Cassini–Huygens

''Cassini–Huygens'' ( ), commonly called ''Cassini'', was a 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, including its rings and natural satellites. The Flagship-class robotic spacecraft comprised both NASA's ''Cassini'' space probe and ESA's ''Huygens'' lander, which landed on Saturn's largest moon, Titan. ''Cassini'' was the fourth space probe to visit Saturn and the first to enter its orbit, where it stayed from 2004 to 2017. The two craft took their names from the astronomers Giovanni Cassini and Christiaan Huygens.

Launched aboard a Titan IVB/Centaur on October 15, 1997, ''Cassini'' was active in space for nearly 20 years, with 13 years spent orbiting Saturn and studying the planet and its system after entering orbit on July 1, 2004. The voyage to Saturn included flybys of Venus (April 1998 and July 1999), Earth (August 1999), the asteroid 2685 Masursky, and Jupiter (December 2000). The mission ended on September 15, 2017, when ''Cassini's'' trajectory took it into Saturn's upper atmosphere and it burned up in order to prevent any risk of contaminating Saturn's moons, which might have offered habitable environments to stowaway terrestrial microbes on the spacecraft. The mission was successful beyond expectations – NASA's Planetary Science Division Director, Jim Green, described ''Cassini-Huygens'' as a "mission of firsts" that has revolutionized human understanding of the Saturn system, including its moons and rings, and our understanding of where life might be found in the Solar System.

''Cassini''s planners originally scheduled a mission of four years, from June 2004 to May 2008. The mission was extended for another two years until September 2010, branded the ''Cassini Equinox Mission''. The mission was extended a second and final time with the ''Cassini Solstice Mission'', lasting another seven years until September 15, 2017, on which date ''Cassini'' was de-orbited to burn up in Saturn's upper atmosphere.

The ''Huygens'' module traveled with ''Cassini'' until its separation from the probe on December 25, 2004; Huygens landed by parachute on Titan on January 14, 2005. The separation was facilitated by the SED (Spin/Eject device), which provided a relative separation speed of 0.35 m/s and a spin rate of 7.5 rpm. It returned data to Earth for around 90 minutes, using the orbiter as a relay. This was the first landing ever accomplished in the outer Solar System and the first landing on a moon other than Earth's Moon.

At the end of its mission, the ''Cassini'' spacecraft executed its "Grand Finale": a number of risky passes through the gaps between Saturn and its inner rings.

This phase aimed to maximize ''Cassini'' scientific outcome before the spacecraft was intentionally destroyed to prevent potential contamination of Saturn's moons if ''Cassini'' were to unintentionally crash into them when maneuvering the probe was no longer possible due to power loss or other communication issues at the end of its operational lifespan. The atmospheric entry of ''Cassini'' ended the mission, but analysis of the returned data will continue for many years.

Overview

Scientists and individuals from 27 countries made up the joint team responsible for designing, building, flying and collecting data from the ''Cassini'' orbiter and the ''Huygens'' probe.

NASA's Jet Propulsion Laboratory in the United States, where the orbiter was assembled, managed the mission. The European Space Research and Technology Centre developed ''Huygens''. The centre's prime contractor, Aérospatiale of France (part of Thales Alenia Space from 2005), assembled the probe with equipment and instruments supplied by many European countries (including ''Huygens'' batteries and two scientific instruments from the United States). The Italian Space Agency (ASI) provided the ''Cassini'' orbiter's high-gain radio antenna, with the incorporation of a low-gain antenna (to ensure telecommunications with the Earth for the entire duration of the mission), a compact and lightweight radar, which also used the high-gain antenna and served as a synthetic-aperture radar, a radar altimeter, a radiometer, the radio science subsystem (RSS), and the visible-channel portion VIMS-V of VIMS spectrometer.

NASA provided the VIMS infrared counterpart, as well as the Main Electronic Assembly, which included electronic sub-assemblies provided by CNES of France.

On April 16, 2008, NASA announced a two-year extension of the funding for ground operations of this mission, at which point it was renamed the Cassini Equinox Mission.

The round of funding was again extended in February 2010 with the Cassini Solstice Mission.

Naming

The mission consisted of two main elements: the ASI/NASA ''Cassini'' orbiter, named for the Italian astronomer Giovanni Domenico Cassini, discoverer of Saturn's ring divisions and four of its satellites; and the ESA-developed ''Huygens'' probe, named for the Dutch astronomer, mathematician and physicist Christiaan Huygens, discoverer of Titan.

The mission was commonly called Saturn Orbiter Titan Probe (SOTP) during gestation, both as a Mariner Mark II mission and generically.

''Cassini-Huygens'' was a Flagship-class mission to the outer planets. The other planetary flagships include ''Galileo'', Voyager, and Viking.

Objectives

''Cassini'' had several objectives, including:
* Determining the three-dimensional structure and dynamic behavior of the rings of Saturn.
* Determining the composition of the satellite surfaces and the geological history of each object.
* Determining the nature and origin of the dark material on Iapetus's leading hemisphere.
* Measuring the three-dimensional structure and dynamic behavior of the magnetosphere.
* Studying the dynamic behavior of Saturn's atmosphere at cloud level.
* Studying the time variability of Titan's clouds and hazes.
* Characterizing Titan's surface on a regional scale.

''Cassini–Huygens'' was launched on October 15, 1997, from Cape Canaveral Air Force Station's Space Launch Complex 40 using a U.S. Air Force Titan IVB/Centaur rocket. The complete launcher was made up of a two-stage Titan IV booster rocket, two strap-on solid rocket engines, the Centaur upper stage, and a payload enclosure, or fairing.

The total cost of this scientific exploration mission was about US$3.26  billion, including $1.4 billion for pre-launch development, $704 million for mission operations, $54 million for tracking and $422 million for the launch vehicle. The United States contributed $2.6 billion (80%), the ESA $500 million (15%), and the ASI $160 million (5%). However, these figures are from the press kit which was prepared in October 2000. They do not include inflation over the course of a very long mission, nor do they include the cost of the extended missions.

