Paula Cleggett-Haleim
Headquarters, Washington, D.C.                                      
Oct. 4, 1993
(Phone: 202/358-0883)

Michael Finneran
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-5565)

RELEASE:  93-179

COMPTON GAMMA-RAY OBSERVATORY TO GET ORBIT BOOST

   NASA's Compton Gamma-Ray Observatory will receive a scheduled 
boost to a higher orbit beginning today to prevent the spacecraft 
from reentering the Earth's atmosphere.

   The observatory will be nudged to the higher orbit in two 
phases using computer commands sent to onboard rocket thrusters 
from NASA's Goddard Space Flight Center, Greenbelt, Md., which 
manages the spacecraft.

   "The reboost is necessary to compensate for orbit decay, which 
is the result of solar activity slowly pushing the satellite 
toward Earth over a period of time," said Goddard's Tom LaVigna, 
the former Deputy Project Manager for the observatory who has 
retained management responsibility for the reboost.  "This was a 
known occurrence and was taken into consideration in the design
of the observatory."

   The first phase of the reboost is scheduled for Oct. 4 to 17; 
the second from Nov. 22 to Dec. 3, LaVigna said.  The first phase 
will lift the 2-year-old satellite to 280 miles (450 kilometers) 
at its apogee or highest point in orbit. The second phase will 
circularize the orbit by boosting the spacecraft's perigee, or 
lowest point, to the same elevation.

   As of late September, the spacecraft was 217 miles (345 
kilometers) above the Earth in an orbit that follows the equator.  
Below 180 miles (290 kilometers), the satellite could lose the 
stability required for reboost, LaVigna said.  If the observatory 
was not reboosted, he said, it would reach that altitude in late 
April of 1994.

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   The October-November reboost will follow an attempt that began 
June 15 but was terminated the same day because of a problem with 
one of the thrusters on the satellite.  NASA officials discovered 
that the B2 attitude control thruster was performing well below 
the other thrusters on the "B" side of the observatory, making it 
difficult to properly control the spacecraft.  While the B2
thruster performance remains low, it has been taken into account 
for the reboost.

   "Extensive analysis and simulations have been performed to 
develop the plan for reboost," LaVigna said.
  
   The observatory has eight attitude control thrusters and four 
larger orbit adjust thrusters, with half of each split between the 
"A" and "B" sides of the spacecraft. The B-side attitude control 
and orbit adjust thrusters will be used for both phases of the 
reboost.  During the previous reboost attempt, only the
attitude control thrusters were planned for use.

   Science operations on the observatory will be suspended only 
during the actual reboost operations.

   The Compton Gamma-Ray Observatory is managed by Goddard for the 
Office of Space Science at NASA Headquarters in Washington, D.C.  
The 17-ton observatory was launched April 5, 1991, on the Space 
Shuttle Atlantis and is the heaviest civilian spacecraft deployed 
by a shuttle.

   The $550-million observatory, a collaboration between the 
United States, Germany and The Netherlands, has a mission goal of 
2 to 6 years and carries four science-gathering instruments. The 
satellite exceeded its minimum life in April this year.

   The observatory was sent aloft to study the gamma-ray universe.   
Gamma rays are the highest energy radiations in the 
electromagnetic spectrum, created by some of the most violent 
events in the Universe, such as the explosion of stars.  
Gamma-rays can be studied effectively only from space because the 
Earth's atmosphere prevents them from reaching the ground.


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