What is ISO?

1. What is ISO?

The European Space Agency's (ESA) Infrared Space Observatory (ISO) is an astronomical satellite that was operational between November 1995 and May 1998. It operated at wavelengths from 2.5 to 240 microns, in the infrared range of the electromagnetic spectrum. Because the atmosphere acts as an 'umbrella' for most infrared wavelengths -preventing them from reaching the ground- a space telescope is needed to detect this kind of radiation invisible to the human eye and to optical telescopes. The satellite essentially consists of: a large liquid-helium cryostat; a telescope with a 60-cm diameter primary mirror; four scientific instruments and the service module. The overall dimensions of ISO are:

  • Height: 5.3 m
  • Width: 3.6 m
  • Depth: 2.8 m
  • Its launch mass was 2400 kg.

2. ISO's predecessors. What are ISO's advantages?

In 1983 the US-Dutch-British IRAS satellite inaugurated infrared space astronomy by mapping 250,000 cosmic infrared sources and large areas of extended emission. But that was only the beginning. ISO's detectors, with an enhanced sensitivity and resolution (by 1000 in sensitivity and by 100 in angular resolution at 12 microns), have allowed a much closer look, a much more detailed perception of the 'infrared scenery'. Thus, ISO has provided astronomers with a unique facility to see both familiar objects in an unusual way and objects that are invisible at other wavelengths.

3. Why observe in the infrared?

Infrared radiation is primarily 'heat', or thermal radiation. Even objects that we think of as being very cold, such as an ice cube, emit infrared radiation. For this reason ISO, operating at wavelengths from 2.5 to 240 microns, could observe astronomical objects that remain hidden for optical telescopes, such as cool objects that are unable to emit in visible light. Opaque objects, those surrounded by clouds of dust, are another specialty of ISO because the longer IR wavelengths can penetrate the dust, allowing us to see deeper into such clouds.

4. Who developed ISO?

The scientific instruments were developed by multinational teams, with leaders in France, Germany, the Netherlands and the United Kingdom. The satellite was developed, manufactured, integrated and tested by an industrial consortium made up of 32 companies, mostly from Europe, headed by Aérospatiale, France.

5. The instruments and their function:

The single 0.6-metre telescope in ISO fed infrared beams via a pyramidal mirror to four instruments. The fields of view and the selection of wavelengths are varied, to suit the nature of the object examined. The four instruments are:

The Infrared Camera (ISOCAM), covering the 2.5-17 micron band with two different detectors. It can be compared to a normal photo camera, taking pictures of the 'infrared face' of astronomical objects at a high resolution (so as to distinguish very fine details). ISOCAM Principal Investigator: Catherine Cesarsky, CEN-SACLAY, France.

The photo-polarimeter (ISOPHOT), designed to detect the amount of infrared radiation emitted by an astronomical object. The broad range of wavelengths at which ISOPHOT operated (between 2.5 and 240 microns) allowed it to 'see' objects as cool as the clouds of dust lying among stars and galaxies, whose temperature may be just a few degrees above absolute zero (minus 273 degrees C). ISOPHOT Principal Investigator: D. Lemke, MPI für Astronomie, Heidelberg, Germany.

The Short-Wave Spectrometer (SWS), covering the 2.4 to 45 micron band. It has provided valuable information about the little known chemistry of the Universe, since many molecules emit copiously in the infrared. Moreover, SWS has been able to find out the physical conditions of those chemical constituents, such as temperature or density. SWS Principal Investigator: Th. de Graauw, Lab. for Space Research, Groningen, The Netherlands.

The Long-Wave Spectrometer (LWS), able to operate at the 45 to 196.8 micron band. LWS has focused on cooler objects than SWS. It is especially useful studying the physical condition in very cold dust clouds in the space between stars. LWS Principal Investigator: P. E. Clegg, Queen Mary and Westfield College, London, United Kingdom.

6. ISO's lifetime

ISO was successfully launched by an Ariane 44P launcher from Europe's spaceport in Kourou, on 17 November 1995. Initially it was supposed to be operational for 20 months, but thanks to meticulous engineering and some good fortune the satellite's working life was stretched to more than 28 months: ISO unveiled the infrared universe until May 1998. Observing the cool universe requires cool instruments, ones working at temperatures close to absolute zero, minus 273 degrees C. Keeping the temperature this low is the task of the large liquid-helium cryostat on board ISO, filled before launch with 2286 litres of superfluid helium. This cryostat has made ISO one of the coolest objects in the universe. ISO's lifetime was limited by its helium supply; hence nearly all observations had to stop when this coolant liquid got depleted on April 8.

