Dolores Beasley

Headquarters, Washington, DC

(Phone: 202/358-1753)

Nancy Neal

Goddard Space Flight Center, Greenbelt, MD

(Phone: 301/286-0039)

RELEASE: 00-44

NASA’s extremely productive and long-lived Compton Gamma-Ray
Observatory mission — which exceeded its mission by four years
and completely changed ideas on the most important unsolved
puzzles in astrophysics — has come to end with the failure of one
of the satellite’s three gyroscopes.

NASA plans to safely direct the satellite back into Earth’s
atmosphere no earlier than June 1 with the remaining two
gyroscopes, which are used to steer the craft. As an extra
precaution, Compton engineers are also developing a method to
control the satellite without any gyroscopes, for use as backup
during the reentry maneuvers in case an anomaly is encountered
with the gyroscopes. Compton’s four instruments are still in
working order.

“Compton has been a workhorse for nine years, far exceeding
our expectations for a two- to five-year mission,” said Dr. Alan
Bunner, director of NASA’s Structure and Evolution of the Universe
science theme, NASA Headquarters, Washington, DC. “New discoveries
made by Compton changed our view of the Universe in fundamental
ways.”

Compton’s lasting legacy will be its impact on gamma ray
astronomy. The telescope detected more than 400 gamma ray
sources, 10 times more than were previously known. Compton
recorded more than 2,500 gamma ray bursts; before Compton, only
about 300 had been detected.

“NASA must have a controlled reentry to direct Compton
towards an uninhabited area in the Pacific ocean, ” said Dr. Ed
Weiler, Associate Administrator for the Office of Space Science,
NASA Headquarters. “NASA decided before Compton was launched that,
due to its size, it would be returned to Earth by controlled
reentry when the mission was over. This was always NASA’s plan. ”

The propulsion system on Compton lacks sufficient fuel to
boost the spacecraft to a higher, longer-lived orbit. Left alone,
Compton will eventually fall from orbit due to a minute drag from
the Earth’s tenuous atmosphere at Compton’s orbital height. Unlike
most other satellites, Compton is too large to burn up entirely in
the atmosphere during reentry. An uncontrolled reentry would
expose some area under its orbital path (28.5 degrees north and
south latitude) to the risk of falling debris.

The decision to reenter Compton before a second gyroscope
fails, even though the satellite is functioning normally, was made
at NASA Headquarters on March 23, 2000, after extensive study to
consider all options. Research showed it was significantly safer
to perform a controlled reentry than any other method of dealing
with the satellite. “We actively pursued the option that provided
the lowest risk to human lives,” said Weiler.

Debris from the reentry will be scattered over an area
estimated to be 16 miles wide and 962 miles long. The center of
the reentry area is on the equator approximately 2,500 miles
southeast of Hawaii (about 120 degrees west longitude). A large
portion of the satellite will vaporize as it transits the
atmosphere, and most of the pieces that survive will be tiny,
about the size of a pea or a grain of sand. However, Compton
contains structures made of titanium, which are expected to fall
as larger pieces.

“Enough will survive to present a small but still
unacceptable risk to populated areas if Compton were allowed to
reenter in an uncontrolled manner,” said Preston Burch, Deputy
Program Manager for Space Science Operations at NASA’s Goddard
Space Flight Center, Greenbelt, MD. “NASA will work closely with
aviation and maritime authorities to ensure the impact area is
free from traffic during reentry.”

Compton flight controllers, stationed at Goddard, will fire
Compton’s propulsion system thrusters in the direction opposite to
its orbital motion, which will slow the spacecraft down and cause
its orbital height to decrease so that it reenters the atmosphere.
There will be four separate firings of the propulsion system
thrusters, each about a day apart. After each firing, Compton’s
new orbit will be determined precisely, and the performance of the
thrusters will be evaluated. The thruster performance varies
according to the pressure of the propellant, so the thrusters will
not perform the same way because each firing consumes propellant,
which decreases its pressure.

NASA and international space agencies plan several upcoming
missions to continue where Compton left off. The Compton Gamma Ray
Observatory was the second of NASA’s Great Observatories and the
gamma-ray equivalent to the Hubble Space Telescope and the Chandra
X-ray Observatory. Compton was launched aboard the Space Shuttle
Atlantis in April 1991, and, at 17 tons, was the largest
astrophysical payload ever flown at that time.

More information is available on the Internet at:

http://pao.gsfc.nasa.gov/gsfc/spacesci/structure/cgro.htm

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