The primary mission for ''Cassini'' was completed on July 30, 2008. The mission was extended to June 2010 (''Cassini'' Equinox Mission). This studied the Saturn system in detail during the planet's equinox, which happened in August 2009.

On February 3, 2010, NASA announced another extension for ''Cassini'', lasting 6 years until 2017, ending at the time of summer solstice in Saturn's northern hemisphere (''Cassini'' Solstice Mission). The extension enabled another 155 revolutions around the planet, 54 flybys of Titan and 11 flybys of Enceladus.
In 2017, an encounter with Titan changed its orbit in such a way that, at closest approach to Saturn, it was only above the planet's cloudtops, below the inner edge of the D ring. This sequence of "proximal orbits" ended when its final encounter with Titan sent the probe into Saturn's atmosphere to be destroyed.

Itinerary

History

''Cassini–Huygens''s origins date to 1982, when the European Science Foundation and the American National Academy of Sciences formed a working group to investigate future cooperative missions. Two European scientists suggested a paired Saturn Orbiter and Titan Probe as a possible joint mission. In 1983, NASA's Solar System Exploration Committee recommended the same Orbiter and Probe pair as a core NASA project. NASA and the European Space Agency (ESA) performed a joint study of the potential mission from 1984 to 1985. ESA continued with its own study in 1986, while the American astronaut Sally Ride, in her influential 1987 report '' NASA Leadership and America's Future in Space'', also examined and approved of the ''Cassini'' mission.

While Ride's report described the Saturn orbiter and probe as a NASA solo mission, in 1988 the Associate Administrator for Space Science and Applications of NASA, Len Fisk, returned to the idea of a joint NASA and ESA mission. He wrote to his counterpart at ESA, Roger Bonnet, strongly suggesting that ESA choose the ''Cassini'' mission from the three candidates at hand and promising that NASA would commit to the mission as soon as ESA did.

At the time, NASA was becoming more sensitive to the strain that had developed between the American and European space programs as a result of European perceptions that NASA had not treated it like an equal during previous collaborations. NASA officials and advisers involved in promoting and planning ''Cassini–Huygens'' attempted to correct this trend by stressing their desire to evenly share any scientific and technology benefits resulting from the mission. In part, this newfound spirit of cooperation with Europe was driven by a sense of competition with the Soviet Union, which had begun to cooperate more closely with Europe as ESA drew further away from NASA. Late in 1988, ESA chose Cassini–Huygens as its next major mission and the following year the program received major funding in the US.

The collaboration not only improved relations between the two space programs but also helped ''Cassini–Huygens'' survive congressional budget cuts in the United States. ''Cassini–Huygens'' came under fire politically in both 1992 and 1994, but NASA successfully persuaded the United States Congress that it would be unwise to halt the project after ESA had already poured funds into development because frustration on broken space exploration promises might spill over into other areas of foreign relations. The project proceeded politically smoothly after 1994, although citizens' groups concerned about its potential environmental impact attempted to derail it through protests and lawsuits until and past its 1997 launch.

Spacecraft design

The spacecraft was planned to be the second three-axis stabilized, RTG-powered Mariner Mark II, a class of spacecraft developed for missions beyond the orbit of Mars. ''Cassini'' was developed simultaneously with the '' Comet Rendezvous Asteroid Flyby'' (''CRAF'') spacecraft, but budget cuts and project rescopings forced NASA to terminate CRAF development to save ''Cassini''. As a result, ''Cassini'' became more specialized. The Mariner Mark II series was cancelled.

The combined orbiter and probe is the third-largest unmanned interplanetary spacecraft ever successfully launched, behind the Phobos 1 and 2 Mars probes, as well as being among the most complex. The orbiter had a mass of , the probe . With the launch vehicle adapter and of propellants at launch, the spacecraft had a mass of .

The ''Cassini'' spacecraft was high and wide. Spacecraft complexity was increased by its trajectory (flight path) to Saturn, and by the ambitious science at its destination. ''Cassini'' had 1,630 interconnected electronic components, 22,000 wire connections, and of cabling. The core control computer CPU was a redundant MIL-STD-1750A system. The main propulsion system consisted of one prime and one backup R-4D bipropellant rocket engine. The thrust of each engine was and the total spacecraft delta-v was about . Smaller monopropellant rockets provided attitude control.

''Cassini'' was powered by of nuclear fuel, mainly plutonium dioxide (containing of pure plutonium). The heat from the material's radioactive decay was turned into electricity. ''Huygens'' was supported by ''Cassini'' during cruise, but used chemical batteries when independent.

The probe contained a DVD with more than 616,400 signatures from citizens in 81 countries, collected in a public campaign.

Until September 2017 the ''Cassini'' probe continued orbiting Saturn at a distance of between from the Earth. It took 68 to 84 minutes for radio signals to travel from Earth to the spacecraft, and vice versa. Thus ground controllers could not give "real-time" instructions for daily operations or for unexpected events. Even if response were immediate, more than two hours would have passed between the occurrence of a problem and the reception of the engineers' response by the satellite.

Instruments

Summary

Instruments:
* Optical Remote Sensing ("Located on the remote sensing pallet")
** Composite Infrared Spectrometer (CIRS)
** Imaging Science Subsystem (ISS)
** Ultraviolet Imaging Spectrograph (UVIS)
** Visible and Infrared Mapping Spectrometer (VIMS)
* Fields, Particles and Waves (mostly in situ)
** Cassini Plasma Spectrometer (CAPS)
** Cosmic Dust Analyzer (CDA)
** Ion and Neutral Mass Spectrometer (INMS)
** Magnetometer (MAG)
** Magnetospheric Imaging Instrument (MIMI)
** Radio and Plasma Wave Science (RPWS)
* Microwave Remote Sensing
** Radar
** Radio Science (RSS)

Description

''Cassini''s instrumentation consisted of: a synthetic aperture radar mapper, a charge-coupled device imaging system, a visible/infrared mapping spectrometer, a composite infrared spectrometer, a cosmic dust analyzer, a radio and plasma wave experiment, a plasma spectrometer, an ultraviolet imaging spectrograph, a magnetospheric imaging instrument, a magnetometer and an ion/neutral mass spectrometer. Telemetry from the communications antenna and other special transmitters (an S-band transmitter and a dual-frequency K_a-band system) was also used to make observations of the atmospheres of Titan and Saturn and to measure the gravity fields of the planet and its satellites.

