A NEW kind of astronomical satellite is ready to leave Cape Canaveral next week on a mission that is rooted in the 1975 Apollo-Soyuz flight.
The diplomatic symbolism of that joint Soviet-American venture captured public attention. But astronomers remember it for opening a branch of their science they had once thought impossible to pursue.
This is the study of cosmic objects by the extreme ultraviolet (EUV) radiation they emit. Astronomers wanted to observe with this kind of "light." Its range of wavelengths fills a gap between the ordinary ultraviolet and X-ray regions where observations already were being made. They knew that they could gain new insight into energetic processes in the atmospheres of stars and in other cosmic objects that are best seen with EUV "light." But they also knew that interstellar hydrogen absorbs EUV radiation s trongly. They assumed that, if they tried to look at the universe through the EUV window, they would see only an opaque fog.
On July 22, 1975 - the seventh day of the Apollo-Soyuz flight - astronauts pointed an EUV telescope at a target in the constellation Coma Berenices and changed that assumption. A strong EUV signal was detected from the hot white dwarf called HZ43, a highly condensed type of star. Several other EUV sources were also detected.
The interstellar gas turned out to be patchy. Furthermore, the solar system is in a region where that gas is so thin EUV sources can be seen up to at least several hundred light-years distance. In some directions, it may be possible to detect sources well beyond our galaxy.
"This was a true surprise," says EUV astronomer Roger Malina of the University of California at Berkeley. He says it turned around a negative attitude toward the prospect for EUV astronomy at the National Aeronautics and Space Administration (NASA).
Stuart Bowyer, who leads the Berkeley EUV group, and colleagues had a detailed proposal for an EUV-observing satellite ready for NASA by December 1976. The result of that proposal is the Extreme Ultraviolet Explorer (EUVE) satellite scheduled for launch June 4. Remembering the many years of design, development, construction, and delays it took to reach this point, Dr. Malina says, "We're really getting excited now."
EUV wavelengths range 1 to 100 nanometers (billionths of a meter). The satellite is the first observatory to cover most of that band, from about 8 to 80 nanometers. It has three telescopes to sweep the sky for EUV sources. A fourth instrument can look at specific objects 10 to 50 times fainter than the mapping telescopes can detect. EUVE is to spend its first six months of operation mapping the entire sky. Then it will settle down to a year and a half or more of detailed studies of specific objects.
EUVE is also helping NASA test a new concept in satellites - the multimission spacecraft. The astronomical package is mounted on a general purpose unit that supplies power, attitude control, and other services. NASA had planned to have shuttle astronauts retrieve the satellite when EUVE's work is over and replace the observatory package with another scientific payload. NASA dropped this plan for budgetary and logistical reasons.
But James Barroman, project manager for the Explorer satellite program, notes the satellite retains its multimission capability. "We were very careful not to preclude reuse," he says. He speculates that NASA planners may think about possible further use of the main spacecraft unit as the EUVE mission nears its end.