New Challenge to the Big Bang?

By JEROME CRAMER/CAPE CANAVERAL

Call it an unexpected bonus from the cold war. During top-secret monitoring of the dark side of the moon 25 years ago, U.S. scientists discovered what they feared might be clandestine Soviet nuclear tests in space. Spy satellites picked up massive bursts of gamma rays similar to those released during the explosion of atom bombs. But these bursters, as gamma-ray scientists began to call them, did not match any known pattern. They were brief, lasting from only a fraction of a second up to 100 seconds. Civilian experts were called in to study the data, and the Soviet-nuclear-test theory was eventually ruled out. But scientists remained puzzled: What were those fleeting yet powerful flashes of gamma rays, and where did they come from?

Astronomers will get a chance to answer some of these questions -- and more -- over the next two to eight years as a result of last week's NASA launch of the Gamma Ray Observatory on board the space shuttle Atlantis. The 17.5-ton GRO will circle the earth at a height of 450 km (280 miles), mapping the heavens as it peers to the very edges of the universe. "Gamma-ray scientists are starved for information," says Richard Lingenfelter, an astronomer at the University of California at San Diego. Data gathered on such violent but poetic-sounding celestial bodies as neutron stars, supernovas and black holes could force astronomers to revise or even discard popular notions on the origin of the universe.

Gamma rays are the most powerful type of radiation, thought to have been created during the explosion that launched the universe and its subsequent expansion. As distant heavenly bodies continue to collapse and explode, the only signals earth may receive of this activity are in the form of gamma rays. For example, gamma-ray bursters have been measured releasing more energy in a matter of seconds than the sun does in thousands of years. Since they carry no electric charge, gamma rays can plow through space unchanged, giving scientists a clear record of cosmic events. The atmosphere shields the earth from most gamma radiation, but this shield has forced scientists studying the rays to rely on instruments lofted aboard huge balloons or rockets. Until now researchers have only peeked through the veil of the universe; last week's GRO launch gives them a powerful tool and years to probe the outer limits.

The GRO satellite will rely on four sophisticated instruments, three of which are the size of small automobiles, to record the full range of gamma-ray activity. The devices will also conduct tests of the skies throughout the electromagnetic spectrum, using X rays, visible light and infrared light. These sensitive instruments were developed by a team of scientists from Germany, the Netherlands, the U.S. and the European Space Agency. The four monitors will all use liquid and solid crystals to record the origin of gamma- ) ray sources. As the rays smash into the crystals, they produce flashes of light called scintillations. Those data will be measured and sent back to earth. One of the instruments will track gamma-ray bursts, events that until recently some scientists did not believe existed. "It's a little like trying to catch and study lightning," a GRO scientist explains.

During the first 15 months, the mission will systematically create a gamma- ray map of the universe. Sources of energy that have long puzzled scientists will be recorded and cataloged. Data will be fed back to earth, where scientists poring over the information will zero in on particularly interesting phenomena or request NASA to point the GRO to a specific corner of the sky. NASA has already accepted 50 proposals, and is funding experiments suggested by theoretical astronomers from around the world. Earth's own Milky Way will get a thorough working over; NASA astronomers are intent on discovering whether the energy at the center of the galaxy is a single black hole or a series of smaller objects. "There are weird collisions of matter and antimatter going on in the center of the Milky Way," says NASA astrophysicist Alan Bunner.

Scientists know so little about the forces in the universe that the real goal of the GRO is to gather basic data to guide astronomers in what to look for. "We're on a fishing expedition in outer space," says University of New Hampshire professor James Ryan. "We can hear the fish jumping, but we don't know what they look like." Ryan, who helped develop one of the instruments on the GRO, says the gamma-ray information, when matched with data streaming in from the Hubble Space Telescope launched last April, is sure to shake up currently accepted theories on the origin and scope of the universe. This new information, combined with data that will be compiled from future observations by X-ray and infrared satellites, could significantly change the Big Bang theory and send scientists back to the drawing board.

NASA engineers say they are sure the GRO will not run into any of the difficulties that have plagued the Hubble telescope. Launch delays caused by the Challenger disaster and other problems have given them a chance to "end- to-end" test all four satellite components. But scientists admit that they are eagerly waiting for the first bits of information on the violent world of gamma rays to begin streaming back to earth.