GRO Satellite Will Allow Scientists To Study Cosmic Gamma Rays

WHILE optical astronomers pore over the first useful images from the flawed Hubble Space Telescope, their colleagues who study the cosmos through gamma rays prepare for the launch of Hubble's companion - the massive Gamma Ray Observatory. The $550 million Gamma Ray Observatory (GRO) now sits at the Kennedy Space Center in Cape Canaveral, Fla., waiting a space shuttle lift into orbit. Shuttle schedule delays have slipped its launch from April to November to next March. But the satellite is ready to give astronomers their first comprehensive look at the universe in the ``light'' of the most energetic kind of radiation cosmic objects generate.

GRO is the second of the four ``Great Observatory'' satellites the National Aeronautics and Space Administration (NASA) plans to launch in the 1990s. The next two will observe X-rays and infrared radiation.

Weighing in at 17 tons, measuring 15 feet high by 30 feet long with a solar area span of 70 feet, GRO is also the largest and heaviest scientific satellite yet built. Stanley Rieb, GRO project manager at TRW Inc., the manufacturer, says his team had to develop a new set of techniques. ``The mechanical ground support equipment was a major build in itself,'' he explains.

Mr. Rieb says that GRO is big because the nature of gamma rays forces observers to use big instruments.

Unlike light or even X-rays, gamma rays are too energetic and penetrating to be focused in telescopic fashion.

Astronomers use techniques familiar to physicists who work with speeding particles produced by high-energy accelerators. Various crystals, liquids, and other materials interact with gamma rays to produce flashes of light which sensors record. Observing instruments have to be large to accommodate these arrays of scintillators and sensors and to capture enough gamma rays for meaningful observations.

Penetrating as they are, cosmic gamma rays are hard to study from the ground. They interact with the atmosphere so that ground-based instruments see only secondary effects.

But observations by a few instruments carried by satellites and high-flying balloons have convinced astronomers that gamma rays will give them unique cosmic views, allowing them to ``see'' into the core of this galaxy as well as observe the results of violent action in distant parts of the universe.

Electrons and other other particles accelerated by powerful gravitational and magnetic forces interact in ways that produce gamma rays. Such forces exist in solar flares. They are characteristic of black holes, which are objects that have collapsed to a density whose gravity is so strong that even light cannot escape it. They are prominent in the cores of many galaxies, including the Milky Way.

Because cosmic gamma rays have been so hard to observe, astronomers have had only hints of what the GRO satellite may discover. For example, unknown objects produce bursts of gamma rays in various parts of the sky. Another tantalizing mystery has arisen as several balloon flights have detected strong gamma radiation from the core of this galaxy while other flights have seen nothing.

In October 1988, a balloon-borne detector flown by a team from the NASA Goddard Space Flight Center found intense gamma ray radiation from the galaxy center. But a joint French and American flight in May found the radiation had decreased by 50 percent.

Reporting the findings in June 1989, project leader James Matteson of the University of California at San Diego said ``this is consistent with the idea that this powerful source at the center of the galaxy does flicker on and off, and now it's off again.''

The source could be a black hole intermittently sucking in matter. However, no one knows what it is.

Gamma ray astronomers have had serious handicaps in trying to solve such mysteries. Their detectors - whether carried by balloons or satellites - have not been able to point accurately enough to allow astronomers easily to identify gamma ray sources with visible objects. Also, these detectors have covered only a limited range of gamma ray energies.

GRO is designed to overcome both limitations. Its instruments, which include German equipment, cover the full range of known cosmic gamma ray energies and will be able to pinpoint gamma ray sources closely enough to ease identification. During its basic two-year mission in a 279-mile-high orbit, GRO should give astronomers a full-sky overview of the universe as revealed by gamma rays. The satellite, however, is designed and fueled for at least eight years active service.

Rieb says he is confident GRO can live up to expectations.

But, he adds, ``we've looked at a lot of things in the light of Hubble [flaws] ... and have written procedures to deal with them.''