However, ISO's farewell included a further last gift for the astronomers. A few of the detectors in the Short Wavelength Spectrometer (SWS), one of the four instruments on-board the satellite, could still be used up to May 10, long after the exhaustion of the liquid helium.

7. Ground stations tracking ISO

The Science Operations Centre at the ESA's Satellite Tracking Station at Villafranca (Spain) has been responsible for the control of the satellite. This is also where observations were scheduled. However, for scientific use ISO needed to be in continuous contact with a ground station. NASA's station at Goldstone (US) tracked ISO when it was obscured by the Earth from Villafranca.

8. ISO orbit

ISO's highly-elliptical orbit had a perigee at around 1000 km; an apogee at 70500 km; and a period of almost 24 hours. The lowest parts of the orbit lay inside the Earth's van Allen belts of trapped electrons and protons. Inside these regions ISO's detectors were scientifically unusable due to effects caused by radiation impacts. ISO spent almost 17 hours per day outside the radiation belts and during this time all detectors could be operated.

9. How many observations has ISO been performing per day?

On average, ISO performed 45 observations per revolution (a period of almost 24 hours). Througout all of its lifetime -more than 900 revolutions- ISO completed successfully well over 26.000 scientific observations.

10. How many scientific teams have made observations with ISO?

Astronomers from all over the world asked for observing time on ISO. Their proposals went through a peer review process during which a time allocation commitee composed of independent outstanding scientists decided on the scientific merit of the proposed programmes and observations. Only proposals of the highest quality were accepted. On the whole, about one thousand proposals were accepted (each one consisting on average of 50 observations). Over 500 principal investigators have made observations with ISO.

11. Some questions ISO is helping to answer:

The birth and death of the stars

As if stars wished to keep their privacy during these critical stages of their lives, their births and deaths usually occur within thick and opaque clouds of dust (the nebulae). But infrared beams manage to escape these dusty regions, enabling ISO to see them.

The origin of planets

ISO searched for discs -rings- of matter around stars, which are considered to be the first stage of planet formation -hence they are called 'protoplanetary discs'.

The chemistry of the Universe

When scrutinizing selected objects ISO could detect the emission or absorption of infrared rays at particular wavelengths, or "lines" in a spectrum, revealing the presence of identifiable atoms, molecules and solids. The Short Wavelength Specrometer and the Long Wavelength Spectrometer, along with the photometer ISOPHOT and the camera ISOCAM, provided detailed chemical diagnoses of celestial objects.

What are the planets' atmospheres made of?

ISO spectrometers have also concentrated on our closest neighbours, the planets in the Solar System. In several cases -such as in the giant planets and even in Mars- the study of the atmospheres' chemical constituents has produced unexpected surprises.

The evolving galaxies

Having ISO in space gave astronomers special opportunities for the study of the history of the galaxies. By detecting infrared wavelengths that are hard to observe from the Earth, ISO has picked out very clearly the galaxies that are evolving most rapidly via periods of intensive starmaking, and has been able to detect infrared galaxies powered by active galactic nuclei. ISO has observed many galaxies at about half the age of the Universe by staring through a window in the dust of our own Milky Way Galaxy, called the Lockman Hole. In those galaxies that glow most brightly in the infrared, astronomers suspect that frantic star formation is in progress, in episodes called starbursts. In nearer galaxies, ISO's astronomers can relate strong infrared emissions to collisions and to violent eruptions in the galactic cores, which have punctuated the evolution of the galaxies.

The farthest known galaxy observed by ISO is a quasar called BR 1202-0725, dating from a time when the Universe was less than one-tenth of its present age. Already it is dusty, indicating star birth and death has occured by this early stage.

What is a comet made of and how does it behave on approaching the sun?

The infrared wavelength region is useful for investigating comets because they are cold -the thermal emission of the comet nucleus and dusty atmosphere peaks at infrared wavelengths. In this emission region the chemical composition of the comet can be studied: the volatile molecular species, sublimated from cometary nucleus ices, can be identified through their fundamental bands of vibration, and are therefore the subject for high resolution spectroscopic observations.

Infrared observations from the ground are limited to a few atmospheric windows, and this is an annoying restriction for investigating changes in the comet on its approaching to the sun. IR spectra of comets have been obtained for Comet Halley, but they were limited in spectral coverage, resolution or both.