; Cassini Plasma Spectrometer (CAPS): CAPS was an in situ instrument that measured the flux of charged particles at the location of the spacecraft, as a function of direction and energy. The ion composition was also measured using a time-of-flight mass spectrometer. CAPS measured particles produced by ionisation of molecules originating from Saturn's and Titan's ionosphere, as well as the plumes of Enceladus. CAPS also investigated plasma in these areas, along with the solar wind and its interaction with Saturn's magnetosphere. CAPS was turned off in June 2011, as a precaution due to a "soft" electrical short circuit that occurred in the instrument. It was powered on again in March 2012, but after 78 days another short circuit forced the instrument to be shut down permanently.

; Cosmic Dust Analyzer (CDA):The CDA was an in situ instrument that measured the size, speed, and direction of tiny dust grains near Saturn. It could also measure the grains' chemical elements. Some of these particles orbited Saturn, while others came from other star systems. The CDA on the orbiter was designed to learn more about these particles, the materials in other celestial bodies and potentially about the origins of the universe.

; Composite Infrared Spectrometer (CIRS):The CIRS was a remote sensing instrument that measured the infrared radiation coming from objects to learn about their temperatures, thermal properties, and compositions. Throughout the ''Cassini–Huygens'' mission, the CIRS measured infrared emissions from atmospheres, rings and surfaces in the vast Saturn system. It mapped the atmosphere of Saturn in three dimensions to determine temperature and pressure profiles with altitude, gas composition, and the distribution of aerosols and clouds. It also measured thermal characteristics and the composition of satellite surfaces and rings.

; Ion and Neutral Mass Spectrometer (INMS):The INMS was an in situ instrument that measured the composition of charged particles (protons and heavier ions) and neutral particles (atoms and molecules) near Titan and Saturn to learn more about their atmospheres. The instrument used a quadrupole mass spectrometer. INMS was also intended to measure the positive ion and neutral environments of Saturn's icy satellites and rings.

; Imaging Science Subsystem (ISS):The ISS was a remote sensing instrument that captured most images in visible light, and also some infrared images and ultraviolet images. The ISS took hundreds of thousands of images of Saturn, its rings, and its moons. The ISS had both a wide-angle camera (WAC) and a narrow-angle camera (NAC). Each of these cameras used a sensitive charge-coupled device (CCD) as its electromagnetic wave detector. Each CCD had a 1,024 square array of pixels, 12 μm on a side. Both cameras allowed for many data collection modes, including on-chip data compression, and were fitted with spectral filters that rotated on a wheel to view different bands within the electromagnetic spectrum ranging from 0.2 to 1.1 μm.

; Dual Technique Magnetometer (MAG):The MAG was an in situ instrument that measured the strength and direction of the magnetic field around Saturn. The magnetic fields are generated partly by the molten core at Saturn's center. Measuring the magnetic field is one of the ways to probe the core. MAG aimed to develop a three-dimensional model of Saturn's magnetosphere, and determine the magnetic state of Titan and its atmosphere, and the icy satellites and their role in the magnetosphere of Saturn.

; Magnetospheric Imaging Instrument (MIMI):The MIMI was both an in situ and remote sensing instrument that produces images and other data about the particles trapped in Saturn's huge magnetic field, or magnetosphere. The in situ component measured energetic ions and electrons while the remote sensing component (the Ion And Neutral Camera, INCA) was an energetic neutral atom imager. This information was used to study the overall configuration and dynamics of the magnetosphere and its interactions with the solar wind, Saturn's atmosphere, Titan, rings, and icy satellites.

; Radar:The on-board radar was an active and passive sensing instrument that produced maps of Titan's surface. Radar waves were powerful enough to penetrate the thick veil of haze surrounding Titan. By measuring the send and return time of the signals it is possible to determine the height of large surface features, such as mountains and canyons. The passive radar listened for radio waves that Saturn or its moons may emit.

; Radio and Plasma Wave Science instrument (RPWS):The RPWS was an in situ instrument and remote sensing instrument that receives and measures radio signals coming from Saturn, including the radio waves given off by the interaction of the solar wind with Saturn and Titan. RPWS measured the electric and magnetic wave fields in the interplanetary medium and planetary magnetospheres. It also determined the electron density and temperature near Titan and in some regions of Saturn's magnetosphere using either plasma waves at characteristic frequencies (e.g. the upper hybrid line) or a Langmuir probe. RPWS studied the configuration of Saturn's magnetic field and its relationship to Saturn Kilometric Radiation (SKR), as well as monitoring and mapping Saturn's ionosphere, plasma, and lightning from Saturn's (and possibly Titan's) atmosphere.

; Radio Science Subsystem (RSS):The RSS was a remote-sensing instrument that used radio antennas on Earth to observe the way radio signals from the spacecraft changed as they were sent through objects, such as Titan's atmosphere or Saturn's rings, or even behind the Sun. The RSS also studied the compositions, pressures and temperatures of atmospheres and ionospheres, radial structure and particle size distribution within rings, body and system masses and the gravitational field. The instrument used the spacecraft X-band communication link as well as S-band downlink and K_a-band uplink and downlink.

; Ultraviolet Imaging Spectrograph (UVIS):The UVIS was a remote-sensing instrument that captured images of the ultraviolet light reflected off an object, such as the clouds of Saturn and/or its rings, to learn more about their structure and composition. Designed to measure ultraviolet light over wavelengths from 55.8 to 190 nm, this instrument was also a tool to help determine the composition, distribution, aerosol particle content and temperatures of their atmospheres. Unlike other types of spectrometer, this sensitive instrument could take both spectral and spatial readings. It was particularly adept at determining the composition of gases. Spatial observations took a wide-by-narrow view, only one pixel tall and 64 pixels across. The spectral dimension was 1,024 pixels per spatial pixel. It could also take many images that create movies of the ways in which this material is moved around by other forces.
:UVIS consisted of four separate detector channels, the Far Ultraviolet (FUV), Extreme Ultraviolet (EUV), High Speed Photometer (HSP) and the Hydrogen-Deuterium Absorption Cell (HDAC). UVIS collected hyperspectral imagery and discrete spectra of Saturn, its moons and its rings, as well as stellar occultation data. The Cassini Ultraviolet Imaging Spectrograph Investigation
:The HSP channel is designed to observe starlight that passes through Saturn's rings (known as stellar occultations) in order to understand the structure and optical depth of the rings. Stellar occultation data from both the HSP and FUV channels confirmed the existence of water vapor plumes at the south pole of Enceladus, as well as characterized the composition of the plumes.

