NASA Preparing Plans for Destructive Reentry to End Compton Gamma Ray Observatory’s Mission

[14 Jan 2000] NASA is considering a deliberate deorbit (hence destruction) of the Compton Gamma Ray Observatory (CGRO). Some investigators have already been informed that CGRO may not be around much longer – and they have contacted NASA Watch. Sadly, CGRO was designed, as was Solar Max, to be repaired/refurbished on-orbit by a Space Shuttle mission. Apparently no one at NASA feels like fixing it.

NASA Watch has learned that Gyroscope Number 3 on CGRO experienced a complete failure on 6 December 1999. None the less, given that one gyroscope’s failure, the spacecraft can still handle 3 axis control using the two remaining (and functional) gyroscopes. However, should an additional gyroscope fail, the spacecraft would not be able to maintain 3 axis control. This would preclude further scientific observations. It would also preclude the ability for CGRO to perform a controlled reentry. There is some work underway at NASA that examines whether alternative means of orienting the spacecraft could be used if another gyroscope fails.

There is a firm requirement levied on this mission that the return of CGRO be done in a controlled fashion – either by Space Shuttle or by virtue of being commanded to perform a targeted destructive reentry over non-populated areas. Given the size of the spacecraft (35,000 pounds – 15,876
kilograms), NASA Watch has learned that NASA has calculated that some of it will survive a destructive reentry and that program statistics show a 1/1000 chance of human casualty if the spacecraft were allowed to make an uncontrolled reentry.

In addition to guidance problems with CGRO, there are apparently long standing fuel line anomalies with the spacecraft that affect its ability to fire attitude control thrusters. These issues might also pose flight rule violations for a Space Shuttle visit and capture.

NASA is still examining its options. If no solutions to guidance, navigation, and control issues are found, it is almost certain that NASA will decide avoid the risk of further failures and the prospect of an uncontrolled reentry and will command the spacecraft to de-orbit. Apparently, the prospect of a Shuttle repair mission is simply out of the question.

The de-orbit is being planned for the middle of March 2000 with a projected impact point south east of Hawaii.

CGRO is a NASA cooperative program with participation from Germany, the
Netherlands, the European Space Agency, and the United Kingdom. It was launched aboard the STS-37 Space Shuttle mission in April 1991. The spacecraft was designed to be refueled and serviced in orbit by a future Space Shuttle mission – indeed, according to NASA’s STS-37 press kit, “The Gamma Ray Observatory is the first scientific payload with a refuelable onboard propulsion system.”

According to the STS-37 press kit: “the instruments onboard GRO, with sensitivities 10 times greater than that of earlier instruments, will scan active galaxies for new information on celestial objects. GRO also can detect the very high temperature emissions from the vicinity of stellar black holes, thereby providing evidence for the existence of these exotic objects. GRO observations of diffuse radiation will not only help resolve questions relating to the large scale distribution of matter in the universe, but also about the processes that may have taken place shortly after the universe began in the theoretical energetic explosion or “Big Bang”.

LINKS

° Reboosting the Compton Gamma-Ray Observatory, NASA GSFC

° Compton Gamma Ray Observatory (CGRO), NASA GSFC

° STS-37 Mission, Deployed CGRO in April 1991, NASA KSC

BACKGROUND INFORMATION