The Frigid World of IRAS — I
F OR SEVERAL YEARS, the astronomica! news has been dominated by reve-lations concerning the hottest and most en-ergetic objects in the universe. Satellites such as the International Ultraviolet Explorer and the Einstein X-ray observatory made us aware of many kinds of these barely imaginable celestial powerhouses.
The latest revolutionary observatory to fly in space was the Infrared Astronomica! Satellite (IRAS). It, too, looked at an un-fa miliar vista, but one that can be more readily glimpsed by the mind's eye. This spacecraft sampled radiation from matter at temperaturas that are familiar to Earth-lings — from frozen water down to the liq-uefaction of hydrogen (—423° Fahrenheit or only 37° above absolute zero).
IRAS' main mission was to map the far-infrared sky, to produce a catalogue of objects that later could be scrutinized by tele-scopes in space or on the ground. In this respect it was a pioneer like Celescope, Uhuru, and COS B, which surveyed, re-spectively, the ultraviolet, X-ray, and gam-ma-ray universe. But IRAS could also stop its monotonous scanning routine to follow fast-moving targets, repeatedly image some special field, or take a spectrum.
Launched January 25, 1983, IRAS was a joint project among the United States, the United Kingdom, and the Netherlands. A 560-mile-high, nearly polar orbit was cho-sen for the satellite, one that would precess 360° a year. Thus, IRAS was able to remata over the Earth's terminator while its 22.4-inch telescope scanned the sky perpendicular to the direction of the Sun. In the course of about six months, the new spacecraft was able to view virtually the entire celestial sphere. To suppress un-wanted far-infrared radiation from the satellite itself, the entire telescope had to be cooled by superfluid helium to a temperature of 2Vj° Kelvin (-455° F). The end of the mission carne when this coolant ran out on November 21 st, a month or so earli-er than anticipated.
At IRAS' heart were four sets of far-infrared detectors arranged in rows across the telescope's 0°5-wide focal piane. They were so sensitive that a single speck of dust would register from two miles away. As the table shows, each set responded to a speciflc range of far-infrared wave-lengths. For convenience, these are called the 12-, 25-, 60-, and 100-micron bands, and together they provide a measure of the observed source's temperature. The short-er wavelengths were especially useful for examining relatively warm material in the solar system, while the longer ones were better suited to studies of colder, inter-stellar sources.
Twice daily IRAS relayed the data ac-quired by its detectors to a ground station
at the Rutherford Appleton Laboratory in England. Similarly, commands sent from that station to IRAS guided its opérations for the next 12 hours. Final processing of the data is being carried out at the Jet Propulsion Laboratory in California.
WHAT IRAS FOUND
Although IRAS began sending high-quality data shortly after launch, until late last year little information was made avail-able to either the scientifîc community or the public. In fact, IRAS policy-makers chose to prohibit scientists from distribut-ing preprints of papers scheduled to ap-pear this March in the Astrophysical Journal Letters prior to a showcase press conference on November 9th. This unusual action caused much displeasure among scientists and journalists alike.
At that preliminary science briefing in Washington, D. C., NASA administrator James Beggs acknowledged that IRAS had produced "a steady stream of stunning scientifîc discoveries." Once fully processed, the IRAS data are expected to yield positions and other information on more than 200,000 far-infrared point sources. A catalogue of these is scheduled to appear this summer.
The NASA scientists made it clear that many of the results presented were preliminary. Planning IRAS' observing program
and keeping up with the flood of incoming data left them little time for analysis and interprétation. They were forced to do "science on the run," noted Nancy Bog-gess, the IRAS program scientist. She re-ported that only a few percent of the data had been examined in detail. In fact, many of the results came from observations of only one percent of the sky — areas that had been observed frequently early in the mission.
The descriptions of IRAS' findings begin here with the solar system and move out-ward — next month's installment will re-view relatively nearby star-forming clouds, the center of our Milky Way, and other galaxies.
Cornets. The public first became aware of IRAS* potential for exciting scientifîc discoveries early last May, with the an-nouncement of the discovery of Cornet IRAS-Araki-Alcock. A couple of weeks later this object skimmed past the Earth at a distance of only 3,000,000 miles, closer than any other cornet since Lexell's in 1770. For observers with dark skies, Cornet 1983d was spectacular, brightening to magnitude 1.5 and sporting a coma up to 2°-3° across (S&T: July, 1983, page 26).
By early November, 1983, IRAS had dis-covered four other cornets: 1983f, 1983j, 1983k, and 1983o. All of these were first picked up when they were 17th magnitude