotite, Fe1−xS (where x = 0 to 0.2) (monocli ...
.
Another opportunity to observe Io arose during the ''Galileo'' Europa Mission (GEM), when ''Galileo'' flew past Io on orbits I24 and I25, and it would revisit Io during the ''Galileo'' Millennium Mission (GMM) on orbits I27, I31, I32 and I33. As ''Galileo'' approached Io on I24 at 11:09 UTC on October 11, 1999, it entered safe mode. Apparently, high-energy electrons had altered a bit on a memory chip. When it entered safe mode, the spacecraft turned off all non-essential functions. Normally it took seven to ten days to diagnose and recover from a safe mode incident; this time the ''Galileo'' Project team at JPL had nineteen hours before the encounter with Io. After a frantic effort, they managed to diagnose a problem that had never been seen before, and restore the spacecraft systems with just two hours to spare. Not all of the planned activities could be carried out, but ''Galileo'' obtained a series of high-resolution color images of the Pillan Patera
Pillan Patera is a patera, or a complex crater with scalloped edges, on Jupiter's moon Io. It is located at , south of Pillan Mons and west of Reiden Patera. It is named after the Araucanian thunder, fire, and volcano god. Its name was approve ...
, and Zamama, Prometheus
In Greek mythology, Prometheus (; , , possibly meaning "forethought")Smith"Prometheus". is a Titans, Titan. He is best known for defying the Olympian gods by taking theft of fire, fire from them and giving it to humanity in the form of technol ...
, and Pele volcanic eruption centers.
When ''Galileo'' next approached Io on I25 at 03:40 UTC on November 26, 1999, JPL were eating their Thanksgiving dinner
The centerpiece of contemporary Thanksgiving in the United States and Canada is Thanksgiving dinner, a large meal generally centered on a large roasted turkey. Thanksgiving is the largest eating event in the United States as measured by retail ...
at the ''Galileo'' Mission Control Center when, with the encounter with Io just four hours away, the spacecraft again entered safe mode. This time the problem was traced to a software patch implemented to bring ''Galileo'' out of safe mode during I24. Fortunately, the spacecraft had not shut down as much as it had on I24, and the team at JPL were able to bring it back online. During I24 they had done so with two hours to spare; this time, they had just three minutes. Nonetheless, the flyby was successful, with ''Galileo'' NIMS and SSI camera capturing an erupting volcano that generated a long plume of lava that was sufficiently large and hot to have also been detected by the NASA Infrared Telescope Facility
The NASA Infrared Telescope Facility (NASA IRTF) is a telescope optimized for use in infrared astronomy and located at the Mauna Kea Observatory in Hawaii. It was first built to support the Voyager program, Voyager missions and is now the US n ...
atop Mauna Kea
Mauna Kea (, ; abbreviation for ''Mauna a Wākea''); is a dormant Shield volcano, shield volcano on the Hawaii (island), island of Hawaii. Its peak is above sea level, making it the List of U.S. states by elevation, highest point in Hawaii a ...
in Hawaii
Hawaii ( ; ) is an island U.S. state, state of the United States, in the Pacific Ocean about southwest of the U.S. mainland. One of the two Non-contiguous United States, non-contiguous U.S. states (along with Alaska), it is the only sta ...
. While such events were more common and spectacular on Io than on Earth, it was extremely fortuitous to have captured it; planetary scientist
Planetary science (or more rarely, planetology) is the scientific study of planets (including Earth), celestial bodies (such as moons, asteroids, comets) and planetary systems (in particular those of the Solar System) and the processes of their ...
Alfred McEwen
Alfred McEwen is a professor of planetary geology at the University of Arizona. McEwen is a member of the Lunar and Planetary Laboratory where he directs the director of the Planetary Image Research Laboratory. He is a member of the imaging scie ...
estimated the odds at 1 in 500.
The safe-mode incidents on I24 and I25 left some gaps in the data, which I27 targeted. This time ''Galileo'' passed over the surface of Io. At this time, the spacecraft was nearly at the maximum distance from Earth, and there was a solar conjunction
Solar conjunction generally occurs when a planet or other Solar System object is on the opposite side of the Sun from the Earth. From an Earth reference, the Sun will pass between the Earth and the object. Communication with any spacecraft in sol ...
