A crucial science satellite, the Compton Gamma Ray Observatory (CGRO), one of NASAs four Great Observatories, is facing the possible end of its mission as its systems have begun to fail.
The CGRO will likely have to be discarded following the failure of one of its three gyroscopes on December 6. This has made the continuation of the flight too dangerous a NASA official who did not want to be identified told space.com.
While failure of one gyroscope has no effect on the science research conducted with CGROs instruments, the loss of a second unit would make the observatory virtually uncontrollable. As a result, ground controllers would not be able to control the spacecraft's braking maneuvers.
The CGRO, which weighs 36,643 pounds (16,621 kilograms), was launched from Space Shuttle Atlantis in 1991.
There is no doubt that fiery debris of a spacecraft this size would reach the Earth's surface upon reentry. Unless controlled reentry is achieved, there is no way to predict where the debris would hit, the NASA source said.
Experts estimate that the odds for human casualty as a result of the observatory's uncontrolled reentry could be as high as one in a thousand.
NASA is taking no chances. At present, emergency plans are being formulated. The agency has ordered the observatory's science team to be prepare by February 16 a plan to maneuver the lab into the atmosphere by mid-March.
Born to be fixed
The Compton Observatory was originally envisioned as a repairable spacecraft. Its four main on-board fuel tanks were designed to allow refueling by spacewalking shuttle astronauts and all the crucial sub-systems of the lab were packed into easily replaceable modules plugged to the spacecraft's sides.
NASA gained experience servicing such replaceable modules in 1984 when the crew of Space Shuttle Challenger successfully retrieved, fixed and released back into orbit the Solar Maximum satellite. However, when the Gamma Ray Observatory was launched in 1991, NASA made no plans to service the lab due to a lack of funding.
In the face of the latest problems, NASA is re-visiting the idea of sending a shuttle to fix CGRO, however a number of issues make such a mission unlikely. Even if the agency decides to race with the time to prepare a repair crew, it will be retrieving a satellite plagued with propulsion problems.
Unlike the Hubble Space Telescope, which relies solely on electrically driven gyroscopes to maneuver, the CGRO is equipped with four orbital correction engines and eight small attitude control thrusters all fueled by toxic hydrazine. At the time of the launch, the spacecraft was carrying 4,200 pounds (1.9 tons) of hydrazine.
Soon after the CGRO's launch in 1991, one of its two main propulsion lines failed to pressurize and the backup line has been used since then. Also, one of the attitude thrusters on board showed signs of problem in 1993.
The fuel leak was suspected as the source of the thruster problems, and as a result the observatory's control team could not guarantee a safe environment for the shuttle to rendezvous with the lab.
The propulsion system on board the Compton has not been used since 1997, when the lab's orbit was last raised.
Around 1,000 pounds (454 kilograms) of propellant would be needed to de-orbit the CGRO.
The CGRO science team is considering alternative ways of orienting the satellite if gyroscope-based navigation should fail. One possibility is to use the sun sensors or magnetometers on board to determine the spacecraft's position in space. In the past this method was used to orient the International Ultraviolet Explorer satellite, however no algorithms or software exist to apply the technique to the Compton.
One of four
The Compton Gamma Ray Observatory is one of four in NASAs Great Observatories series of orbiting telescopes. First conceived as far back as the 1970s, its purpose is to study the universe across the electromagnetic spectrum.
The spacecraft complements the Hubble Space Telescope, which has been concentrating mostly on the visible part of the spectrum since 1990; the Chandra X-ray telescope, launched last year and a yet-to-be-launched infrared telescope.
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