Donald L. Savage
  Headquarters, Washington, D.C.                                    
  December 20, 1993
  (Phone:  202/358-1547)
  
  Michael Finneran 
  Goddard Space Flight Center, Greenbelt, Md.
  (Phone: 301/286-5565)
  
  
  RELEASE:  93-224
  
  NASA SUCCEEDS WITH GAMMA-RAY OBSERVATORY REBOOST
  
  
      NASA has successfully boosted its Compton Gamma-Ray 
  Observatory into a higher orbit around the Earth, a move that 
  will keep the 17-ton satellite from reentering the atmosphere.
  
      The reboost is a significant accomplishment because it 
  extends the mission life of the observatory by 5 years and 
  prevents a reentry in which large parts from the spacecraft could 
  have struck Earth, said officials at the Goddard Space Flight 
  Center in Greenbelt, Md., which manages the spacecraft.
  
      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 
  Earth's atmosphere prevents them from reaching the ground. 
  
      The reboost, which was concluded Dec. 17, had been 
  anticipated when the 2-year-old spacecraft was designed.  It was 
  necessary because of solar activity that slowly pushed the 
  satellite back toward Earth.  Initially scheduled earlier in the 
  mission, the reboost was delayed because of the high risk of 
  operating the spacecraft's propulsion system after a problem that 
  occurred shortly after launch.
  
      "Although this reboost was always planned, its successful 
  completion involved solving some complex problems never before 
  encountered by NASA," said Goddard's Tom LaVigna, former deputy 
  project manager for the observatory, who retained management 
  responsibility for the reboost.
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      NASA managers had to compensate for a balky rocket thruster 
  on the 
  observatory, LaVigna said, and devise a new way to inject fuel 
  into the sensitive plumbing lines leading to the thrusters.
  
      The complex procedure for boosting Compton was developed by 
  Goddard engineers after extensive analysis and simulations.  It 
  employed a novel way to operate the observatory propulsion and 
  the attitude control subsystems together to achieve a controlled 
  boost.
  
      The observatory has the largest propulsion system ever 
  launched by NASA, carrying 4,200 pounds (1,910 kilograms) of 
  propellant.  It has 8 attitude control thrusters and four larger 
  orbit adjust thrusters, with half of each split between the "A" 
  and "B" sides of the spacecraft. 
  
      Through mathematical analyses and tests, it was determined 
  that very high fuel-line pressure surges had compromised the 
  A-side propulsion system when the system was first operated, 
  shortly after launch.  After extensive study and testing, a 
  unique fuel-line priming procedure was developed to prepare the 
  redundant B-side thrusters for use in the reboost. 
  
      The priming worked well, but the reboost was terminated when 
  the B2 attitude control thruster performed significantly below 
  the other thrusters and caused the observatory to go unstable and 
  tumble.  Quick action by ground controllers terminated the 
  thruster firings and, using a contingency procedure, brought the 
  observatory back into a stable mode. 
  
      With the standard procedure no longer usable, Goddard 
  engineers developed a novel way around the problem.  The 
  procedure involved firing 2 of the 4 100-pound orbit adjust 
  thrusters for 60 to 90 seconds.  The 5-pound attitude control 
  thrusters and reaction wheels were used together to limit 
  unstable conditions and errors in pointing the observatory.  A 
  total of 18 propulsion system burns were used, one each day.
  
      The reboost was accomplished in two phases, one that 
  concluded in October and another spanning November and December.  
  The first phase lifted the observatory's apogee -- or highest 
  point in orbit -- to 280 miles (452 kilometers).  The apogee had 
  descended to 214 miles (346 kilometers).  The two-part second 
  phase of the reboost nudged the spacecraft's perigee -- lowest 
  point -- to the same 280 miles, making Compton's orbit around the 
  Earth almost perfectly circular.
      
       If the satellite had descended below 180 miles (290 
  kilometers), it could have lost the stability required for 
  reboost, LaVigna said.  If the observatory was not reboosted, he 
  added, it would have dropped to that altitude in late April of 
  1994.
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      When the gamma-ray observatory's life is over, NASA managers 
  plan a controlledreentry so that the unburned debris falls 
  harmlessly on an uninhabited area. 
  
      The $550-million observatory, a collaboration between the 
  United States, Germany and The Netherlands, has a mission goal of 
  2 to 8 years and carries four science-gathering instruments.  The 
  satellite exceeded its minimum life requirement of 2 years in 
  April 1993.
  
      The Compton Gamma-Ray Observatory is managed by Goddard for 
  the Office of Space Science at NASA Headquarters in Washington, 
  D.C.  The observatory was launched April 5, 1991, on the Space 
  Shuttle Atlantis and is the heaviest civilian spacecraft ever 
  deployed by a shuttle. 
  
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