SKYWATCH ALERT
NASA Decides Observatory Fate
From: Ndunlks@aol.com
By PAUL RECER
.c The Associated Press
WASHINGTON (AP) - An ailing space observatory, already past its expected lifetime, will be crashed deliberately into a remote area of the Pacific Ocean to avoid a risk that the 17-ton spacecraft might smash into a populated area, NASA said Friday.
A series of rocket firings starting in May will lower the orbit of the Compton Gamma-Ray Observatory, and a final rocket thrust will send the craft into its fiery plunge on June 3. The crash is expected to scatter debris along a corridor of ocean 2,547 miles long and 16 miles wide.
The impact lane starts about 2,500 miles southeast of Hawaii, crosses the equator in a northwest to southeast angle and ends about 680 miles south of the Galapagos Islands.
Compton's return to Earth will occur in darkness and is not expected to be observed, although military radar may follow the craft for part of its long fall. The Compton will be the largest spacecraft every brought down in a deliberate, controlled crash.
The $670 million Compton was launched April 5, 1991, with an expected mission life of five years. It carries four instruments that can detect natural gamma rays, a powerful form of invisible light that bathes the universe. In nine years of operation, the craft detected more than 400 gamma ray sources and more than 2,500 gamma ray bursts, a powerful type of celestial explosion that still puzzles scientists.
Scientists still use the spacecraft, but Ed Weiler, NASA's chief scientist, decided it was too risky to keep the heavyweight craft in orbit.
``This was a difficult decision, but the safest option is to bring it down in June,'' said Ed Weiler, NASA's chief scientist.
If NASA did nothing and let the craft eventually fall from natural forces, he said, there was one chance in 1,000 for a human fatality on Earth. The Compton's 317-mile orbit carries it craft over some of the most populated areas of the world, including Mexico City, Bangkok and Miami.
A controlled crash, with remaining onboard guidance and control equipment working, would lower the risk of a fatality to about one in 29 million. Even with the loss of the primary control system, he said, a controlled crash using a back-up system would keep the risk at about one in 4.1 million.
Weiler, who made the end-of-mission decision, said some potential astronomy research will be lost due to the controlled crash, but he felt safety was more important.
``How much science is worth the risk of even one human life?'' he asked at a news conference.
Weiler said NASA engineers began planning a controlled crash for Compton in February 1999, when one of the craft's three gyroscopes began ailing. When the gyro failed altogether last year, engineers starting making specific end-of-mission plans.
The spacecraft uses gyroscopes to control its position in space. It is capable of operating with just the two gyros, but one more failure and the craft would be more difficult to control.
By planning the spacecraft suicide while the gyros are still working, engineers can more precisely target the impact zone.
Weiler said other options were considered, including sending astronauts to the spacecraft to attempt repairs. But none was considered worth the risk, he said.
If NASA did nothing, the Compton could remain in orbit for about 11 more years. As it ages, however, the chances increase that equipment will fail and engineers would lose the ability to control its re-entry. This happened with the Skylab, a 78-ton abandoned U.S. space station that crashed in 1979. Debris fell harmlessly into the Indian Ocean and across a remote section of western Australia.
Engineers expect the Compton to break up as it smashes into the upper atmosphere from space. The heat of friction will vaporize many parts, turning most into pea-size grains. But Weiler said some metal chunks of up to a ton in weight could hit the ocean.
Among such parts are six 1,800-pound aluminum I-beams that make up the spacecraft's spine.
The end-of-mission plan calls for all science studies to stop on May 26. Rocket firings on May 31 and June 1 will lower the orbit from 317 miles to 156 miles. On June 3, a third rocket thrust will drop the craft to a 93-mile orbit, and a final rocket firing 90 minutes later will send Compton into a long, fiery fall toward the Pacific Ocean.
On the Net: NASA Web site: http://www.nasa.gov
Gamma rays Web site:
http://pao.gsfc.nasa.gov/gsfc/spacesci/structure/cgro.htm
AP-NY-03-24-00 1707EST