; Visible and Infrared Mapping Spectrometer (VIMS):The VIMS was a remote sensing instrument that captured images using visible and infrared light to learn more about the composition of moon surfaces, the rings, and the atmospheres of Saturn and Titan. It consisted of two cameras - one used to measure visible light, the other infrared. VIMS measured reflected and emitted radiation from atmospheres, rings and surfaces over wavelengths from 350 to 5100 nm, to help determine their compositions, temperatures and structures. It also observed the sunlight and starlight that passes through the rings to learn more about their structure. Scientists used VIMS for long-term studies of cloud movement and morphology in the Saturn system, to determine Saturn's weather patterns.

Plutonium power source

Because of Saturn's distance from the Sun, solar arrays were not feasible as power sources for this space probe. To generate enough power, such arrays would have been too large and too heavy. Instead, the ''Cassini'' orbiter was powered by three GPHS-RTG radioisotope thermoelectric generators, which use heat from the decay of about of plutonium-238 (in the form of plutonium dioxide) to generate direct current electricity via thermoelectrics.
The RTGs on the ''Cassini'' mission have the same design as those used on the ''New Horizons'', ''Galileo'', and ''Ulysses'' space probes, and they were designed to have very long operational lifetimes.
At the end of the nominal 11-year ''Cassini'' mission, they were still able to produce 600 to 700 watts of electrical power. (Leftover hardware from the ''Cassini'' RTG Program was modified and used to power the ''New Horizons'' mission to Pluto and the Kuiper belt, which was designed and launched later.)

Power distribution was accomplished by 192 solid-state power switches, which also functioned as circuit breakers in the event of an overload condition. The switches used MOSFETs that featured better efficiency and a longer lifetime as compared to conventional switches, while at the same time eliminating transients. However, these solid-state circuit breakers were prone to erroneous tripping (presumably from cosmic rays), requiring them to reset and causing losses in experimental data.

To gain momentum while already in flight, the trajectory of the ''Cassini'' mission included several gravitational slingshot maneuvers: two fly-by passes of Venus, one more of the Earth, and then one of the planet Jupiter. The terrestrial flyby was the final instance when the probe posed any conceivable danger to human beings. The maneuver was successful, with ''Cassini'' passing by above the Earth on August 18, 1999.
Had there been any malfunction causing the probe to collide with the Earth, NASA's complete environmental impact study estimated that, in the worst case (with an acute angle of entry in which ''Cassini'' would gradually burn up), a significant fraction of the 33 kg of nuclear fuel inside the RTGs would have been dispersed into the Earth's atmosphere so that up to five billion people (i.e. almost the entire terrestrial population) could have been exposed, causing up to an estimated 5,000 additional cancer deaths over the subsequent decades, see , Table 2-8 (0.0005 per cent, i.e. a fraction 0.000005, of a billion cancer deaths expected anyway from other causes; the product is incorrectly calculated elsewhere as 500,000 deaths). However, the chance of this happening were estimated to be less than one in one million, i.e. a chance of one person dying (assuming 5,000 deaths) as less than 1 in 200.

NASA's risk analysis to use plutonium was publicly criticized by Michio Kaku on the grounds that casualties, property damage, and lawsuits resulting from a possible accident, as well as the potential use of alternate energy sources, such as solar and fuel cells, were underestimated.

Telemetry

The ''Cassini'' spacecraft was capable of transmitting in several different telemetry formats. The telemetry subsystem is perhaps the most important subsystem, because without it there could be no data return.

The telemetry was developed from the ground up, due to the spacecraft using a more modern set of computers than previous missions. Therefore, ''Cassini'' was the first spacecraft to adopt mini-packets to reduce the complexity of the Telemetry Dictionary, and the software development process led to the creation of a Telemetry Manager for the mission.

There were around 1088 channels (in 67 mini-packets) assembled in the ''Cassini'' Telemetry Dictionary. Out of these 67 lower complexity mini-packets, 6 mini-packets contained the subsystem covariance and Kalman gain elements (161 measurements), not used during normal mission operations. This left 947 measurements in 61 mini-packets.

A total of seven telemetry maps corresponding to 7 AACS telemetry modes were constructed. These modes are: (1) Record; (2) Nominal Cruise; (3) Medium Slow Cruise; (4) Slow Cruise; (5) Orbital Ops; (6) Av; (7) ATE (Attitude Estimator) Calibration. These 7 maps cover all spacecraft telemetry modes.

''Huygens'' probe

The ''Huygens'' probe, supplied by the European Space Agency (ESA) and named after the 17th century Dutch astronomer who first discovered Titan, Christiaan Huygens, scrutinized the clouds, atmosphere, and surface of Saturn's moon Titan in its descent on January 15, 2005. It was designed to enter and brake in Titan's atmosphere and parachute a fully instrumented robotic laboratory down to the surface.

The probe system consisted of the probe itself which descended to Titan, and the probe support equipment (PSE) which remained attached to the orbiting spacecraft. The PSE includes electronics that track the probe, recover the data gathered during its descent, and process and deliver the data to the orbiter that transmits it to Earth. The core control computer CPU was a redundant MIL-STD-1750A control system.

The data were transmitted by a radio link between ''Huygens'' and ''Cassini'' provided by Probe Data Relay Subsystem (PDRS). As the probe's mission could not be telecommanded from Earth because of the great distance, it was automatically managed by the Command Data Management Subsystem (CDMS). The PDRS and CDMS were provided by the Italian Space Agency (ASI).

