The Infrared Space Observatory (ISO) was a

space telescope A space telescope or space observatory is a telescope in outer space used to observe astronomical objects. Suggested by Lyman Spitzer in 1946, the first operational telescopes were the American Orbiting Astronomical Observatory, OAO-2 launch ...

for

infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from aroun ...

light designed and operated by the

European Space Agency , owners = , headquarters = Paris, Île-de-France, France , coordinates = , spaceport = Guiana Space Centre , seal = File:ESA emblem seal.png , seal_size = 130px , image = Views in the Main Control Room (120 ...

(ESA), in cooperation with ISAS (now part of

JAXA The is the Japanese national air and space agency. Through the merger of three previously independent organizations, JAXA was formed on 1 October 2003. JAXA is responsible for research, technology development and launch of satellites into o ...

) and

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedi ...

. The ISO was designed to study infrared light at wavelengths of 2.5 to 240 micrometres and operated from 1995 to 1998. The €480.1- million satellite was launched on 17 November 1995 from the ELA-2

launch pad A launch pad is an above-ground facility from which a rocket-powered missile or space vehicle is vertically launched. The term ''launch pad'' can be used to describe just the central launch platform ( mobile launcher platform), or the ent ...

at the

Guiana Space Centre The Guiana Space Centre (french: links=no, Centre spatial guyanais; CSG), also called Europe's Spaceport, is a European spaceport to the northwest of Kourou in French Guiana, a region of France in South America. Kourou is located approximat ...

near Kourou in French Guiana. The

launch vehicle A launch vehicle or carrier rocket is a rocket designed to carry a payload (spacecraft or satellites) from the Earth's surface to outer space. Most launch vehicles operate from a launch pads, supported by a launch control center and syste ...

, an Ariane 44P rocket, placed ISO successfully into a highly elliptical

geocentric orbit A geocentric orbit or Earth orbit involves any object orbiting Earth, such as the Moon or artificial satellites. In 1997, NASA estimated there were approximately 2,465 artificial satellite payloads orbiting Earth and 6,216 pieces of space debr ...

, completing one revolution around the

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surf ...

every 24 hours. The

primary mirror A primary mirror (or primary) is the principal light-gathering surface (the objective) of a reflecting telescope. Description The primary mirror of a reflecting telescope is a spherical or parabolic shaped disks of polished reflective met ...

of its Ritchey-Chrétien telescope measured 60 cm in diameter and was cooled to 1.7

kelvin The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and ph ...

s by means of

superfluid Superfluidity is the characteristic property of a fluid with zero viscosity which therefore flows without any loss of kinetic energy. When stirred, a superfluid forms vortices that continue to rotate indefinitely. Superfluidity occurs in two ...

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

. The ISO satellite contained four instruments that allowed for imaging and photometry from 2.5 to 240

micrometre The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer ( American spelling), also commonly known as a micron, is a unit of length in the International System of ...

s and

spectroscopy Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter ...

from 2.5 to 196.8 micrometers. ESA and the Infrared Processing and Analysis Center made efforts to improve the data pipelines and specialized software analysis tools to yield the best quality calibration and data reduction methods from the mission. IPAC supports ISO observers and data archive users through in-house visits and workshops.

History and development

In 1983, the US-Dutch-British IRAS inaugurated space-based infrared astronomy by performing the first-ever 'all-sky survey' at infrared

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...

s. The resulting map of the infrared sky pinpointed some 350,000 infrared sources waiting to be explored by IRAS' successors. In 1979, IRAS was in an advanced stage of planning and the expected results from IRAS led to the first proposal for ISO made to ESA in the same year. With the rapid improvements in infrared detector-technology, ISO was to provide detailed observations for some 30,000 infrared sources with much improved

sensitivity Sensitivity may refer to: Science and technology Natural sciences * Sensitivity (physiology), the ability of an organism or organ to respond to external stimuli ** Sensory processing sensitivity in humans * Sensitivity and specificity, statisti ...

and resolution. ISO was to perform 1000 times better in sensitivity and 100 times better in angular resolution at 12 micrometres compared to IRAS. A number of follow-up studies resulted in the selection of ISO as the next installment for the ESA Scientific Programme in 1983. Next came a Call for Experiment and Mission Scientist Proposals to the scientific community, resulting in the selection of the

scientific instrument A scientific instrument is a device or tool used for scientific purposes, including the study of both natural phenomena and theoretical research. History Historically, the definition of a scientific instrument has varied, based on usage, laws, a ...

s in 1985. The four instruments chosen were developed by teams of researchers from France, Germany, the Netherlands and United Kingdom.