, a period when the Sun blocked the line of sight between Earth and Jupiter. As a consequence, three quarters of the observations had to be taken over a period of three hours. NIMS images revealed fourteen active volcanoes in a region thought to contain just four. Images of Loki Patera
Loki Patera is the largest volcanic depression on Jupiter's moon Io, in diameter. It contains an active lava lake, with an episodically overturning crust. The level of activity seen is similar to a superfast spreading mid-ocean ridge on Earth ...
showed that in the four and half months between I24 and I27, some had been covered in fresh lava. A series of observations of extreme ultraviolet
Extreme ultraviolet radiation (EUV or XUV) or high-energy ultraviolet radiation is electromagnetic radiation in the part of the electromagnetic spectrum spanning wavelengths shorter than the hydrogen Lyman-alpha line from 121 nm down to ...
(EUV) had to be cancelled due to yet another safe-mode event. Radiation exposure caused a transient bus
A bus (contracted from omnibus, with variants multibus, motorbus, autobus, etc.) is a motor vehicle that carries significantly more passengers than an average car or van, but fewer than the average rail transport. It is most commonly used ...
reset, a computer hardware error resulting in a safe mode event. A software patch implemented after the Europa encounter on orbit E19 guarded against this when the spacecraft was within 15 Jupiter radii of the planet, but this time it occurred at 29 Jupiter radii. The safe mode event also caused a loss of tape playback time, but the project managers decided to carry over some Io data into orbit G28, and play it back then. This limited the amount of tape space available for that Ganymede encounter, but the Io data was considered to be more valuable.
The discovery of Io's iron core raised the possibility that it had a magnetic field. The I24, I25 and I27 encounters had involved passes over Io's equator, which made it difficult to determine whether Io had its own magnetic field or one induced by Jupiter. Accordingly, on orbit I31, ''Galileo'' passed within of the surface of the north pole of Io, and on orbit I32 it flew over the south pole. After examining the magnetometer results, planetary scientist Margaret G. Kivelson, announced that Io had no intrinsic magnetic field, which meant that its molten iron core did not have the same convective
Convection is single or multiphase fluid flow that occurs spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...
properties as that of Earth.
On I31 ''Galileo'' sped through an area that had been in the plume of the Tvashtar Paterae volcano, and it was hoped that the plume could be sampled. This time, Tvashtar was quiet
QUIET was an astronomy experiment to study the polarization of the cosmic microwave background radiation. QUIET stands for Q/U Imaging ExperimenT. The Q/U in the name refers to the ability of the telescope to measure the Q and U Stokes parameters ...
, but the spacecraft flew through the plume of another, previously unknown, volcano away. What had been assumed to be hot ash from the volcanic eruption turned out to be sulfur dioxide snowflakes, each consisting of 15 to 20 molecules clustered together. ''Galileo'' final return to Io on orbit I33 was marred by another safe mode incident, and much of the hoped-for data was lost.
Europa
Although the smallest of the four Galilean moons, with a radius of , Europa is the sixth-largest moon in the solar system. Observations from Earth indicated that it was covered in ice. Like Io, Europa is tidally locked with Jupiter. It is in orbital resonance with Io and Ganymede, with its 85-hour orbit being twice that of Io, but half that of Ganymede. Conjunctions with Io always occur on the opposite side of Jupiter to those with Ganymede. Europa is therefore subject to tidal effects. There is no evidence of volcanism like on Io, but ''Galileo'' revealed that the surface ice was covered in cracks.
Some observations of Europa were made during orbits G1 and G2. On C3, ''Galileo'' conducted a "nontargeted" encounter of Europa—meaning a secondary flyby at a distance of up to —on November 6, 1996. During E4 from December 15 to 22, 1996, ''Galileo'' flew within of Europa, but data transmission was hindered by a Solar occultation
An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks f ...
that blocked transmission for ten days.
''Galileo'' returned to Europa on E6 in January 1997, this time at a height of , to analyze oval-shaped features in the infrared and ultraviolet spectra. Occultations by Europa, Io and Jupiter provided data on the atmospheric profiles of them, and measurements were made of Europa's gravitational field. On E11 from November 2 to 9, 1997, data was collected on the magnetosphere. Due to the problems with the HGA, only about two percent of the anticipated number of images of Europa were obtained by the primary mission. On the GEM, the first eight orbits (E12 through E19) were all dedicated to Europa, and ''Galileo'' paid it a final visit on E26 during the GMM.