After ''Cassini'' launch, it was discovered that data sent from the ''Huygens'' probe to ''Cassini'' orbiter (and then re-transmitted to Earth) would be largely unreadable. The cause was that the bandwidth of signal processing electronics was too narrow and the anticipated Doppler shift between the lander and the mother craft would put the signals out of the system's range. Thus, ''Cassini'' receiver would be unable to receive the data from ''Huygens'' during its descent to Titan.

A work-around was found to recover the mission. The trajectory of ''Cassini'' was altered to reduce the line of sight velocity and therefore the doppler shift. ''Cassinis subsequent trajectory was identical to the previously planned one, although the change replaced two orbits prior to the ''Huygens'' mission with three, shorter orbits.

Selected events and discoveries

Venus and Earth fly-bys and the cruise to Jupiter

The ''Cassini'' space probe performed two gravitational-assist flybys of Venus on April 26, 1998, and June 24, 1999. These flybys provided the space probe with enough momentum to travel all the way out to the asteroid belt, while the Sun's gravity pulled the space probe back into the inner Solar System.

On August 18, 1999, at 03:28 UTC, the craft made a gravitational-assist flyby of the Earth. One hour and 20 minutes before closest approach, ''Cassini'' made its closest approach to the Earth's Moon at 377,000 kilometers, and it took a series of calibration photos.

On January 23, 2000, ''Cassini'' performed a flyby of the asteroid 2685 Masursky at around 10:00 UTC. It took photos in the period five to seven hours before the flyby at a distance of and a diameter of was estimated for the asteroid.

Jupiter flyby

''Cassini'' made its closest approach to Jupiter on December 30, 2000, at 9.7 million kilometers, and made many scientific measurements. About 26,000 images of Jupiter, its faint rings, and its moons were taken during the six-month flyby. It produced the most detailed global color portrait of the planet yet (see image at right), in which the smallest visible features are approximately across.

A major finding of the flyby, announced on March 6, 2003, was of Jupiter's atmospheric circulation. Dark "belts" alternate with light "zones" in the atmosphere, and scientists had long considered the zones, with their pale clouds, to be areas of upwelling air, partly because many clouds on Earth form where air is rising. But analysis of ''Cassini'' imagery showed that individual storm cells of upwelling bright-white clouds, too small to see from Earth, pop up almost without exception in the dark belts. According to Anthony Del Genio of NASA's Goddard Institute for Space Studies, "the belts must be the areas of net-rising atmospheric motion on Jupiter, othe net motion in the zones has to be sinking."

Other atmospheric observations included a swirling dark oval of high atmospheric haze, about the size of the Great Red Spot, near Jupiter's north pole. Infrared imagery revealed aspects of circulation near the poles, with bands of globe-encircling winds, with adjacent bands moving in opposite directions.

The same announcement also discussed the nature of Jupiter's rings. Light scattering by particles in the rings showed the particles were irregularly shaped (rather than spherical) and likely originate as ejecta from micrometeorite impacts on Jupiter's moons, probably Metis and Adrastea.

Tests of general relativity

On October 10, 2003, the mission's science team announced the results of tests of Albert Einstein's general theory of relativity, performed by using radio waves transmitted from the ''Cassini'' space probe. The radio scientists measured a frequency shift in the radio waves to and from the spacecraft, as they passed close to the Sun. According to the general theory of relativity, a massive object like the Sun causes space-time to curve, causing a beam of radiowaves travelling out of its gravitational well to decrease in frequency and radiowaves travelling into the gravitational well to increase in frequency, referred to as gravitational redshift / blueshift.

Although some measurable deviations from the values calculated using the general theory of relativity are predicted by some unusual cosmological models, no such deviations were found by this experiment. Previous tests using radiowaves transmitted by the ''Viking'' and ''Voyager'' space probes were in agreement with the calculated values from general relativity to within an accuracy of one part in one thousand. The more refined measurements from the ''Cassini'' space probe experiment improved this accuracy to about one part in 51,000. The data firmly support Einstein's general theory of relativity.

New moons of Saturn

In total, the ''Cassini'' mission discovered seven new moons orbiting Saturn. Using images taken by ''Cassini'', researchers discovered Methone, Pallene and Polydeuces in 2004, although later analysis revealed that Voyager 2 had photographed Pallene in its 1981 flyby of the ringed planet.

On May 1, 2005, a new moon was discovered by ''Cassini'' in the Keeler gap. It was given the designation S/2005 S 1 before being named Daphnis. A fifth new moon was discovered by ''Cassini'' on May 30, 2007, and was provisionally labeled S/2007 S 4. It is now known as Anthe. A press release on February 3, 2009, showed a sixth new moon found by ''Cassini''. The moon is approximately in diameter within the G-ring of the ring system of Saturn, and is now named Aegaeon (formerly S/2008 S 1). A press release on November 2, 2009, mentions the seventh new moon found by ''Cassini'' on July 26, 2009. It is presently labeled S/2009 S 1 and is approximately 300 m (1000 ft) in diameter in the B-ring system.

On April 14, 2014, NASA scientists reported the possible beginning of a new moon in Saturn's A Ring.

Phoebe flyby

On June 11, 2004, ''Cassini'' flew by the moon Phoebe. This was the first opportunity for close-up studies of this moon (Voyager 2 performed a distant flyby in 1981 but returned no detailed images). It also was ''Cassini's'' only possible flyby for Phoebe due to the mechanics of the available orbits around Saturn.

The first close-up images were received on June 12, 2004, and mission scientists immediately realized that the surface of Phoebe looks different from asteroids visited by spacecraft. Parts of the heavily cratered surface look very bright in those pictures, and it is currently believed that a large amount of water ice exists under its immediate surface.

Saturn rotation

In an announcement on June 28, 2004, ''Cassini'' program scientists described the measurement of the rotational period of Saturn. Because there are no fixed features on the surface that can be used to obtain this period, the repetition of radio emissions was used. This new data agreed with the latest values measured from Earth, and constituted a puzzle to the scientists. It turns out that the radio rotational period had changed since it was first measured in 1980 by ''Voyager 1'', and it was now 6 minutes longer. This, however, does not indicate a change in the overall spin of the planet. It is thought to be due to variations in the upper atmosphere and ionosphere at the latitudes which are magnetically connected to the radio source region.