Design A design is a plan or specification for the construction of an object or system or for the implementation of an activity or process or the result of that plan or specification in the form of a prototype, product, or process. The verb ''to design'' ...

and development of the

satellite A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioiso ...

started in 1986 with

Aérospatiale Aérospatiale (), sometimes styled Aerospatiale, was a French state-owned aerospace manufacturer that built both civilian and military aircraft, rockets and satellites. It was originally known as Société nationale industrielle aérospatiale ( ...

's space division (currently absorbed into Thales Alenia Space) leading an international consortium of 32 companies responsible for manufacture,

integration Integration may refer to: Biology * Multisensory integration * Path integration * Pre-integration complex, viral genetic material used to insert a viral genome into a host genome *DNA integration, by means of site-specific recombinase technolo ...

and testing of the new satellite. Final assembly took place at the Cannes Mandelieu Space Center.

The satellite

Animation of Infrared Space Observatory's orbit

The basic design of ISO was strongly influenced by that of its immediate predecessor. Like IRAS, ISO was composed of two major components: * Payload module, composed of a large

cryostat A cryostat (from ''cryo'' meaning cold and ''stat'' meaning stable) is a device used to maintain low cryogenic temperatures of samples or devices mounted within the cryostat. Low temperatures may be maintained within a cryostat by using various r ...

holding the telescope and the four scientific instruments. * Service module, supports the activities of the payload module by providing

electrical power Electric power is the rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt, one joule per second. Standard prefixes apply to watts as with other SI units: thousands, millions and billions ...

, thermal control, attitude and orbit control and

telecommunication Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than tha ...

s. The payload module also held a conical sun shade, to prevent stray light from reaching the telescope, and two large star trackers. The latter were part of the Attitude and Orbit Control Subsystem (AOCS) which provided three-axis stabilisation of ISO with a pointing

accuracy Accuracy and precision are two measures of '' observational error''. ''Accuracy'' is how close a given set of measurements (observations or readings) are to their '' true value'', while ''precision'' is how close the measurements are to each ot ...

of one arc second. It consisted of Sun and Earth sensors, the before-mentioned star trackers, a quadrant star sensor on the telescope axis, gyroscopes and reaction wheels. A complementary

reaction control system A reaction control system (RCS) is a spacecraft system that uses thrusters to provide attitude control and translation. Alternatively, reaction wheels are used for attitude control. Use of diverted engine thrust to provide stable attitude con ...

(RCS), using hydrazine

propellant A propellant (or propellent) is a mass that is expelled or expanded in such a way as to create a thrust or other motive force in accordance with Newton's third law of motion, and "propel" a vehicle, projectile, or fluid payload. In vehicles, the ...

, was responsible for orbital direction and finetuning shortly after launch. The complete satellite weighed just under 2500 kg, was 5.3 m high, 3.6 m wide and measured 2.3 m in depth. The service module held all the warm

electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...

, the hydrazine propellant tank and provided up to 600

watt The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James Wa ...

s of electrical power by means of

solar cell A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon.cryostat A cryostat (from ''cryo'' meaning cold and ''stat'' meaning stable) is a device used to maintain low cryogenic temperatures of samples or devices mounted within the cryostat. Low temperatures may be maintained within a cryostat by using various r ...

of the payload module surrounded the telescope and science instrument with a large dewar containing a toroidal tank loaded with 2268

litre The litre (international spelling) or liter (American English spelling) (SI symbols L and l, other symbol used: ℓ) is a metric unit of volume. It is equal to 1 cubic decimetre (dm3), 1000 cubic centimetres (cm3) or 0.001 cubic metre (m3 ...

s of superfluid helium. Cooling by slow evaporation of the helium kept the

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied on ...

of the telescope below 3.4 K and the science instruments below 1.9 K. These very low temperatures were required for the scientific instruments to be sensitive enough to detect the small amount of infrared radiation from cosmic sources. Without this extreme cooling, the telescope and instruments would see only their own intense infrared emissions rather than the faint ones from afar.