Images of Europa also showed few impact craters. It seemed unlikely that it had escaped the meteor and comet impacts that scarred Ganymede and Callisto, so this indicated Europa has an active geology that renews the surface and obliterates craters. Astronomer Clark Chapman argued that, assuming a crater occurs in Europa once every million years, and given only about twenty have been spotted on Europa, the implication is that the surface must only be about 10 million years old. With more data on hand, in 2003 a team led by Kevin Zahle at NASA's Ames Research Center
The Ames Research Center (ARC), also known as NASA Ames, is a major NASA research center at Moffett Federal Airfield in California's Silicon Valley. It was founded in 1939 as the second National Advisory Committee for Aeronautics (NACA) laborat ...
arrived at a figure of 30 to 70 million years. Tidal flexing of up to per day was the most likely culprit. But not all scientists were convinced; Michael Carr, a planetologist from the US Geological Survey
The United States Geological Survey (USGS), founded as the Geological Survey, is an agency of the U.S. Department of the Interior whose work spans the disciplines of biology, geography, geology, and hydrology. The agency was founded on March ...
, argued that, on the contrary, Europa's surface age was closer to a billion years. He compared the craters on Ganymede with those on Earth's moon, and concluded that the satellites of Jupiter were not subject to the same amount of cratering.
Evidence of surface renewal hinted at the possibility of a viscous layer below the surface of warm ice or liquid water. NIMS observations by ''Galileo'' indicated that the surface of Europa appeared to contain magnesium- and sodium-based salts. A likely source was brine
Brine (or briny water) is a high-concentration solution of salt (typically sodium chloride or calcium chloride) in water. In diverse contexts, ''brine'' may refer to the salt solutions ranging from about 3.5% (a typical concentration of seawat ...
below the ice crust. Further evidence was provided by the magnetometer, which reported that the magnetic field was induced by Jupiter. This could be explained by the existence of a spherical shell of conductive material like salt water. Since the surface temperature on Europa was , any water breaching the surface ice would instantly freeze over. Heat required to keep water in a liquid state could not come from the Sun, which at that distance had only 4 percent of the intensity it had on Earth, but ice is a good insulator, and the heat could be provided by the tidal flexing. ''Galileo'' also yielded evidence that the crust of Europa had slipped over time, moving south on the hemisphere facing Jupiter, and north on the far side.
There was acrimonious debate among scientists over the thickness of the ice crust, and those who presented results indicating that it might be thinner than the proposed by the accredited scientists on the ''Galileo'' Imaging Team faced intimidation, scorn, and reduced career opportunities. The ''Galileo'' Imaging Team was led by Michael J. Belton from the Kitt Peak National Observatory
The Kitt Peak National Observatory (KPNO) is a United States astronomy, astronomical observatory located on Kitt Peak of the Quinlan Mountains in the Arizona-Sonoran Desert on the Tohono Oʼodham Nation, west-southwest of Tucson, Arizona. With ...
. Scientists who planned imaging sequences had the exclusive right to the initial interpretation of the ''Galileo'' data, most which was performed by their research students. The scientific community did not want a repetition of the 1979 Morabito incident, when Linda A. Morabito, an engineer at JPL working on ''Voyager 1'', discovered the first active extraterrestrial volcano on Io. The Imaging Team controlled the manner in which discoveries were presented to the scientific community and the public through press conferences, conference papers and publications.
Observations by the Hubble Space Telescope in 1995 reported that Europa had a thin oxygen atmosphere. This was confirmed by ''Galileo'' in six experiments on orbits E4 and E6 during occultations when Europa was between ''Galileo'' and the Earth. This allowed Canberra and Goldstone to investigate the ionosphere
The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays ...
of Europa by measuring the degree to which the radio beam was diffracted by charged particles. This indicated the presence of water ions, which were most likely water molecules that had been dislodged from the surface ice and then ionized by the Sun or the Jovian magnetosphere. The presence of an ionosphere was sufficient to deduce the existence of a thin atmosphere on Europa.
On December 11, 2013, NASA reported, based on results from the ''Galileo'' mission, the detection of " clay-like minerals" (specifically, phyllosilicates
Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust.
In mineralogy, the crystalline forms of silica (silicon dio ...
), often associated with organic materials
Organic matter, organic material or natural organic matter is the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have come fro ...
, on the icy crust of Europa
Europa may refer to:
Places
* Europa (Roman province), a province within the Diocese of Thrace
* Europa (Seville Metro), Seville, Spain; a station on the Seville Metro
* Europa City, Paris, France; a planned development
* Europa Cliffs, Alexan ...