In 2019 NASA announced Saturn's rotational period as 10 hours, 33 minutes, 38 seconds, calculated using Saturnian ring seismology. Vibrations from Saturn's interior cause oscillations in its gravitational field. This energy is absorbed by ring particles in specific locations, where it accumulates until it is released in a wave. Scientists used data from more than 20 of these waves to construct a family of models of Saturn's interior, providing basis for calculating its rotational period.

Orbiting Saturn

On July 1, 2004, the spacecraft flew through the gap between the F and G rings and achieved orbit, after a seven-year voyage. It was the first spacecraft to ever orbit Saturn.

The Saturn Orbital Insertion (SOI) maneuver performed by ''Cassini'' was complex, requiring the craft to orient its High-Gain Antenna away from Earth and along its flight path, to shield its instruments from particles in Saturn's rings. Once the craft crossed the ring plane, it had to rotate again to point its engine along its flight path, and then the engine fired to decelerate the craft by 622 m/s to allow Saturn to capture it. ''Cassini'' was captured by Saturn's gravity at around 8:54 pm Pacific Daylight Time on June 30, 2004. During the maneuver ''Cassini'' passed within of Saturn's cloud tops.

When Cassini was in Saturnian orbit, departure from the Saturn system was evaluated in 2008 during end of mission planning.

Titan flybys

''Cassini'' had its first flyby of Saturn's largest moon, Titan, on July 2, 2004, a day after orbit insertion, when it approached to within of Titan. Images taken through special filters (able to see through the moon's global haze) showed south polar clouds thought to be composed of methane and surface features with widely differing brightness. On October 27, 2004, the spacecraft executed the first of the 45 planned close flybys of Titan when it passed a mere above the moon. Almost four gigabits of data were collected and transmitted to Earth, including the first radar images of the moon's haze-enshrouded surface. It revealed the surface of Titan (at least the area covered by radar) to be relatively level, with topography reaching no more than about in altitude. The flyby provided a remarkable increase in imaging resolution over previous coverage. Images with up to 100 times better resolution were taken and are typical of resolutions planned for subsequent Titan flybys. Cassini collected pictures of Titan and the lakes of methane were similar to the lakes of water on Earth.

''Huygens'' lands on Titan

''Cassini'' released the ''Huygens'' probe on December 25, 2004, by means of a spring and spiral rails intended to rotate the probe for greater stability. It entered the atmosphere of Titan on January 14, 2005, and after a two-and-a-half-hour descent landed on solid ground. Although ''Cassini'' successfully relayed 350 of the pictures that it received from ''Huygens'' of its descent and landing site, a software error failed to turn on one of the ''Cassini'' receivers and caused the loss of another 350 pictures. While landing, for caution, NASA loaded Huygens with 3 parachutes.

Enceladus flybys

During the first two close flybys of the moon Enceladus in 2005, ''Cassini'' discovered a deflection in the local magnetic field that is characteristic for the existence of a thin but significant atmosphere. Other measurements obtained at that time point to ionized water vapor as its main constituent. ''Cassini'' also observed water ice geysers erupting from the south pole of Enceladus, which gives more credibility to the idea that Enceladus is supplying the particles of Saturn's E ring. Mission scientists began to suspect that there may be pockets of liquid water near the surface of the moon that fuel the eruptions.

On March 12, 2008, ''Cassini'' made a close fly-by of Enceladus, passing within 50 km of the moon's surface. The spacecraft passed through the plumes extending from its southern geysers, detecting water, carbon dioxide and various hydrocarbons with its mass spectrometer, while also mapping surface features that are at much higher temperature than their surroundings with the infrared spectrometer. ''Cassini'' was unable to collect data with its cosmic dust analyzer due to an unknown software malfunction.

On November 21, 2009, ''Cassini'' made its eighth flyby of Enceladus, this time with a different geometry, approaching within of the surface. The Composite Infrared Spectrograph (CIRS) instrument produced a map of thermal emissions from the Baghdad Sulcus 'tiger stripe'. The data returned helped create a detailed and high resolution mosaic image of the southern part of the moon's Saturn-facing hemisphere.

On April 3, 2014, nearly ten years after ''Cassini'' entered Saturn's orbit, NASA reported evidence of a large salty internal ocean of liquid water in Enceladus. The presence of an internal salty ocean in contact with the moon's rocky core, places Enceladus "among the most likely places in the Solar System to host alien microbial life". On June 30, 2014, NASA celebrated ten years of ''Cassini'' exploring Saturn and its moons, highlighting the discovery of water activity on Enceladus among other findings.

In September 2015, NASA announced that gravitational and imaging data from ''Cassini'' were used to analyze the librations of Enceladus' orbit and determined that the moon's surface is not rigidly joined to its core, concluding that the underground ocean must therefore be global in extent.

On October 28, 2015, ''Cassini'' performed a close flyby of Enceladus, coming within of the surface, and passing through the icy plume above the south pole.

Radio occultations of Saturn's rings

In May 2005, ''Cassini'' began a series of radio occultation experiments, to measure the size-distribution of particles in Saturn's rings, and measure the atmosphere of Saturn itself. For over four months, the craft completed orbits designed for this purpose. During these experiments, it flew behind the ring plane of Saturn, as seen from Earth, and transmitted radio waves through the particles. The radio signals received on Earth were analyzed, for frequency, phase, and power shift of the signal to determine the structure of the rings.

Spokes in rings verified

In images captured September 5, 2005, ''Cassini'' detected spokes in Saturn's rings, previously seen only by the visual observer Stephen James O'Meara in 1977 and then confirmed by the Voyager space probes in the early 1980s.

Lakes of Titan

Radar images obtained on July 21, 2006, appear to show lakes of liquid hydrocarbon (such as methane and ethane) in Titan's northern latitudes. This is the first discovery of currently existing lakes anywhere besides on Earth. The lakes range in size from one to one-hundred kilometers across.

