Super-Cool Satellite Unlocks The Hidden Universe

ISO, the Infrared Space Observatory, concludes its successful mission

European scientists in London, Tuesday 7 April 1998, described the new view of the Universe that has come from the work of the European Space AgencyŽs infrared space observatory (ISO). Launched in 1995, ISO has uncovered stars, galaxies, and black holes that astronomers had been unable to see before and revealed water in the deserts of space. Because ISO operates in the infrared, it can see through the dust clouds which obscure the vision of optical telescopes like Hubble.

ISOŽs most recently discovered water vapour around the Saturn moon Titan. This is relevant to the Cassini-Huygens mission to Saturn, launched last year, which will parachute into TitanŽs atmosphere to investigate itŽs primitive atmosphere which is thought to be like that of young Earth.

Another amazing discovery made possible by ISO concerns Centurus A, a flat anonymous dusty band in visible light which is one of the UniverseŽs strongest sources of cosmic radio waves. ISO shows us that it is a galactic traffic accident with two galaxies colliding. At the centre of one of the galaxies is a giant black hole producing twin jets of electrons.

Roger Bonnet, Director of Science at the European Space Agency (ESA), used the opportunity to make a point about ESAŽs ability to do innovative space research. He said that ESA was committed to doing only such projects which were unique. Since NASA was already doing everything that seems possible, ESA consequently could only try to do the impossible.

But besides such joking remarks the moment of triumph was also a moment of sadness, since ISO will soon cease to be functionable. Observing the cool universe requires cool instruments, working at temperatures close to absolute zero, minus 273 degrees C. Keeping this low temperatures 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 is limited by its helium supply and ISO is running out of helium right now, in April 98.

However, ISO is not just a scientific but also a technical success. The designed operation time of 18 months was expanded to 28 months. During the extra time the count of ISOŽs observations of cosmic objects has risen from 16.000 to about 26.000.

Among the benefits of ISOŽs longevity has been the chance to examine an important region in the sky, in and around the constellation of Orion. Behind the cosmic dust of The Horsehead nebula in Orion previously unseen young stars have been discovered.

The primary source of infrared radiation is '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, which operates at wavelengths from 2.5 to 240 microns, can see astronomical objects that remain hidden for optical telescopes: cool objects that are unable to emit in visible light; opaque objects, those surrounded by opaque clouds of dust, like the Horsehead nebula.

The instruments and their function

The scientific instruments were developed by multinational teams, with leaders in France, Germany, the Netherlands and the United Kingdom.

A single 0.6-metre telescope in ISO feeds infrared beams via a pyramidal mirror to four instruments. The fields of view and the selection of wavelengths can vary to suit the nature of the object examined.

The Infrared Camera (ISOCAM), covers 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. (CEN-SACLAY, France)

The photo-polarimeter (ISOPHOT), detects the amount of radiation emitted by an astronomical object. The broad range of wavelengths at which ISOPHOT operates (between 2.5 and 240 microns) allows 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). (MPI für Astronomie, Heidelberg, Germany)

The Short-Wave Spectrometer (SWS), operates at the 2.4 to 45 micron band. One of its tasks is to distinguish chemical components of the target (most molecules' peak of emission is in the infrared), as well as to provide valuable information about the physical conditions at which these constituents are (such as temperature or density). (Lab. for Space Research, Groningen, The Netherlands)

The Long-Wave Spectrometer (LWS), operates at the 45 to 196.8 micron band. LWS can study cooler objects than SWS. And is especially useful to study the physical condition in very cold dust clouds in the space between stars. (Queen Mary and Westfield College, London, United Kingdom)

ISO is controlled from the Science Operations Center, at the ESA's Satellite Tracking Station at Villafranca (Spain). This is also where observations are scheduled. However, for scientific use ISO needs to be in continuous contact with a ground station. NASA's station at Goldstone (US) tracks ISO when it is obscured by the Earth from Villafranca.

The Future of Infrared Space Telescopes

The discoveries presented so far represent only a small part of discoveries still to be made. Terrabytes of data are waiting now at the Science Operation Center at Villafranca to be archived and made available for the scientific community. More than 1000 astronomers working on ca. 1200 research projects will analyze this huge data pool in the next years.

ISO will not have extinguished all of its fuel by the time its operating life had finished. Some of the fuel will be saved in order to lead the satellite to a much lower orbit, to avoid ISO becoming a piece of useless and dangerous space debris. Early in the 21st century ISO will be so close to Earth that the upper part of the atmosphere will burn it up.

More often than not, the number of questions a scientific mission solves is not as good an indicator of its success as the number of new questions it generates. And ISO has been a succesful mission from every point of view: no doubt more space telescopes are needed to do more infrared astronomy.

Thus, NASA's infrared space telescope called Space Infrared Telescope Facility (SIRTF) is already scheduled for launch in December 2001. Further in time ESA is preparing the Far Infrared and Submillimiter Telescope (FIRST), a space telescope for far infrared and submillimetre astronomy due to be launched in 2005.

This will allow insights into deeper spaces of the universe as well as into the earlier history of the universe.

For up-to-date information, many more images and even video clips please refer to ISO's Homepage on the Web.