. The presence of the minerals may have been the result of a collision with an asteroid
An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...
or comet
A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or Coma (cometary), coma surrounding ...
.
Ganymede
The largest of the Galilean moons with a radius of , Ganymede is larger than Earth's moon, the dwarf planet
A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun, massive enough to be hydrostatic equilibrium, gravitationally rounded, but insufficient to achieve clearing the neighbourhood, orbital dominance like the ...
Pluto
Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of Trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Su ...
or the planet Mercury. It is the largest of the moons in the Solar system that are characterized by large amounts of water ice, which also includes Saturn's moon Titan
Titan most often refers to:
* Titan (moon), the largest moon of Saturn
* Titans, a race of deities in Greek mythology
Titan or Titans may also refer to:
Arts and entertainment
Fictional entities
Fictional locations
* Titan in fiction, fictiona ...
, and Neptune's moon Triton
Triton commonly refers to:
* Triton (mythology), a Greek god
* Triton (moon), a satellite of Neptune
Triton may also refer to:
Biology
* Triton cockatoo, a parrot
* Triton (gastropod), a group of sea snails
* ''Triton'', a synonym of ''Triturus' ...
. Ganymede has three times as much water for its mass as Earth has.
When ''Galileo'' entered Jovian orbit, it did so at an orbital inclination
Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object.
For a satellite orbiting the Earth ...
to the Jovian equator, and therefore in the orbital plane of the four Galilean moons. To transfer orbit while conserving propellant, two slingshot maneuvers were performed. On G1, the gravity of Ganymede was used to slow the spacecraft's orbital period from 210 to 72 days to allow for more encounters and to take ''Galileo'' out of the more intense regions of radiation. On G2, the gravity assist was employed to put it into a coplanar orbit to permit subsequent encounters with Io, Europa and Callisto.
Although the primary purpose of G1 and G2 was navigational, the opportunity to make some observations was not missed. The plasma-wave experiment and the magnetometer detected a magnetic field with a strength of about , more than strong enough to create a separate magnetosphere within that of Jupiter. This was the first time that a magnetic field had ever been detected on a moon contained within the magnetosphere of its host planet. This discovery led naturally to questions about its origin. The evidence pointed to an iron or iron sulfide core and mantle below the surface, encased in ice. Margaret Kivelson, the scientist in charge of the magnetometer experiment, contended that the induced magnetic field required an iron core, and speculated that an electrically conductive layer was required, possibly a brine ocean below the surface.
''Galileo'' returned to Ganymede on orbits G7 and G9 in April and May 1997, and on G28 and G29 in May and December 2000 on the GMM. Images of the surface revealed two types of terrain: highly cratered dark regions and grooved terrain sulcus. Images of the Arbela Sulcus taken on G28 made Ganymede look more like Europa, but tidal flexing could not provide sufficient heat to keep water in liquid form on Ganymede, although it may have made a contribution. One possibility was radioactivity, which might provide sufficient heat for liquid water to exist below the surface. Another possibility was volcanism. Slushy water or ice reaching the surface would quickly freeze over, creating areas of a relatively smooth surface.
Callisto
Callisto is the outermost of the Galilean moons, and the most pockmarked, indeed the most of any body in the Solar system. So many craters must have taken billions of years to accumulate, which gave scientists the idea that its surface was as much as four billion years old, and provided a record of meteor activity in the Solar system. ''Galileo'' visited Callisto on orbits C3, C9 and C100 during the prime mission, and then on C20, C21, C22 and C23 during the GEM. When the cameras observed Callisto close up, there was a puzzling absence of small craters. The surface features appeared to have been eroded, indicating that they had been subject to active geological processes.
''Galileo'' flyby of Callisto on C3 marked the first time that the Deep Space Network operated a link between its antennae in Canberra and Goldstone that allowed them to operate as a gigantic array, thereby enabling a higher bit rate. With the assistance of the antenna at Parkes, this raised the effective bandwidth to as much as 1,000 bits per second.
Data accumulated on C3 indicated that Callisto had a homogeneous composition, with heavy and light elements intermixed. This was estimated to be composed of 60 percent silicate
A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used ...
, iron and iron sulfide rock and 40 percent water ice. This was overturned by further radio Doppler observations on C9 and C10, which indicated that rock had settled towards the core, and therefore that Callisto indeed has a differentiated internal structure, although not as much so as the other Galilean moons.
