Gamma ray ‘watchdog’ ends a stellar career

Contact: Steve Roy
Media Relations Department
(256) 544-0034
steve.roy@msfc.nasa.gov

RELEASE: 00-174

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.”