Optical telescope

The ISO telescope was mounted on the

center line Center line, centre line or centerline may refer to: Sports * Center line, marked in red on an ice hockey rink * Centre line (football), a set of positions on an Australian rules football field * Centerline, a line that separates the service cour ...

of the dewar, near the bottom-side of the torodial helium tank. It was of the Ritchey-Chrétien type with an effective

entrance pupil In an optical system, the entrance pupil is the optical image of the physical aperture stop, as 'seen' through the front (the object side) of the lens system. The corresponding image of the aperture as seen through the back of the lens system i ...

of 60 cm, a focal length ratio of 15 and a resulting focal length of 900 cm. Very strict control over straylight, particularly that from bright infrared sources outside the telescope's field of view, was necessary to ensure the guaranteed sensitivity of the scientific instruments. A combination of light-tight shields, baffles inside the telescope and the sunshade on top of the cryostat accomplished full protection against straylight. Furthermore, ISO was constrained from observing too close to the Sun, Earth and Moon; all major sources of infrared radiation. ISO always pointed between 60 and 120 degrees away from the Sun and it never pointed closer than 77 degrees to Earth, 24 degrees to the

Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width ...

or closer than 7 degrees to

Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandt ...

. These restrictions meant that at any given time only about 15

percent In mathematics, a percentage (from la, per centum, "by a hundred") is a number or ratio expressed as a fraction of 100. It is often denoted using the percent sign, "%", although the abbreviations "pct.", "pct" and sometimes "pc" are also us ...

of the sky was available to ISO. A pyramid-shaped mirror behind the

of the telescope distributed the infrared light to the four instruments, providing each of them with a 3 arc-minute section of the 20 arc-minute field of view of the telescope. Thus, pointing of a different instrument to the same cosmic object meant repointing the entire ISO satellite. Iso lsw flight spare

Instruments

ISO carried an array of four scientific instruments for observations in the infrared: * Infrared Camera (ISOCAM) – A high-resolution camera covering 2.5 to 17 micrometre wavelength with two different detectors. Like a visible-light camera it takes pictures of astronomical objects, but the image shows what the object looks like in infrared light. * Photo-polarimeter (ISOPHOT) – An instrument designed to measure the polarisation of infrared radiation emitted from an astronomical object. The very broad wavelength range from 2.4 to 240 micrometre allowed this instrument to see the infrared emissions of even the coldest astronomical objects such as interstellar dust clouds * Short Wave Spectrometer (SWS) – A

spectrometer A spectrometer () is a scientific instrument used to separate and measure spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon where th ...

covering the 2.4 to 45 micrometre wavelength. Observations with this instrument provided valuable information about the chemical composition, density and temperature of the universe. * Long Wave Spectrometer (LWS) – A spectrometer covering the 45 to 196.8 micrometre wavelength. All four instruments were mounted directly behind the primary mirror of the telescope, in a circular arrangement, with each instrument taking up an 80

degree Degree may refer to: As a unit of measurement * Degree (angle), a unit of angle measurement ** Degree of geographical latitude ** Degree of geographical longitude * Degree symbol (°), a notation used in science, engineering, and mathemati ...

segment of the cylindrical space. The field of view for each instrument was offset to the central axis of the telescope's field of view. This means that every instrument 'saw' a different portion of the sky at a given moment. In standard operational mode one instrument was in primary operation.

Launch and operations

After a very successful development and integration phase ISO was finally launched into orbit on November 17, 1995, on board an Ariane-44P launch vehicle. Performance of the launch vehicle was very good with the apogee only 43 km lower than expected. ESA's Space Operations Centre in

Darmstadt Darmstadt () is a city in the States of Germany, state of Hesse in Germany, located in the southern part of the Frankfurt Rhine Main Area, Rhine-Main-Area (Frankfurt Metropolitan Region). Darmstadt has around 160,000 inhabitants, making it th ...

in Germany had full control over ISO in the first four days of flight. After early commissioning primary control over ISO was handed over to the Spacecraft Control Centre (SCC) at

Villafranca Villafranca (Basque: ''Alesbes'') is a town and municipality located in the province and the autonomous community (Comunidad Foral) of Navarre, northern Spain , image_flag = Bandera de España.svg , image_coat = Escudo ...

in Spain ( VILSPA) for the remainder of the mission. In the first three weeks after launch the

orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such a ...

