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(meteorobs) NASA's Successful Compton Gamma-Ray Telescope Mission Comes To An End

Keep this in mind if there are any fireball sightings in early June.

Ron Baalke
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Dolores Beasley  
Headquarters, Washington, DC                March 24, 2000
(Phone: 202/358-1753)

Nancy Neal 
Goddard Space Flight Center, Greenbelt, MD 
(Phone: 301/286-0039)

RELEASE:  00-44

NASA'S SUCCESSFUL COMPTON GAMMA-RAY TELESCOPE MISSION COMES TO AN END

     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.nasadot gov/gsfc/spacesci/structure/cgro.htm

                            -end-


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