ISO, unique explorer of the invisible cool universe is ready for its mission

That mission is now set to start on 8 November. Rising into the Kourou sky, launched by Ariane, will be a satellite keenly awaited by astronomers wound the world: ISO, the first real infrared space observatory, built and launched under the responsibility of ESA. its task will be long-duration observation of celestial radiation sources, studying them with unparalleled sensitivity and precision. It will view them in the invisible and cool light of infrared radiation, as yet very largely unexplored. ISO will provide an entirely fresh perspective on the universe. This should provide a major and significant boost to scientists working in many areas of astrophysics, from nearby planets to the most distant quasars, taking in star formation, the dark matter of the universe and superluminous galaxies.

As Roger Bonnet, director of ESA's science programme, explains: "The ISO launch will be the culmination of twelve years of intensive effort to build the most powerful and precise infrared space telescope to date. We fully expect astronomers in Europe, and all over the world, to be rewarded with unprecedented insights into the particularly rich and fertile sources of the universe hidden in the infrared."

For the mission looks set to mark a new step forward for astronomy. To date, infrared astronomy has always been hampered by two major difficulties.

The first is that infrared radiation is thermal, calorific. This means that any BODY which is not absolutely cold naturally emits this type of "heat" or "hidden" radiation, Consequently, at the corresponding wavelengths, all telescopes, detectors and the Earth's atmosphere appear to shine spontaneously, Infrared observation is therefore tantamount to attempting to observe the stars in broad daylight using a telescope a million times more luminous than the stars themselves.

Moreover, the atmosphere is partly or totally opaque to infrared light. Hence the great attraction of conducting such observations from space, above the Earth's atmospheric layers. using a very low-temperature telescope.

The first - and, so far, truly - Satellite actually to have done so is the Infrared Astronomical Satellite (IRAS) - a joint UK/Dutch/US mission - which surveyed and catalogued the sky far ten months in 1983. A corner of the curtain was thus lifted, The scene was set. Now, the task is to observe each character, each object.

And here, infrared astronomy is hampered by a second major difficulty: the very detection of light. For while infrared radiation was discovered back in 1800 (in the solar spectrum). infrared-sensitive photoelectric detectors did not materialise until the Second World War, IRAS itself had to make do without the multi-dimensional set-ups that would have allowed it to record true images. It had to build up its images dot by dot, line by line.

One of ISO's major contributions will be the first in-orbit use of just such arrays of detectors, which can be compared to the CCD optics of video cameras now available to the general public. ISO will therefore be able to record very long photographic exposures and provide very detailed high- definition images. It will be able to observe a given object for up to ten hours continuously, exceeding IRAS performance with a thousand-fold increase in sensitivity and resolution ten times higher.

As the project managers maintain, "with its sensitivity to thermal radiation and its sharp images, the space, telescope should be able to detect an ice-cold object the size of human being 100 km away "

Of course, the technical development and technology for so innovative and powerful an instrument as this have thrown up enormous challenges. Hans Steinz, ESA project manager, explains; "This is the first satellite of its kind to be built in Europe and the work has drown an the efforts of no fewer than thirty-five highly specialised firms." The prime challenge was to develop the cryogenic systems, which now make ISO akin to a giant thermos flask filled with 2100 litres of superfluid helium chilled to -271^°C (1.8^° above absolute zero).

Another challenge was the telescope's ultralight gold-coated quartz mirror. The finish required for this centrepiece of the observatory is such that, if its 60 cm diameter were artificially expanded to the size of the Earth's diameter, the residual "bumps" and "dips" of the reflecting surface would be no more than one metre up or down, the size of a child.

All these problems have left the development project two years behind schedule - which is not in fact bad going, given its scope and complexity.

Now, however, all the problems have been overcome. The satellites capabilities correspond fully to expectations and since late June ISO has been in Kourou at the Guiana Space Centre, where it is undergoing all the necessary pre-launch tests.

The ISO spacecraft takes the form of a white cylinder 3.5 metres in diameter and 5.3 metres long. Inside are the actual telescope, the four scientific instruments, the electronics, power and radio communication systems. On the side, wide solar panels supply the 600 watts of electrical power and at the same time serve as a baffle shielding the satellite from solar radiation.

The spacecraft is fully covered with highly effective thermal insolation. if - purely hypothetically - its cryostat were filled with boiling water instead of liquid helium, it would take six years for ISO to drop to ambient temperature. This insulation is vital for minimising the space telescope's sensitivity to thermal disturbance caused by solar and terrestrial radiation.

The four focal-plane instruments mounted behind the telescope's mirror represent the scientific heart of the satellite. This set of instruments, built by the principal investigators and industrial consortia in ESA Member States, will provide measurements of unprecedented sensitivity and precision over a very wide wavelength range: 2.5 to 250 microns, of which the 120 to 250 micron region is as yet entirely unexplored.

The Isophot photometer, built under the supervision of the Max Planck Institute, Germany, is the instrument providing the most far-ranging wavelength coverage. When operating at very long wavelengths, a region undetected to date, it should reveal extremely cold stars and cosmic matter. Isophot measures the intensity and spatial distribution of light and the spectral components in the near infrared.