On March 13, 2007, the Jet Propulsion Laboratory announced that it had found strong evidence of seas of methane and ethane in the northern hemisphere of Titan. At least one of these is larger than any of the Great Lakes in North America.

Saturn hurricane

In November 2006, scientists discovered a storm at the south pole of Saturn with a distinct eyewall. This is characteristic of a hurricane on Earth and had never been seen on another planet before. Unlike a terrestrial hurricane, the storm appears to be stationary at the pole. The storm is across, and high, with winds blowing at .

Iapetus flyby

On September 10, 2007, ''Cassini'' completed its flyby of the strange, two-toned, walnut-shaped moon, Iapetus. Images were taken from above the surface. As it was sending the images back to Earth, it was hit by a cosmic ray that forced it to temporarily enter safe mode. All of the data from the flyby were recovered.

Mission extension

On April 15, 2008, ''Cassini'' received funding for a 27-month extended mission. It consisted of 60 more orbits of Saturn, with 21 more close Titan flybys, seven of Enceladus, six of Mimas, eight of Tethys, and one targeted flyby each of Dione, Rhea, and Helene. The extended mission began on July 1, 2008, and was renamed the Cassini Equinox Mission as the mission coincided with Saturn's equinox.

Second mission extension

A proposal was submitted to NASA for a second mission extension (September 2010 – May 2017), provisionally named the extended-extended mission or XXM. This ($60M pa) was approved in February 2010 and renamed the Cassini Solstice Mission. It included ''Cassini'' orbiting Saturn 155 more times, conducting 54 additional flybys of Titan and 11 more of Enceladus.

Great Storm of 2010 and aftermath

On October 25, 2012, ''Cassini'' witnessed the aftermath of the massive Great White Spot storm that recurs roughly every 30 years on Saturn. Data from the composite infrared spectrometer (CIRS) instrument indicated a powerful discharge from the storm that caused a temperature spike in the stratosphere of Saturn above normal. Simultaneously, a huge increase in ethylene gas was detected by NASA researchers at Goddard Research Center in Greenbelt, Maryland. Ethylene is a colorless gas that is highly uncommon on Saturn and is produced both naturally and through man-made sources on Earth. The storm that produced this discharge was first observed by the spacecraft on December 5, 2010, in Saturn's northern hemisphere. The storm is the first of its kind to be observed by a spacecraft in orbit around Saturn as well as the first to be observed at thermal infrared wavelengths, allowing scientists to observe the temperature of Saturn's atmosphere and track phenomena that are invisible to the naked eye. The spike of ethylene gas that was produced by the storm reached levels that were 100 times more than those thought possible for Saturn. Scientists have also determined that the storm witnessed was the largest, hottest stratospheric vortex ever detected in the Solar System, initially being larger than Jupiter's Great Red Spot.

Venus transit

On December 21, 2012, ''Cassini'' observed a transit of Venus across the Sun. The VIMS instrument analyzed sunlight passing through the Venusian atmosphere. VIMS previously observed the transit of exoplanet HD 189733 b.

''The Day the Earth Smiled''

On July 19, 2013, the probe was pointed towards Earth to capture an image of the Earth and the Moon, as part of a natural light, multi-image portrait of the entire Saturn system. The event was unique as it was the first time NASA informed the public that a long-distance photo was being taken in advance. The imaging team said they wanted people to smile and wave to the skies, with ''Cassini'' scientist Carolyn Porco describing the moment as a chance to "celebrate life on the Pale Blue Dot".

Rhea flyby

On February 10, 2015, the ''Cassini'' spacecraft visited Rhea more closely, coming within . The spacecraft observed the moon with its cameras producing some of the highest resolution color images yet of Rhea.

Hyperion flyby

''Cassini'' performed its latest flyby of Saturn's moon Hyperion on May 31, 2015, at a distance of about .

Dione flyby

''Cassini'' performed its last flyby of Saturn's moon Dione on August 17, 2015, at a distance of about . A previous flyby was performed on June 16.

Hexagon changes color

Between 2012 and 2016, the persistent hexagonal cloud pattern at Saturn's north pole changed from a mostly blue color to more of a golden color. One theory for this is a seasonal change: extended exposure to sunlight may be creating haze as the pole swivels toward the Sun. It was previously noted that there was less blue color overall on Saturn between 2004 and 2008.

Grand Finale and destruction

''Cassini'' end involved a series of close Saturn passes, approaching within the rings, then an entry into Saturn's atmosphere on September 15, 2017, to destroy the spacecraft. This method was chosen because it is imperative to ensure protection and prevent biological contamination to any of the moons of Saturn thought to offer potential habitability.

In 2008 a number of options were evaluated to achieve this goal, each with varying funding, scientific, and technical challenges. A short period Saturn impact for an end of mission was rated "excellent" with the reasons "D-ring option satisfies unachieved AO goals; cheap and easily achievable" while collision with an icy moon was rated "good" for being "cheap and achievable anywhere/time".

There was budgetary drama in 2013-14 about NASA receiving U.S. government funding for the Grand Finale. The two phases of the Grand Finale ended up being the equivalent of having two separate Discovery Program-class missions in that the Grand Finale was completely different from the main ''Cassini'' regular mission. The U.S. government in late 2014 approved the Grand Finale at the cost of $200 million. This was far cheaper than building two new probes in separate Discovery-class missions.

On November 29, 2016, the spacecraft performed a Titan flyby that took it to the gateway of F-ring orbits: This was the start of the Grand Finale phase culminating in its impact with the planet. A final Titan flyby on April 22, 2017, changed the orbit again to fly through the gap between Saturn and its inner ring days later on April 26. ''Cassini'' passed about above Saturn's cloud layer and from the visible edge of the inner ring; it successfully took images of Saturn's atmosphere and began returning data the next day. After a further 22 orbits through the gap, the mission was ended with a dive into Saturn's atmosphere on September 15; signal was lost at 11:55:46 UTC on September 15, 2017, just 30 seconds later than predicted. It is estimated that the spacecraft burned up about 45 seconds after the last transmission.

In September 2018, NASA won an Emmy Award for Outstanding Original Interactive Program for its presentation of the ''Cassini'' mission's Grand Finale at Saturn.