was fine-tuned and all satellite systems were activated and tested. Cool-down of the cryostat proved to be more efficient than previously calculated, so the anticipated mission length was extended to 24 months. Between November 21 and November 26 all four science instruments were switched on and thoroughly checked out. Between December 9, 1995, and February 3, 1996, the 'Performance Verification Phase' took place, dedicated to commissioning all instruments and fixing problems. Routine observations started from February 4, 1996, and lasted until the last helium coolant depleted on April 8, 1998. The perigee of ISO's orbit lay well inside the Van Allen radiation belt, forcing the science instruments to be shut down for seven hours during each pass through the radiation belt. Thus, 17 hours in each orbit remained for scientific observation. A typical 24-hour orbit of ISO can be broken down into six phases: * Acquisition-of-Signal (AOS) by the primary Mission Control Center VILSPA in Spain and activation of the satellite. * Science operations during the VILSPA window, starting four hours after perigee, and lasting for up to nine hours. * Handover of operations to the secondary mission control center at Goldstone at apogee. During this 15 minute periode the science instruments could not be operated. * Science operations during the Goldstone window, lasting up to eight hours. * De-activation of the instruments upon approach of the Van Allen radiation belt and Loss-of-Signal (LOS) at Goldstone. * Perigee passage. Contrary to IRAS, no science data was recorded on-board ISO for later transmission to the ground. All data, both science data and housekeeping data were transmitted to the ground in real-time. The perigee point of ISO's orbit was below the radio horizon of the mission control centers at both VILSPA and Goldstone, thus forcing the science instruments to be switched off at perigee.

End of mission

At 07:00 UTC on April 8, 1998 flight controllers at VILSPA noticed a rise in temperature of the telescope. This was a clear sign that the load of superfluid helium coolant had depleted. At 23:07 UTC the same day, the temperature of the science instruments had risen above 4.2 K and science observations were ceased. A few detectors in the SWS instrument were capable of making observations at higher temperatures and remained in use for another 150 hours to make detailed measurements of an additional 300

star A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Earth make ...

s. In the month following depletion of coolant the 'Technology Test Phase' (TTP) was initiated to test several elements of the satellite in off-nominal conditions. After completion of TTP, the perigee of ISO's orbit was lowered sufficiently enough to ensure ISO will burn up in Earth's atmosphere in 20 to 30 years after shutdown. ISO was then permanently switched off on May 16, 1998, at 12:00 UTC.

Results

On average, ISO performed 45 observations in each 24-hour orbit. Throughout its

lifetime Lifetime may refer to: * Life expectancy, the length of time a person is expected to remain alive Arts, entertainment, and media Music * Lifetime (band), a rock band from New Jersey * ''Life Time'' (Rollins Band album), by Rollins Band * ...

of over 900 orbits ISO performed more than 26,000 successful scientific observations. The huge amounts of scientific data generated by ISO was subject to extensive archiving activities up to 2006. The full data-set has been available to the scientific community since 1998 and many discoveries have been made, with probably many more still to come: * ISO detected the presence of water vapour in starforming regions, in the vicinity of stars at the end of their lives, in sources very close to the Galactic Centre, in the atmospheres of

planet A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a ...

s in the

Solar System The Solar System Capitalization 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 "Solar ...

and in the Orion Nebula. * Planet formation was detected around old, dying stars. This discovery contradicted theories that planet formation was only possible around young stars. * Hydrogen fluoride gas was for the first time detected in interstellar gas clouds. * The first detection of the earliest stages of stellar formation. The pre-stellar core L1689B was found and studied in great detail with ISO's LWS instrument. * ISO discovered large amounts of cosmic dust in the previously thought empty space between galaxies. * Observations of the most-luminous object in the universe,

Arp 220 Arp 220 is the result of a collision between two galaxies which are now in the process of merging. It is the 220th object in Halton Arp's ''Atlas of Peculiar Galaxies''. Features Arp 220 is the closest Ultraluminous Infrared Galaxy (ULIRG) to ...

, revealed that the source for its enormous emission of infrared radiation is an outburst of star formation. * Observations with the LWS instrument confirmed the previous discovery by IRAS of large cloud-like structures of very cold

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...

s radiating primarily in the infrared. These infrared cirrus clouds affect the energy balance of the entire universe, acting as a kind of galactic refrigerator. * ISO searched for, and found several

protoplanetary disk A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may also be considered an accretion disk for the star itself, ...

s: rings or disks of material around stars which are considered to be the first stage of planet formation. * ISO pointed its sensitive instruments on several of the planets in the Solar System to determine the chemical composition of their atmospheres.