Observations made with ''Galileo'' magnetometer indicated that Callisto had no magnetic field of its own, and therefore lacked an iron core like Ganymede's, but that it did have an induced field from Jupiter's magnetosphere. Because ice is too poor a conductor to generate this effect, it pointed to the possibility that Callisto, like Europa and Ganymede, might have a subsurface ocean of brine. ''Galileo'' made its closest encounter with Callisto on C30, when it made a pass over the surface, during which it photographed the Asgard
In Nordic mythology, Asgard (Old Norse: ''Ásgarðr''; "Garden of the Æsir") is a location associated with the gods. It appears in several Old Norse sagas and mythological texts, including the Eddas, however it has also been suggested to be refe ...
, Valhalla
In Norse mythology, Valhalla ( , ; , )Orchard (1997:171–172) is described as a majestic hall located in Asgard and presided over by the god Odin. There were five possible realms the soul could travel to after death. The first was Fólkvang ...
and Bran craters. This was used for slingshot maneuvers to set up the final encounters with Io on I31 and I32.
Amalthea
Although ''Galileo'' main mission was to explore the Galilean moons, it also captured images of four of the inner moons, Thebe, Adrastea, Amalthea, and Metis
Metis or Métis, meaning "mixed" in French, may refer to:
Ethnic groups
* Métis, recognized Indigenous communities in Canada and the United States whose distinct culture and language emerged after early intermarriage between First Nations peopl ...
. Such images were only possible from a spacecraft; to Earth-based telescopes they were merely specks of light. Two years of Jupiter's intense radiation took its toll on the spacecraft's systems, and its fuel supply was running low in the early 2000s. ''Galileo'' cameras were deactivated on January 17, 2002, after they had sustained irreparable radiation damage.
NASA engineers were able to recover the damaged tape-recorder electronics, and ''Galileo'' continued to return scientific data until it was deorbited in 2003, performing one last scientific experiment: a measurement of Amalthea's mass as the spacecraft swung by it. This was tricky to arrange; to be useful, ''Galileo'' had to fly within of Amalthea, but not so close as to crash into it. This was complicated by its irregular potato-like shape. It was tidally locked, pointing its long axis towards Jupiter. A successful flyby meant knowing which direction the asteroid was pointed in relation to ''Galileo'' at all times.
''Galileo'' flew by Amalthea on November 5, 2002, during its 34th orbit, allowing a measurement of the moon's mass as it passed within of its surface. The results startled the scientific team; they revealed that Amalthea had a mass of , and with a volume of , it therefore had a density of 857 ± 99 kilograms per cubic meter, less than that of water.
A final discovery occurred during the last two orbits of the mission. When the spacecraft passed the orbit of Amalthea, the star scanner detected unexpected flashes of light that were reflections from seven to nine moonlets. None of the individual moonlets was reliably sighted twice, so no orbits were determined. It is believed that they were most likely debris ejected from Amalthea that formed a tenuous, and perhaps temporary, ring around Jupiter.
Star scanner
''Galileo'' star scanner was a small optical telescope that provided an absolute attitude reference, but it made several scientific discoveries serendipitously. In the prime mission, it was found that the star scanner was able to detect high-energy particles as a noise signal. This data was eventually calibrated to show the particles were predominantly > electrons that were trapped in the Jovian magnetic belts, and released to the Planetary Data System.
A second discovery occurred in 2000. The star scanner was observing a set of stars that included the second magnitude
Magnitude may refer to:
Mathematics
*Euclidean vector, a quantity defined by both its magnitude and its direction
*Magnitude (mathematics), the relative size of an object
*Norm (mathematics), a term for the size or length of a vector
*Order of ...
star Delta Velorum
Delta Velorum (δ Velorum, abbreviated Delta Vel, δ Vel) is a triple star system in the southern constellation of Vela, near the border with Carina, and is part of the False Cross. Based on parallax measurements, it is approximate ...
. At one point, this star dimmed for 8 hours below the star scanner's detection threshold. Subsequent analysis of ''Galileo'' data and work by amateur and professional astronomers showed that Delta Velorum is the brightest known eclipsing binary
A binary star or binary star system is a Star system, system of two stars that are gravity, gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved ...
, brighter at maximum than Algol
ALGOL (; short for "Algorithmic Language") is a family of imperative computer programming languages originally developed in 1958. ALGOL heavily influenced many other languages and was the standard method for algorithm description used by the ...