The Isocam camera, responsibility for which lies with the Service d'Astrophysique de Saclay, France, will take photographs in the 2.5 to 17 micron range with two arrays, each of 1024 individual infrared detectors (32 x 32 pixel). Some twenty filters select the infrared spectral range of the observations ("colours" ) and four optical lenses allow various magnification factors (from 1 to 8). Depending on the lens chosen, the finest detail observable will be from 1.5 to 12 arcsec per pixel, i.e. 1/1200 to 1/150 times the full diameter of the moon.

The SWS and LWS spectrometers, built under the supervision of the Groningen Laboratory for Space Research (Netherlands) and the Queen Mary and Westfield College, London (GB), respectively, form a single unit that will analyse the composition of the radiation spectrum from 2.5 to 200 microns. They will thereby provide access to an extraordinarily extensive range of physical phenomena which can then be studied with very great sharpness and precision (resolving power: 50 to 30 000).

At Kourou final preparations are currently underway: electromechanical checks, gyroscope alignment, fitting of the thermal protection, and helium and fuel filling. ISO will then be placed in the fairing of its Ariane 44P launcher in preparation for the countdown.

The launch is scheduled for 8 November 1995 and could in fact take place as late as 21 February 1996. ISO will be lifted by Ariane into a very elongated 24-hour orbit with a perigee of 1000 km and an apogee of 70 000 km. This elliptical trajectory will yield very high-quality scientific data for the 16 hours a day that the satellite is outside the Van Allen belts, whose radiation produces stray signals that hinder measurement-taking.

Ground control and radio tracking operations will be carried by a hundred or so experts working in shifts round the clock at ESA's Villafranca station near Madrid. Reflecting the project's international dimension, Japanese scientists will be involved in these operations. The NASA station at Goldstone, California, will relay communications when the satellite is out of Europe's view. Full coverage will thereby be provided with real-time links throughout the 18-month mission.

The Villafranca station will receive 170 million bytes of data daily, and ultimately 90 billion units of information over the total duration of the mission - enough to fill 10 million typed pages of A4 paper which if placed end-to-end could cover the 7000 km between Paris and Miami.

Observing time is already oversubscribed. Even before the satellite has actually come into service, ESA has received proposals from European, American and Japanese scientists amounting to 60 000 requests for utilisation - demand four times in excess of the possible supply under this mission.

Martin Kessler, ESA's project scientist, explains: "ISO will explore a cool, hidden universe inaccessible to conventional optical telescopes. " For example, astronomers will be able to observe lower-temperature stars not observable in the visible or stars hidden by clouds of gas and dust that only infrared light can penetrate. "The telescope will be able to pick out new-born cold stars taking shape among the bright stars of the galaxy. Intensive study of such stars will enhance our understanding of the overall process whereby they - and hence the sun itself - came into existence. "

To begin with, ISO will take the first cool look at nearby planets, satellites, asteroids and comets in our solar. system. It will pay particular attention to Titan, Saturn's intriguing hazy moon (due to be explored by ESA's Huygens probe in 2004). Astronomers suspect that its atmosphere is host to complex organic chemical processes which may be similar to those allowing the emergence of life on Earth. ISO will take precise measurements of the composition of this atmosphere and ascertain the abundance of its minority chemical species and variation with altitude, well before Huygens actually delves into Titan's gassy surroundings and probes them directly.

Is there a similar planetary system elsewhere in the galaxy? The answer to this huge question has a key bearing on any possibility of detecting life forms outside the solar system. ISO will endeavour to provide an answer by examining disks of dust around several hundred stars of varying types, near and far, young and evolved. It is within just such disks that the planets of our solar system are thought to have formed; indeed, the IRAS mission revealed similar structures around ten or so nearby stars, including Vega and Beta Pictoris. With one thousand-fold greater sensitivity, ISO should add further such disks to the catalogue. It will study their mass, chemical composition, dimensions and evolution over time. Scientists will thereby obtain further information on the feasibility of the formation - and very existence - of planetoid bodies around stars in the Milky Way.

Other prime targets for ISO are outer galaxies. Astronomers hope to improve their understanding of how these come into existence and produce their stars. Observation will concentrate on a particularly intriguing category of objects: ultraluminous galaxies, which emit twenty times more radiation in the infrared than in visible light and, overall, ten times more energy than ordinary galaxies. The source of energy feeding these cosmic beacons still remains a mystery: is it some gigantic black hole or sudden burst of star formation? Through detailed examination of the light emitted by such galaxies, ISO will test out these two competing theories and maybe help choose between them.

ISO will also spend over a hundred hours investigating the much-debated and mysterious dark matter of the universe. Detected by indirect methods, such matter appears to account for 90 to 99% of the mass of the universe yet stubbornly refuses to allow itself to be observed in visible light. As a result, its composition and physical properties remain a complete mystery. ISO will try to detect dark matter in the infrared, where there is the greatest chance of observing the "brown dwarfs" in our galaxy or the cool clouds of hydrogen molecules thought to be found on the fringes of outer galaxies. It could well make a decisive contribution to the debate concerning the composition and total mass of the universe.

With its ability to pick up hitherto unobserved very long wavelength radiation, ISO is in effect an amazing time machine. With its great sensitivity, it will observe increasingly distant objects from the correspondingly distant past, whose light - in travelling across an expanding universe - has been shifted into the red -and infrared regions.

Astronomers thus hope to gain access, at the farthest reaches of observable space, to galaxies existing almost eight billion years ago when the universe was only a third of its present age. If such stars really existed, these could be the very first to have formed in the universe and the most distant identifiable ancestors of our galaxy.