Lift-off for SIRTF mission

Physics World, April 2003

On 15 April 2003*, a Delta rocket will take the fourth and last mission of Nasa's Great Observatories Program into space. Following the successes of the Hubble Space Telescope, the Compton Gamma-Ray Observatory and the Chandra X-Ray Observatory, the Space Infrared Telescope Facility (SIRTF) will give astronomers an unprecedented view of the universe at wavelengths of 3-180 microns.

SIRTF - which will be given a more elegant name, chosen from a public competition, after its launch - will be the largest infrared telescope ever launched into space. It carries an 85cm telescope and three cryogenically-cooled imaging and spectroscopy instruments, weighing in at just under a tonne.

Because much of the infrared spectrum is blocked from terrestrial observatories by the Earth's atmosphere, space is the natural location for infrared astronomy. Previous missions include the pioneering Anglo-Dutch-US Infra Red Astronomical Satellite (IRAS) which observed for ten months in 1983, and the European Space Agency's Infrared Space Observatory (ISO), which flew from 1995-98.

"While there are what we refer to as atmospheric windows in infrared, they are somewhat limited," says Tom Soifer, director of the SIRTF Science Center at the California Institute of Technology. "Beyond 20 microns or so, the atmosphere - even from an outstanding ground-based telescope site - is virtually opaque until sub-millimetre. By being in space, one has 100 per cent visibility at all wavelengths."

SIRTF has been on the drawing board since the early 1980s, when it was planned as a reusable facility to be carried aboard the Space Shuttle. By the National Academy of Sciences' 1990 decadal review, the project had evolved into a mammoth $2 billion free-flying satellite. Two major redesigns and much budget trimming later, the final version has a price tag of $720 million, including launch costs.

"I think the need to downsize has mostly affected complexity," notes Soifer. "By means of clever design and clever instrumentation, the observational capabilities are probably 60-70 per cent of what the decadal review proposals looked like, and for much less cost."

At the heart of SIRTF's design is the efficient use of liquid helium to cool its instruments to a few degrees above absolute zero. This is necessary to reduce the thermal noise of the instruments themselves which would otherwise swamp the faint infrared signals they seek to measure. SIRTF will carry 360 litres of liquid helium, less than a tenth of the volume planned for its 1990 incarnation. "One of the really beautiful technical aspects is the very efficient use of helium," says Soifer. "The only activity on the satellite that will boil off helium is the use of the instruments themselves. That's because there's a very clever design that uses the cooling power of space to cool all the other elements that need to be cooled."

The use of helium does limit the lifespan of the mission, but at an expected five to six years SIRTF will have a life far longer than its predecessors. The satellite's store of helium cannot be replenished because of an unusual solar orbit in which it will drift 15 million km further from the Earth each year. As well as being cheaper to achieve than a conventional Earth orbit, this orbit protects SIRTF's instruments from the Earth's own thermal emissions and interference caused by particles trapped in its magnetic field.

Once in orbit, SIRTF will study a wide variety of astronomical phenomena, from measuring the sizes of individual asteroids, to peering deep into the distant universe to study galaxy formation. Around half of its first year will be devoted to the Legacy Science Program, a series of six projects of lasting importance to astronomy. "These are large coherent programmes where the data will be made public as soon as we process it - it will be much more broadly usable than just for the scientific objective of the initial investigators," says Soifer.

The Legacy projects range from studying the evolution of planetary systems to the star-forming interstellar medium. The project that Soifer says he finds most interesting is called GOODS - the Great Observatory Origins Deep Survey, led by Mark Dickinson of the Space Telescope Science Institute, Maryland. This will examine two small patches of sky that have already been probed to great depth by telescopes including the Hubble and Chandra observatories.

"They're going to observe these small patches to almost the ultimate limit of SIRTF's senstitivity," says Soifer. "What's going to be really exciting is the multi-wavelength nature. We're using a vast number of telescopes to explore what the universe is like, very close to where galaxies form. The whole picture of this stage of evolution of the universe will become much clearer with this combined coverage."

Aside from the Legacy program, SIRTF's unique vantage on the infrared universe will be open to the wider astronomical community, with proposals solicited by the SIRTF Science Centre. Its five-year mission does not spell the end of terrestrial infrared observatories, however.

"SIRTF will produce good data - some of it unique and some of it in the goldmine category - but that is not the end of it," says Andy Adamson, science director at the UK Infrared Telescope on Mauna Kea, Hawaii. "I would expect that SIRTF, as do most space missions, will provide a foundation to build on from the ground."

* - following several delays, SIRTF launched on 25 August 2003