. It has a primary period of 45 days and the dimming is just visible with the naked eye.
Radiation-related anomalies
Jupiter's uniquely harsh radiation environment caused over 20 anomalies over the course of ''Galileo'' mission, in addition to the incidents expanded upon below. Despite having exceeded its radiation design limit by at least a factor of three, the spacecraft survived all these anomalies. Work-arounds were found eventually for all of these problems, and ''Galileo'' was never rendered entirely non-functional by Jupiter's radiation. The radiation limits for ''Galileo'' computers were based on data returned from ''Pioneer 10
''Pioneer 10'' (originally designated Pioneer F) is a NASA space probe launched in 1972 that completed the first mission to the planet Jupiter. ''Pioneer 10'' became the first of five artificial objects to achieve the escape velocity needed ...
'' and ''Pioneer 11
''Pioneer 11'' (also known as ''Pioneer G'') is a NASA robotic space probe launched on April 5, 1973, to study the asteroid belt, the environment around Jupiter and Saturn, the solar wind, and cosmic rays. It was the first probe to Exploration ...
'', since much of the design work was underway before the two ''Voyagers'' arrived at Jupiter in 1979.
A typical effect of the radiation was that several of the science instruments suffered increased noise
Noise is sound, chiefly unwanted, unintentional, or harmful sound considered unpleasant, loud, or disruptive to mental or hearing faculties. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrat ...
while within about of Jupiter. The SSI camera began producing totally white images when the spacecraft was hit by the exceptional Bastille Day coronal mass ejection in 2000, and did so again on subsequent close approaches to Jupiter. The quartz crystal used as the frequency reference for the radio suffered permanent frequency shifts with each Jupiter approach. A spin detector failed, and the spacecraft gyro output was biased by the radiation environment.
The most severe effects of the radiation were current leakages somewhere in the spacecraft's power bus, most likely across brushes
A brush is a common tool with bristles, wire or other filaments. It generally consists of a handle or block to which filaments are affixed in either a parallel or perpendicular orientation, depending on the way the brush is to be gripped during u ...
at a spin bearing connecting rotor and stator sections of the orbiter. These current leakages triggered a reset of the onboard computer and caused it to go into safe mode. The resets occurred when the spacecraft was either close to Jupiter or in the region of space magnetically downstream of Jupiter. A change to the software was made in April 1999 that allowed the onboard computer to detect these resets and autonomously recover, so as to avoid safe mode.
Tape recorder problems
Routine maintenance of the tape recorder involved winding the tape halfway down its length and back again to prevent it sticking. In November 2002, after the completion of the mission's only encounter with Jupiter's moon Amalthea, problems with playback of the tape recorder again plagued ''Galileo''. About 10 minutes after the closest approach of the Amalthea flyby, ''Galileo'' stopped collecting data, shut down all of its instruments, and went into safe mode, apparently as a result of exposure to Jupiter's intense radiation environment. Though most of the Amalthea data was already written to tape, it was found that the recorder refused to respond to commands telling it to play back data.
After weeks of troubleshooting of an identical flight spare of the recorder on the ground, it was determined that the cause of the malfunction was a reduction of light output in three infrared Optek OP133 light-emitting diode
A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (corre ...
s (LEDs) located in the drive electronics of the recorder's motor encoder wheel. The gallium arsenide
Gallium arsenide (GaAs) is a III-V direct band gap semiconductor with a Zincblende (crystal structure), zinc blende crystal structure.
Gallium arsenide is used in the manufacture of devices such as microwave frequency integrated circuits, monoli ...
LEDs had been particularly sensitive to proton
A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...
-irradiation-induced atomic lattice displacement defects, which greatly decreased their effective light output and caused the drive motor's electronics to falsely believe the motor encoder wheel was incorrectly positioned.
''Galileo'' flight team then began a series of " annealing" sessions, where current was passed through the LEDs for hours at a time to heat them to a point where some of the crystalline lattice defects would be shifted back into place, thus increasing the LED's light output. After about 100 hours of annealing and playback cycles, the recorder was able to operate for up to an hour at a time. After many subsequent playback and cooling cycles, the complete transmission back to Earth of all recorded Amalthea flyby data was successful.
End of mission and deorbit
When the exploration of Mars was being considered in the early 1960s, Carl Sagan and Sidney Coleman
Sidney Richard Coleman (7 March 1937 – 18 November 2007) was an American theoretical physicist noted for his research in high-energy physics.
