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Release: 00-174

Gamma ray ‘watchdog’ ends a stellar career

One of astronomy’s most prolific tools of the last decade officially ends its career Sunday when NASA plans to command the Compton Gamma Ray Observatory to reenter the atmosphere.

For nearly nine years, Compton’s Burst And Transient Source Experiment, or BATSE -- designed and built by NASA’s Marshall Space Flight Center in Huntsville, Ala. -- kept unblinking watch on the universe to alert scientists to the invisible, mysterious gamma ray bursts that had puzzled them for decades.

By studying gamma rays from objects like black holes, pulsars, quasars, neutron stars and other exotic objects, scientists can discover clues to the birth, evolution and death of stars, galaxies and the universe.

"I think BATSE has done everything a scientific tool can do," said Dr. Gerald Fishman, principal investigator for the instrument at the Marshall Center. "It has answered some of the most perplexing questions in astronomy. It’s answered some questions we didn’t know we should ask, and it has provided us with a new set of questions for the future."

The gamma ray instrument was one of four major science instruments aboard Compton. It consisted of eight detectors, or modules, located at each corner of the rectangular satellite to simultaneously scan the entire universe for bursts of gamma rays ranging in duration from fractions of a second to minutes.

When such bursts were detected, it alerted Compton’s three other instruments, which could point toward the burst for a more detailed look.

With an impressive list of discoveries and diverse accomplishments, BATSE could claim to have rewritten enough astronomy textbooks in its 10-year life to make a famous author jealous.Because gamma rays are so powerful, they pass through conventional telescope mirrors. Instead of a mirror, the heart of each BATSE module was a large, flat, transparent crystal that generates a tiny flash of light when struck by a gamma ray.

The flashes were amplified, recorded and transmitted to the ground. The gamma ray burst position information was provided to Compton’s other three instruments, which have limited fields of view, but could see the sky in more detail than BATSE.

The gamma ray experiment discovered nearly 30 new exotic astrophysical objects and other phenomena stretching from Earth’s own atmosphere to the edge of the universe, Fishman said.

Some 37 universities, observatories and NASA centers in 19 states, and 11 more institutions in Europe and Russia, participated in BATSE’s science program. In 1996, it was one of the most-cited experiments in scientific papers, and gained attention from numerous general interest publications.

BATSE-related research was responsible for 10 scientific prizes by leading astronomers, as well as 18 new Ph.Ds. And it helped pioneer astronomy’s use of the Internet -- becoming a tool for a broader realm of astronomy than the original Compton observatory team.

In January 1999, the instrument -- via the Internet -- cued a computer-controlled telescope at Los Alamos National Laboratory in Los Alamos, N.M., within 20 seconds of registering a burst. Programmed to respond in the absence of human control, the telescope re-pointed itself to observe BATSE’s latest burst.

With that capability, the gamma ray experiment came to serve as a gamma ray burst alert for the Hubble Space Telescope, the Chandra X-ray Observatory and major ground-base observatories around the world.

Although its science instruments were functioning normally, NASA decided March 23 to return Compton to Earth’s atmosphere for safety reasons after one of three on-board gyroscopes used to steer the orbiting observatory malfunctioned.

The propulsion system on Compton lacked sufficient fuel to boost the spacecraft to a higher, longer-lived orbit. Left alone, Compton eventually would have fallen from orbit, slowed by Earth's atmosphere.

Unlike most other satellites, Compton was too large to burn up entirely in the atmosphere during reentry and could have exposed populated areas to the risk of falling debris. The observatory is being safely steered using the two functioning gyroscopes to reenter over an unpopulated area of the Pacific Ocean on Sunday. Its science instruments were turned off earlier this week.

Compton exceeded its planned five-year mission by four years.

Compton was the largest satellite dedicated to peering into deep space in the highest energy part of the spectrum — gamma rays — which reveal the violent processes that occur in the universe.

NASA and international space agencies plan several upcoming missions to continue where Compton left off. Marshall is responsible for the Gamma-Ray Burst Monitor, the main instrument for detecting bursts on Compton’s successor, the Gamma Ray Large Area Space Telescope, or GLAST. The telescope is planned for launch in 2005 to study gamma ray sources at even higher energies.

"The legacy of BATSE and its science team will continue because the sky is constantly changing," Fishman said. "Where one part of the sky today is dark, empty space, tomorrow it may be the scene of a cataclysmic eruption.

"With what we learned from BATSE and its successors," added Fishman, "we will continue to seek answers about the formation of stars and galaxies, the age of the cosmos, how far the universe extends, and, the still-elusive mechanism that causes these mysterious gamma ray bursts."