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1 December 2003
ISO was the world's first space observatory able to see the sky in infrared light. This pioneering position allowed ISO to unveil a new 'face' of the Universe, its infrared face. This has changed radically our view of the Universe. ISO operated between November 1995 and May 1998, and made 30 000 scientific observations.

Infrared light is invisible to the human eye and to optical telescopes, which are sensitive to the same light the human eye can see. Moreover, the Earth's atmosphere prevents most infrared radiation from reaching the ground, so a space telescope is needed to detect it. For these reasons, the infrared universe was very poorly known until ISO. All objects emit infrared radiation. The sky therefore looks very different when seen in the infrared. ISO could see objects and processes that are too cold or too dusty, or both, to appear when you look through optical telescopes, such as clouds in which stars are being born.

ISO's results impact most fields of astronomical research, from comets to cosmology. One of ISO's greatest achievements was the discovery that the water molecule is very common in many regions on the Universe, even in distant galaxies. Astronomers from all over the world continue to make frequent discoveries by studing the observations stored in the ISO data archive.
ISO was designed to unveil the 'cold and dusty' Universe. Astronomers expected ISO observations to help understand phenomena that happen at very low temperatures, or within clouds of dust. Some of these phenomena are:

  • The birth and death of the stars
  • The formation of planetary systems
  • The evolution of galaxies
  • The rate at which stars have been forming throughout the Universe's history
  • The 'chemical evolution' of the Universe (because many molecules emit infrared radiation)

Scientific highlights to date

ISO has more than fulfilled scientists' expectations. More than 1000 papers based on ISO observations have been published so far in refereed scientific journals, and the publishing rate is not decreasing. Some of the most outstanding ISO results are:

  • ISO peered into clouds of dust and gas in which stars are being born, and observed for the first time the earliest steps of star-making. ISO discovered, for example, that stars begin to form at temperatures as low as -250°C or less.
  • ISO analysed the chemical composition of the cosmic dust, thereby opening up a new field of research, 'astromineralogy'. Scientists were able to follow how dust formed from the massive 'dust factories', that is, old stars, to the regions where new planetary systems are born. Most young stars are surrounded by discs of dust that could harbour planets.
  • ISO demonstrated that the water molecule is very common in the Universe: it was detected at Mars, Titan, the giant planets, comets, close to stars, in cold and open interstellar space and in distant galaxies. Also, ISO discovered that complex organic molecules, such as benzene, form easily in the neighbourhood of some stars.
  • The first infrared observations of the sky revealed a new type of galaxy that appeared extremely bright in the infrared range but very faint in optical light. ISO found out that the powerful emission of many of these objects is due to very active star formation. This happens within dust clouds and therefore looks opaque to optical telescopes.

About 615 million Euros. This includes all aspects of the mission (spacecraft, launch, operations, and the current archiving phase until December 2006), except the scientific payload provided by the research institutes that was funded by their national governments.
ISO was launched by an Ariane-44P launcher from Europe's spaceport in Kourou, French Guiana, on 17 November 1995.
ISO orbited the Earth. Its highly elliptical orbit had a perigee at around 1000 kilometres; an apogee at 70 500 kilometres; and a period of almost 24 hours.
Mission lifetime
ISO's lifetime had been estimated initially as 20 months, but it was extended to more than 28 months (until May 1998). ISO's lifetime was limited by its liquid helium supply. The helium was used to keep ISO detectors extremely cold, at temperatures close to absolute zero (- 273°C). If the detectors had been warmer, their own infrared emission would have made it impossible to detect the infrared light from cold astronomical objects. When the liquid helium ran out, ISO had to stop observing.


ISO essentially consisted of: a large liquid-helium tank (a cryostat); a telescope with a 60-centimetre diameter primary mirror; four scientific instruments, and the service module.


2400 kilograms total launch mass.


Height: 5.3 metres Width: 3.6 metres Depth: 2.8 metres

Industrial involvement

The ISO satellite was developed, manufactured, integrated, and tested by an industrial consortium made up of 32 companies, mostly from Europe, headed by Aérospatiale, France.

What's on board?
ISO's four scientific instruments were developed by multinational teams with leaders in France, Germany, the Netherlands, and the United Kingdom.

Infrared Camera - ISOCAM Pictured the 'infrared face' of astronomical objects at a high resolution. It covered the 2.5-17 micron band with two different detectors.

Principal Investigators: C. Cesarsky, CEN-SACLAY, France.

Photo-polarimeter - ISOPHOT Detected the amount of infrared radiation emitted by an astronomical object. Covered a broad range of wavelengths: between 2.5 and 240 micron.

Principal Investigators: D. Lemke, MPI für Astronomie, Heidelberg, Germany.

Short-Wave Spectrometer - SWS Detected many molecules in space and revealed something about their physical conditions, such as temperature or density. Covered the 2.4 to 45 micron band.

Principal Investigators: Th. de Graauw, Lab. for Space Research, Groningen, The Netherlands.

Long-Wave Spectrometer - LWS Observed cooler objects than SWS. It was especially useful studying the physical condition in very cold dust clouds in the space between stars. Covered the 43 to 197 micron band.

Principal Investigator: P. E. Clegg, Queen Mary and Westfield College, London, United Kingdom.

ISO Science Operations Centre at ESA's Satellite Tracking Station at Villafranca (Spain) was responsible for the control of the satellite. Observations were also scheduled here. NASA's station at Goldstone (United States) tracked ISO when it was obscured by the Earth from Villafranca. On average, ISO performed 45 observations per revolution (a period of almost 24 hours).

ESA Mission Manager: Martin Kessler
ESA Project Scientist: Alberto Salama

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