In December 2018, Netflix aired "NASA's Cassini Mission" on their series ''7 Days Out'' documenting the final days of work on the ''Cassini'' mission before the spacecraft crashed into Saturn to complete its Grand Finale.

In January 2019, new research using data collected during ''Cassini's Grand Finale phase was published:

* The final close passes by the rings and planet enabled scientists to measure the length of a day on Saturn: 10 hours, 33 minutes and 38 seconds.
* Saturn's rings are relatively new, 10 to 100 million years old.

Missions

The spacecraft operation was organized around a series of missions. Each is structured according to a certain amount of funding, goals, etc. At least 260 scientists from 17 countries have worked on the ''Cassini–Huygens'' mission; in addition thousands of people overall worked to design, manufacture, and launch the mission.
* Prime Mission, July 2004 through June 2008.
* ''Cassini'' Equinox Mission was a two-year mission extension which ran from July 2008 through September 2010.
* ''Cassini'' Solstice Mission ran from October 2010 through April 2017. (Also known as the XXM mission.)
* Grand Finale (spacecraft directed into Saturn), April 2017 to September 15, 2017.

8423 20181 1saturn2016.jpg, Saturn by ''Cassini'', 2016
Cassini Numbers Final.jpg, ''Cassini-Huygens'' by the numbers
(September 2017)
PIA17218 – A Farewell to Saturn, Annotated Version.jpg, Farewell to Saturn and moons ( Enceladus, Epimetheus, Janus, Mimas, Pandora and Prometheus)
(September 13, 2017)

Glossary

* AACS: Attitude and Articulation Control Subsystem
* ACS: Attitude Control Subsystem
* AFC: AACS Flight Computer
* ARWM: Articulated Reaction Wheel Mechanism
* ASI: Agenzia Spaziale Italiana, the Italian space agency
* BIU: Bus Interface Unit
* BOL: Beginning of Life
* CAM: Command Approval Meeting
* CDS: Command and Data Subsystem—Cassini computer that commands and collects data from the instruments
* CICLOPS
Cassini Imaging Central Laboratory for Operations
* CIMS: ''Cassini'' Information Management System
* CIRS: Composite Infrared Spectrometer
* DCSS: Descent Control Subsystem
* DSCC: Deep Space Communications Center
* DSN: Deep Space Network (large antennas around the Earth)
* DTSTART: Dead Time Start
* ELS: Electron Spectrometer (part of CAPS instrument)
* EOM: End of Mission
* ERT: Earth-received time, UTC of an event
* ESA: European Space Agency
* ESOC: European Space Operations Centre
* FSW: flight software
* HGA: High Gain Antenna
* HMCS: ''Huygens'' Monitoring and Control System
* HPOC: ''Huygens'' Probe Operations Center
* IBS: Ion Beam Spectrometer (part of CAPS instrument)
* IEB: Instrument Expanded Blocks (instrument command sequences)
* IMS: Ion Mass Spectrometer (part of CAPS instrument)
* ITL: Integrated Test Laboratory—spacecraft simulator
* IVP: Inertial Vector Propagator
* LGA: Low Gain Antenna
* NAC: Narrow Angle Camera
* NASA: National Aeronautics and Space Administration, the United States space agency
* OTM: Orbit Trim Maneuver
* PDRS: Probe Data Relay Subsystem
* PHSS: Probe Harness SubSystem
* POSW: Probe On-Board Software
* PPS: Power and Pyrotechnic Subsystem
* PRA: Probe Relay Antenna
* PSA: Probe Support Avionics
* PSIV: Preliminary Sequence Integration and Validation
* PSE: probe support equipment
* RCS: Reaction Control System
* RFS: Radio Frequency Subsystem
* RPX: ring plane crossing
* RWA: Reaction Wheel Assembly
* SCET: Spacecraft Event Time
* SCR: sequence change requests
* SKR: Saturn Kilometric Radiation
* SOI: Saturn Orbit Insertion (July 1, 2004)
* SOP: Science Operations Plan
* SSPS: Solid State Power Switch
* SSR: Solid State Recorder
* SSUP: Science and Sequence Update Process
* TLA: Thermal Louver Assemblies
* USO: UltraStable Oscillator
* VRHU: Variable Radioisotope Heater Units
* WAC: Wide Angle Camera
* XXM: Extended-Extended Mission

See also

* Europlanet, data network
* ''Galileo'', Jupiter orbiter and entry probe (1989–2003)
* '' In Saturn's Rings''
* List of missions to the outer planets
* Planetary Science Decadal Survey
* Timeline of ''Cassini–Huygens''

References

Further reading

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External links

Official websites

''Cassini-Huygens'' website
by the Jet Propulsion Laboratory

''Cassini-Huygens'' website
by NASA

''Cassini-Huygens'' website
by the European Space Agency

''Cassini-Huygens'' website
by NASA's Solar System Exploration division


at NASA's Planetary Data System

Media and telecommunications

CICLOPS.org
''Cassini'' imaging homepage

''Cassini'' Hall of Fame
image galleries by the Jet Propulsion Laboratory

"Cassini at Saturn"
a YouTube playlist by the Jet Propulsion Laboratory
*
"Titan Touchdown"
Depiction of ''Huygens'' descent and landing

DESCANSO DSN Telecom information

''In Saturn's Rings''
film animated from millions of still photographs

''Around Saturn''
film animated from more than 200,000 images taken by ''Cassini'' from 2004 to 2012



Cassini image album
by Kevin M. Gill
* https://www.nasa.gov/sites/default/files/atoms/files/the_saturn_system.pdf - THROUGH THE EYES OF CASSINI

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Spacecraft launched in 1997
Spacecraft decommissioned in 2017
European Space Agency space probes
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Spacecraft launched by Titan rockets
Lunar flybys
Space synthetic aperture radar
Missions to main-belt asteroids
Missions to Saturn
Missions to Jupiter
Jet Propulsion Laboratory
1997 in Florida
Christiaan Huygens
Orbiters (space probe)
Space radars
Giovanni Domenico Cassini
Extraterrestrial atmosphere entry
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