Life and work
Sidney Coleman grew up on the Far North Side of Chicago. In 1957, he received h ...
produced a paper concerning contamination of the red planet. In order that scientists could determine whether native life forms existed before the planet became contaminated by micro-organisms from Earth, they proposed that space missions should aim at a 99.9 percent chance that contamination should not occur. This figure was adopted by the Committee on Space Research
The Committee on Space Research (COSPAR) was established on October 3, 1958 by the International Council for Scientific Unions (ICSU) and its first chair was Hildegard Korf Kallmann-Bijl. Among COSPAR's objectives are the promotion of scienti ...
(COSPAR) of the International Council of Scientific Unions
The International Council for Science (ICSU, after its former name, International Council of Scientific Unions) was an international non-governmental organization devoted to international cooperation in the advancement of science. Its members ...
in 1964, and was subsequently applied to all planetary probes.
The danger was highlighted in 1969 when the Apollo 12
Apollo 12 (November 14–24, 1969) was the sixth crewed flight in the United States Apollo program and the second to land on the Moon. It was launched on November 14, 1969, by NASA from the Kennedy Space Center in Florida. Commander Charles ...
astronauts returned components of the Surveyor 3
Surveyor 3 is the third lander of the American uncrewed Surveyor program sent to explore the surface of the Moon in 1967 and the second to successfully land. It was the first mission to carry a surface-soil sampling-scoop.
Surveyor 3 was vis ...
spacecraft that had landed on the Moon three years before, and it was found that microbes were still viable even after three years in that harsh climate. An alternative was the Prime Directive
In the fictional universe of ''Star Trek'', the Prime Directive (also known as "Starfleet General Order 1", and the "non-interference directive") is a guiding principle of Starfleet that prohibits its members from interfering with the natural dev ...
, a philosophy of non-interference with alien life forms enunciated by the original ''Star Trek'' television series that prioritized the interests of the life forms over those of scientists. Given the (admittedly slim) prospect of life on Europa, scientists Richard Greenberg and Randall Tufts proposed that a new standard be set of no greater chance of contamination than that which might occur naturally by meteorites.
''Galileo'' had not been sterilized prior to launch and could conceivably have carried bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
from Earth. Therefore, a plan was formulated to send the probe directly into Jupiter, in an intentional crash to eliminate the possibility of an impact with Jupiter's moons, particularly Europa, and prevent a forward contamination. On April 14, 2003, the ''Galileo'' orbiter reached its greatest orbital distance from Jupiter for the entire mission since orbital insertion, , before plunging back towards the gas giant for its final impact. At the completion of J35, its final orbit around the Jovian system, ''Galileo'' struck Jupiter in darkness just south of the equator on September 21, 2003, at 18:57 UTC. Its impact speed was approximately .
Major findings
# The composition of Jupiter differs from that of the Sun, indicating that Jupiter has evolved since the formation of the Solar System.
# ''Galileo'' made the first observation of ammonia clouds in another planet's atmosphere. The atmosphere creates ammonia ice particles from material coming up from lower depths.
# Io was confirmed to have extensive volcanic activity that is 100 times greater than that found on Earth. The heat and frequency of eruptions are reminiscent of early Earth.
# Complex plasma interactions in Io's atmosphere create immense electrical currents which couple to Jupiter's atmosphere.
# Several lines of evidence from ''Galileo'' support the theory that liquid oceans exist under Europa's icy surface.
# Ganymede possesses its own, substantial magnetic field – the first satellite known to have one.
# ''Galileo'' magnetic data provided evidence that Europa, Ganymede and Callisto have a liquid salt water layer under the visible surface.
# Evidence exists that Europa, Ganymede, and Callisto all have a thin atmospheric layer known as a "surface-bound exosphere
The exosphere is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the density is so low that the molecules are essentially collision-less. In the case of ...
".
# Jupiter's ring system
A ring system is a disc or torus orbiting an astronomical object that is composed of solid material such as dust, meteoroids, planetoids, moonlets, or stellar objects.
Ring systems are best known as planetary rings, common components of sate ...
is formed by dust kicked up as interplanetary meteoroid
A meteoroid ( ) is a small rocky or metallic body in outer space.
Meteoroids are distinguished as objects significantly smaller than ''asteroids'', ranging in size from grains to objects up to wide. Objects smaller than meteoroids are classifie ...
s smash into the planet's four small inner moons. The outermost ring is actually two rings, one embedded with the other. There is probably a separate ring along Amalthea's orbit as well.
