IRAS - the British-Dutch-United States Infrared (heat-sensing) Astronomical Satellite - has ceased sending scientifically useful data. This ends the data-gathering phase of one of the most successful joint space science projects yet undertaken.
It has given scientists a view of the cosmos unavailable before. Infrared (IR) radiation is heavily absorbed by Earth's atmosphere. This has restricted astronomers' IR viewing to indistinct observations with mountaintop or balloon-borne telescopes and clearer but brief glimpses with rocket-carried instruments.
IRAS, orbiting 900 kilometers (560 miles) out from Earth in an orbit passing nearly over the poles, has made the entire infrared sky available for study. Since its launch on Jan. 25, it has returned ''a steady stream of stunning scientific discoveries,'' according to James M. Beggs, administrator of the US National Aeronautics and Space Administration (NASA).
These include a survey of more than 200,000 IR sources that covered 95 percent of the sky. Many are identified with known objects. The identity of others is a mystery. Galaxies that shine up to 100 times more brightly in the infrared than in visible light challenge astrophysicists to account for their IR brilliance. Wispy clouds, visible at IR wavelengths, cover much of the sky - the so-called infrared ''cirrus.''
The center of our own Milky Way galaxy, which is hidden behind dust clouds, has been seen clearly for the first time, thanks to its dust-penetrating IR emission. And within our solar system, five new comets, new asteroids, and a thick ring of dust between the orbits of Mars and Jupiter have been found.
The IRAS scientific team is, of course, disappointed that the satellite has stopped functioning. In reporting some of their findings at a press conference Nov. 9, they had predicted, hopefully, that its observing capability would last through late December or early January. Nevertheless, they are more than satisfied with the rich data harvest, which Beggs has described as ''nothing short of spectacular.''
Actually, IRAS mechanisms continue to function well. But the satellite has lost the liquid helium needed to cool its IR sensors to a frigid 2.5 degrees above absolute zero (minus 455 degrees F.). The sensors need to be in such a cold environment so that they will not be blinded by the satellite's own heat radiation.
The IRAS team knew that the helium could run out suddenly and with little warning. It had been expected to last only about six months. In fact, the Dewar (insulated container) which held it has done considerably better than that.
Now, a NASA spokesman says, the IR sensors have warmed to a point that their data are scientifically useless. However, it still is valuable to engineers. They are closely studying the satellite's performance as it warms up to learn how to improve the design of future instruments.
The IRAS team would not have wanted the satellite to operate indefinitely anyway. Nancy Boggess, IRAS program scientist, says that this satellite's chief function was to lay out the ''road map'' of the infrared sky. Now astronomers want to study that map and plan how best to explore it in detail with future instruments.
One of the most important of these instruments will be the Shuttle Infrared Telescope Facility (SIRTF) to be flown on the space shuttle in the late 1980s or early 1990s. It will take several years of intensive study of the IRAS data just to prepare to use SIRTF effectively, let alone build and test the instrument, Dr. Boggess explained at the Nov. 9 press conference.
The IRAS project has also demonstrated the great value of international cooperation.
From the US perspective, Beggs says, ''European contributions to our program have been very, very valuable.'' Noting that Britain and the Netherlands paid half of the $250 million IRAS cost, he adds that ''They've enabled us to do things that we could not otherwise do.''
And, he notes, ''it taps another very vital and important resource and that is brains . . . we need access to the best brains in the world if we're going to do the best work. . . .''
Likewise, speaking from the European viewpoint, Jan de Koomen and Eric Dunford, the Dutch and British IRAS project managers, told the Nov. 9 press conference that their countries had also benefited substantially by this sharing of cost and pooling of talent. Thus IRAS has become a landmark project for international cooperation as well as for astronomy.