http://www.spaceviews.com/2000/05/08a.html Scientists Ask NASA to Keep Compton in Orbit Published: 2000 May 8 4:07 am ET (0807 UT) NASA's plans to deorbit the Compton Gamma Ray Observatory next month could prove riskier than simply leaving it in orbit, a small but growing number of scientists and other experts now believe. Indeed, the existence of backup plans to control the spacecraft should its gyroscopes fail have left many in the high-energy astronomy community "mystified" as to why NASA would bring the otherwise functional spacecraft to a fiery death, while NASA documents obtained by SpaceViews hint that the decision to deorbit the spacecraft may have been made months before. The space agency announced March 24 that it had decided to deorbit Compton on June 3 over the eastern Pacific Ocean, after a series of maneuvers starting in late May that will lower the spacecraft's orbit. The justification for the decision was based on safety after one of the three gyroscopes on Compton failed in December. The spacecraft can maintain proper attitude control with two gyros, but an additional failure, NASA officials claimed, could cause the spacecraft to lose attitude control and eventually burn up uncontrollably in the Earth's atmosphere. The large size of the spacecraft -- some 15,000 kg (33,000 lbs.) -- means that large pieces of it would survive any reentry and crash to Earth, endangering people. NASA estimated a 1-in-1000 chance that an uncontrolled reentry could cause a human casualty. "There can be no tradeoff between science and human safety," said Ed Weiler, NASA associate administrator for space science, at the March 24 press conference announcing the decision. To bring the spacecraft down safely, NASA has planned a series of four thruster burns that will lower the spacecraft's orbit starting May 31. After a second burn June 1, a pair of maneuvers June 3 will plunge Compton into the atmosphere above the eastern Pacific, raining debris over an uninhabited section of ocean. The problem with this plan, according to one scientist, is that it fails to take into account the effects of a solar flare. Jim Ryan, a professor of physics at the University of New Hampshire and a co-investigator on one of Compton's instruments, notes that a solar flare can "instantaneously" increase the density of the upper atmosphere by a factor of ten. Such an increase, if it took place late in the Compton reentry procedure, could have a dramatic effect on the reentry plans. "What was controlled becomes uncontrolled," said Ryan. Ryan said that NASA planning for the reentry has not taken this threat into account, and a review of planning documents obtained by SpaceViews shows no explicit mention of the deleterious effects of a solar flare. While the exact probability for a solar flare during the critical reentry procedure wasn't immediately available, Ryan notes the relative probability is highest now as the Sun is at its peak in its 11-year activity cycle. "You'd like to wait until the Sun quiets down," he said, which won't be until at least 2002. Moreover, Compton advocates note that there's no urgent need to deorbit Compton. Backup plans exist for controlling the spacecraft even if all its remaining gyros fail. In that case, the casualty odds from a reentry drop to 1-in-4 million, worse than the 1-in-29 million from a two-gyro reentry but still far better than the 1-in-1000 odds for an uncontrolled reentry. According to a NASA presentation titled "CGRO Reentry - Code S Decision Review", a zero-gyro mode would no longer make a gyro failure critical. "If the zero gyro mode is determined to be feasible, then a gyro failure is no longer a critical failure for controlled reentry," according to the document. The same document notes that NASA may be concerned with other aspects of the spacecraft, though. "The longer we operate, the more likely it is that we could lose a subsystem critical for controlled reentry," the document stated. Still, Ryan said that he and other scientists, as well as even "senior-level" officials at NASA Headquarters, are "mystified at the origin and logic of the decision" to deorbit Compton next month, given the risks and the alternatives. In addition to the zero-gyro mode, NASA looked at other alternatives, including a servicing or retrieval mission by the shuttle, developing software that would allow alternate means of controlling the spacecraft and thus "save" the gyros for reentry, and boosting the spacecraft to a higher orbit to delay reentry. Another NASA presentation obtained by SpaceViews hints that the decision to deorbit Compton was made at the highest levels of NASA management as early as mid-December 1999, just days after the gyro failure. The document quoted Weiler as saying, "For the Record, the decision has been made to reenter..." at a meeting at NASA Headquarters on December 17, but provides no additional information on the context within which that statement was made. Ryan admits there is no "immediate, direct way" to overturn NASA's decision to deorbit Compton, so he and other scientists are turning to the public and to Congress to increase pressure on NASA to subject their decision to an independent, non-NASA review. A "Dear Colleague" letter drafted for members of the High Energy Astrophysics Division of the American Astronomical Society called on astronomers to contact Congress and request they put pressure on NASA to review its decision. "It would be a national tragedy to unnecessarily destroy one of America's Great Observatories," the letter stated. Messages sent by SpaceViews to Compton project officials at NASA's Goddard Space Flight Center on Friday requesting comment on the Compton deorbiting plans were not returned. Related Web site: http://pao.gsfc.nasa.gov/gsfc/spacesci/structure/reentry.htm ________________________________________________________________ YOU'RE PAYING TOO MUCH FOR THE INTERNET! Juno now offers FREE Internet Access! Try it today - there's no risk! For your FREE software, visit: http://dl.www.juno.com/get/tagj. ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www2.satellite.eu.org/seesat/seesatindex.html
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