A long, cool look into deep space

The birthplaces of planets and colliding galaxies will be the focus for a telescope launched this week.
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Beyond the confines of our little solar system swirl huge clouds of dust and gas. These are the stellar nurseries of our galaxy where young stars condense out of the nebulae. Stellar parturition has always been a secret process: these clouds obstruct the transmission of visible light. The heat radiation (infrared "light") is transmitted but, alas, that is absorbed by our own planet's atmosphere.

On Saturday this will all change when a European Ariane rocket takes off from French Guiana. It will carry into orbit an infrared telescope sensitive and powerful enough to pick up the heat radiated by a snowman 100 kilometres away.

But the "eye" of the European Space Agency's Infrared Space Observatory (ISO) will range across our own solar system as well as out into deep space. Astronomers hope to learn more about the atmospheres of the giant gas planets such as Jupiter and Saturn, as well as Saturn's large moon, Titan, and about the dirty snowballs that we see as comets when they approach the Sun, and about the bands of interplanetary dust in the asteroid belt between Mars and Jupiter.

ISO will also be taking a long look at stars cooler than our Sun. These range from red giants millions of kilometres across to elusive brown dwarfs - star-like objects too small for nuclear reactions to take place in their interiors. Some astronomers believe they may be common throughout the universe and may account for much of the invisible dark matter that seems to influence everything around it.

Until now our knowledge of the infrared sky has largely been based on a 12-year-old survey conducted by a US-Dutch-British satellite known as IRAS. Over a 10-month operational lifetime, this spectacularly successful mission completed the first detailed map of the infrared universe, identifying more than 25,000 sources. But "there is an enormous amount left to be found out", says Professor Michael Rowan-Robinson, of Imperial College, London. "IRAS was a fairly crude instrument with just four wavelength bands and not much spatial resolution. ISO has cameras and spectrometers which cover a much greater range. In some wavelengths, ISO will be 10,000 times more sensitive, and there will be an improvement in spectral resolution of at least 1,000 times."

To achieve such sensitivity the instruments have to be supercooled so the satellite's own heat does not distort readings. This is achieved by boiling off 2,100 litres of liquid helium from a tank chilled to-271C (just above absolute zero). The supply of helium is expected to run out after about 18 months, so this sets the limit on the observatory's active life.

Further protection is provided by thick blankets of insulating material and by using the solar panel, which produces electricity, as a sunscreen. Another vital precaution is a slanting sunshield on the telescope. Accidental exposure to heat from the Sun, the Earth or even the Moon would cause the telescope to overheat and might irreparably damage its detectors.

The observatory's gold-coated quartz mirror is a miracle of engineering. If its diameter was expanded from the real figure of 60cm to the size of the Earth, its surface would go up and down by no more than one metre. This remarkably smooth reflecting surface will send the incoming radiation to four instruments. Two spectrometers will split the incoming infrared light to reveal the nature of the atoms and molecules in the source region. A photometer will measure the intensity of the radiation while a camera will take the most detailed infrared pictures yet obtained.

The UK has been heavily involved in this project, helping to develop the camera and photometer, and leading the design team for one of the spectrometers. With astronomers throughout Europe queueing up to make full use of ISO's limited lifespan, UK scientists have been fortunate to obtain 20 per cent of the observing time available.

"We have people interested in the whole gamut of astronomy, from planets to galaxies," says Professor Peter Clegg, principal investigator for the long wavelength spectrometer and head of a consortium which includes scientists from the UK, France, Italy, the United States and Canada.

One of the main discoveries of IRAS was the existence of discs of dust and gas surrounding nearby stars. Many of these may be the birthplaces of new planets, just as our Sun and its retinue of planets condensed from a similar disc five billion years ago. Although ISO does not have the capability directly to image planets hidden by the dust, it should reveal new information on the sizes and velocities of the particles which make up these clouds.

Beyond the Milky Way are the billions of huge star systems known as galaxies. Most of them are powerful emitters at infrared wavelengths: more than 90 per cent of the energy output of many galaxies is in this part of the spectrum. Astronomers believe this bias is the result of large bursts of star formation, often through two galaxies colliding or merging.

In the nearer galaxies, ISO will be able to pick out such star bursts. Further away, the detail will become obscured, but the observatory will still return a wealth of information on the distribution of dust and gas, the types of dust grains and the abundance of carbon and oxygen.

As telescopes such as ISO probe the outer reaches of the universe, they see things as they were billions of years ago when the radiation first set off on its journey towards the Earth. All the galaxies we see are moving away from us. The further the galaxy, the faster it seems to accelerate away. This rapid motion shifts their radiation towards the red end of the spectrum. The faster the galaxy is moving, the more its radiation is shifted towards the infrared.

Whereas IRAS was unable to detect many galaxies at high redshifts, ISO will be able to probe deeper into the past, searching for protogalaxies - huge conglomerations of stars and dust still in the process of formation. The observatory should also be able to detect the background from galaxies too distant to be seen individually.

The first results from ISO will appear next spring. Its contribution to astronomy will be particularly significant since it will be the only infrared observatory to be placed in orbit before the year 2000. While European astronomers enjoy the flood of data from ISO, their American counterparts can only look on with envy as Nasa closes down its Kuiper airborne infrared observatory and struggles to find funding for SIRTE, a new orbital facility.

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