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NEXT week, if everything goes according to plan, the Space Infrared Telescope Facility will be launched on a Delta rocket from Cape Canaveral in Florida. SIRTF is the fourth and final of NASA鈥檚 鈥済reat observatories鈥�, a series of space-based instruments designed to make observations over many different wavelengths. The others 鈥� Hubble, Compton and Chandra 鈥� have been collecting a wealth of data at visible, gamma-ray and X-ray wavelengths, enriching our view of the Universe. SIRTF will detect infrared radiation, lifting up the dusty veil that hangs over the cosmos and completing our picture of the Universe.

Far away from the warm glow of the Earth鈥檚 atmosphere, SIRTF鈥檚 infrared telescope and instruments will get a unique view of the cool Universe, capturing everything from icy comets and moons in our Solar System to the most distant galaxies in the Universe. That鈥檚 the plan at least, but it has been a long time coming. Many of the scientists and engineers who designed and constructed the observatory have been working on it for 20 years, an unprecedented length of time for an unmanned space mission.

Although SIRTF was originally proposed in 1979, it was five years before NASA put together a team of astronomers to define its mission and build the instruments. By 1990 they had designed a satellite weighing a massive 5700 kilograms and costing over $2 billion. Over the following decade NASA鈥檚 financial problems demanded some serious cost-cutting and this, combined with improved technology, means the satellite now weighs a rather more modest 950 kilograms and costs a mere $450 million.

In spite of the financial cutbacks, the mission鈥檚 science capabilities are still remarkably similar to the 1990 proposal. The telescope鈥檚 mirror is 85 centimetres in diameter, compared with 1 metre in the original design. As envisaged, a large infrared camera, a sensitive spectrometer and a photometer for analysing the wavelength and intensity of the radiation are attached to the telescope. And astronomers still hope to operate the telescope for five years.

So how did they achieve the cutbacks? A crucial step was changing SIRTF鈥檚 orbit so that it didn鈥檛 need to carry as much coolant on board. In order to observe the faint glow emitted by interstellar dust and distant galaxies, the entire telescope and its instruments have to be cooled to almost absolute zero. This usually requires huge tanks of liquid helium. For example, the European Space Agency鈥檚 Infrared Space Observatory, launched in 1995, used over 2000 litres of liquid helium during its two-and- a-half-year mission.

But thanks to SIRTF鈥檚 new orbit, it requires just 360 litres of coolant weighing a mere 50 kilograms. Instead of circling around the Earth as Hubble, Chandra and Compton do, SIRTF will trail behind the planet as it orbits the Sun. This orbit allows the telescope to escape the heat of the Earth鈥檚 atmosphere, so that it naturally cools to below 40 kelvin. The liquid helium cools it the rest of the way.

After lift-off, SIRTF will take just 40 minutes to reach its unusual orbit. It will then undergo a series of checks over the next three months. NASA has decided to dedicate much of the first year of observation to six programmes that will provide a lasting legacy of information (see 鈥淗ow SIRTF鈥檚 data will be used鈥�).

Astronomers are eager to see the first images from this sensitive infrared observatory. They will reveal what lies beneath the veil of dust that hides so much of the starlight of the Universe. As much as two-thirds of all light emitted by stars is absorbed by interstellar dust and re-emitted at infrared wavelengths. Regions in the Milky Way and other galaxies where new stars form are completely shrouded by dust, so infrared surveys are crucial in trying to understand the origins of stars and planetary systems. With large-scale surveys, it should be possible to detect galaxies where stars were forming in the first billion years of the Universe鈥檚 history.

Old, cold and dusty

Most of the elements made in stars are stored in small dust grains less than 1 micrometre in diameter. In fact, all the atoms in our bodies 鈥� carbon, nitrogen, oxygen and so on 鈥� were once locked up in these dust grains. Looking at the spectra of infrared light emitted by the grains reveals their composition, and gives an insight into the chemical evolution of interstellar material. The spectra also tell us about the physical conditions, such as the density and temperature of the gas from which stars formed.

The infrared is also the ideal wavelength band to search for direct evidence of giant planets and brown dwarfs. These cool objects are not quite massive enough to ignite nuclear burning at their cores and become visible stars in their own right. As well as searching for these giant planets and brown dwarfs, SIRTF will study the icy comets and satellites of the giant planets in our Solar System.

Within the next few months, astronomers should be analysing SIRTF data in earnest. And there will be a lot to do: the largest survey carried out by ESA鈥檚 Infrared Space Observatory discovered around 3000 infrared-emitting sources, mainly galaxies forming stars and dust-shrouded quasars. SIRTF鈥檚 instruments are so sensitive they should detect over 1 million galaxies. All of which explains why thousands will be watching next week鈥檚 launch with bated breath.