# The ''Galileo'' spacecraft identified the global structure and dynamics of a giant planet's magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior Dynamo ...
.
Follow-on missions
There was a spare ''Galileo'' spacecraft that was considered by the NASA–ESA Outer Planets Study Team in 1983 for a mission to Saturn, but it was passed over in favor of a newer design, which became ''Cassini–Huygens
''Cassini–Huygens'' ( ), commonly called ''Cassini'', was a space research, space-research mission by NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI) to send a space probe to study the planet Saturn and its system, i ...
''. While ''Galileo'' was operating, ''Ulysses
Ulysses is the Latin name for Odysseus, a legendary Greek hero recognized for his intelligence and cunning. He is famous for his long, adventurous journey home to Ithaca after the Trojan War, as narrated in Homer's Odyssey.
Ulysses may also refer ...
'' passed by Jupiter in 1992 on its mission to study the Sun's polar regions, and ''Cassini–Huygens
''Cassini–Huygens'' ( ), commonly called ''Cassini'', was a space research, space-research mission by NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI) to send a space probe to study the planet Saturn and its system, i ...
'' coasted by the planet in 2000 and 2001 en route to Saturn. ''New Horizons
''New Horizons'' is an Interplanetary spaceflight, interplanetary space probe launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institut ...
'' passed close by Jupiter in 2007 for a gravity assist en route to Pluto, and it too collected data on the planet.
''Juno''
The next mission to orbit Jupiter was NASA's ''Juno
Juno commonly refers to:
*Juno (mythology), the Roman goddess of marriage and queen of the gods
* ''Juno'' (film), the 2007 film
Juno may also refer to:
Arts, entertainment and media Fictional characters
*Juno, a character in the book ''Juno of ...
'' spacecraft, which was launched on August 5, 2011, and entered Jovian orbit on July 4, 2016. Although intended for a two-year mission, it is still active in 2024 and expected to continue until September 2025. ''Juno'' provided the first views of Jupiter's north pole and new insights into Jupiter's aurorae, magnetic field, and atmosphere. Information gathered about Jovian lightning prompted revision of earlier theories, and analysis of the frequency of interplanetary dust impacts (primarily on the backs of the solar panels), as ''Juno'' passed between Earth and the asteroid belt, indicated that this dust comes from Mars
Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...
, rather than from comets or asteroids, as was previously thought.
Jupiter Icy Moons Explorer
The European Space Agency is planning to return to the Jovian system with the Jupiter Icy Moons Explorer
The Jupiter Icy Moons Explorer (Juice, formerly JUICE) is an interplanetary spacecraft on its way to orbit and study three icy moons of Jupiter (planet), Jupiter: Ganymede (moon), Ganymede, Callisto (moon), Callisto, and Europa (moon), Europa ...
(JUICE). This was launched from Europe's Spaceport in French Guiana on April 14, 2023, and is expected to reach Jupiter in July 2031.
''Europa Clipper''
Even before ''Galileo'' concluded, NASA considered the Europa Orbiter
The Europa Orbiter was a planned NASA mission to Jupiter's Moon Europa, that was cancelled in 2002. Its main objectives included determining the presence or absence of a subsurface ocean and identifying candidate sites for future lander mis ...
, but it was canceled in 2002. A lower-cost version was then studied, which led to ''Europa Clipper
Europa Clipper (previously known as Europa Multiple Flyby Mission) is a space probe developed by NASA to study Europa (moon), Europa, a Galilean moon of Jupiter. It was launched on October 14, 2024. The spacecraft used a gravity assist from Mar ...
'' being approved in 2015. This mission launched from Kennedy Space Center on October 14, 2024 and is expected to reach Jupiter in April 2030.
''Europa Lander''
A lander, simply called '' Europa Lander'' was assessed by the Jet Propulsion Laboratory. , this mission remains a concept, although some funds were released for instrument development and maturation.
Notes
Citations
References
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
External links
''Galileo'' mission site
by NASA's Solar System Exploration
''Galileo'' legacy site
by NASA's Solar System Exploration
''Galileo'' Satellite Image Mosaics
by Arizona State University
Galileo image album
by Kevin M. Gill
{{DEFAULTSORT:Galileo (Spacecraft)
Galileo Galilei
Galileo program
Missions to Jupiter
Missions to main-belt asteroids
Articles containing video clips