A cool view of the heavens: infrared observatory spies a hidden universe.

Every time the Infrared Space Observatory (ISO) slews from one target to another, it discovers new objects in the seemingly empty voids of space. This telescope, often dubbed Europe's answer to the Hubble Space Telescope, has opened a new window on the universe.

Last month in Villafranca, Spain, the European Space Agency presented the first findings from the mission, which was launched last November and still has over 18 months to run. Among the observations are spectacular images of colliding galaxies and stellar nurseries, as well as a new view of Saturn. Although ISO has only one-sixteenth the light-collecting area of Hubble, the $1 billion observatory can see infrared emissions to which the space telescope is blind. Because these wavelengths are low-energy emissions, they reveal long-duration, subtle activity in their sources. This capability enables ISO to detect some of the chilliest objects in the universe and lay bare parts of the cosmos hidden behind dust.

The coldest bodies in the heavens-ranging from planets and failed stars to interstellar gas clouds-emit little visible light. In ultraviolet light and X rays, they're even fainter. Only in the infrared do these objects boldly announce their presence by radiating energy.

"You're looking at something which is at the temperature of dry ice, and you're measuring the heat coming from that-except that this object is millions of light-years away, and the stuff is distributed so tenuously that you can almost look through it," notes ISO mission scientist Martin Harwit, former director of the Smithsonian Institution's National Air and Space Museum in Washington, D.C.

The telescope reveals other parts of the invisible universe by penetrating the veils of dust that hide newborn stars and shroud the hearts of galaxies. Hot, young stars radiate most of their energy at visible and ultraviolet wavelengths. But the dust surrounding the newborns absorbs these emissions, reradiating their energy in the infrared.

Similarly, the telescope sees through dust that may mask a tumult of activity at the cores of galaxies, where black holes and quasars may lurk. The telescope has also begun a search for youngsters in the cosmos-baby planets and infant galaxies. It lacks the resolution to image directly the dusty disks around stars, widely regarded as the spawning grounds for planets.

But it can identify excess infrared emissions, a likely signature of dust disks, around stars that lie within 100 light-years of Earth. Therefore, ISO may identify places likely to harbor planetary systems.

The observatory also seeks primeval galaxies-distant, young galaxies caught in the act of making their first generation of stars (SN: 2/24/96, p. 120). These stars radiate mostly in the ultraviolet, but the expansion of the universe shifts their light to much longer wavelengths, so they appear bright in the infrared.

The astronomers plan to characterize nearby galaxies in great detail at shorter infrared wavelengths. They can then investigate more distant galaxies by looking for the same features redshifted to longer infrared wavelengths. "We're creating a template of galaxies in the nearby universe so we can know what to expect when we look deeper, at more distant galaxies," says ISO project scientist Martin Kessler of the European Space Agency in Villafranca. Noting that astronomers aren't sure what galaxies look like in the far infrared, the longest infrared wavelengths, Kessler says the ISO observations highlight "the unexplored nature of this part of the electromagnetic spectrum."

Observations such as these are all but impossible from the ground. Several gases in Earth's atmosphere-mainly water vapor and carbon dioxide-provide a double whammy in the infrared. These molecules absorb most infrared light, allowing only a few narrow bands of radiation to reach the ground. In addition, the molecules themselves radiate copious amounts of infrared light, overwhelming the faint emissions from stars and galaxies.

"It's as though you were trying to do astronomy in a snowstorm," notes Harwit.

Short-duration airplane and balloon surveys have flown above much of this atmospheric fog. Many studies have relied on the Kuiper Airborne Observatory, an infrared telescope that flew high in the atmosphere in a converted C-141 airplane. Even so, infrared emissions from the equipment itself as well as the atmosphere have interfered with observations.

Leaving the atmosphere far behind, ISO outperforms its only predecessor in space, the Infrared Astronomical Satellite. During its 1983 survey, the earlier satellite observed infrared emissions in only four broad bands, none of them at wavelengths longer than 120 micrometers. In contrast, ISO's suite of instruments-a sensitive camera, two spectrometers to analyze the components of infrared light, and a photometer to measure brightness-cover a wavelength range spanning the middle and far infrared, from 3 to 240 micrometers. "One can now make observations across the entire infrared spectral band-perhaps that's the most important attribute of the mission," says Harwit.

"Now we have this absolutely clear view of the universe at all infrared wavelengths."

Even in the chilly environs of space, most infrared telescopes would generate infrared emissions some 10,000 times as strong as the radiation they are designed to detect. That's why scientists built ISO as a cryogenic telescope, using a huge tank of superfluid helium to cool it down to 2#251# above absolute zero. The mission ends when the helium runs out, about 20 months from now.

By that time, if all goes according to plan, astronauts will have outfitted the Hubble Space Telescope with an infrared camera, and NASA may be 2 years away from launching a major infrared observatory know as SOFIA (Stratospheric Observatory for Infrared Astronomy). Astronomers hope that as part of its legacy, ISO will have unveiled a variety of cool bodies for these telescopes to examine further.