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Compton Gamma Ray Observatory Safely Returns to Earth

05 June 2000

NASA's Compton Gamma Ray Observatory re-entered the Earth's atmosphere at approximately 2:10 a.m. EDT on June 4, according to calculations made by controllers at NASA's Goddard Space Flight Center, Greenbelt, Md., in coordination with the U.S. Space Command's Control Center.

As planned, pieces of the observatory that survived the re-entry landed in the Pacific Ocean approximately 2,400 miles (3,862 km) southeast of Hawaii.

The fourth and final burn needed to re-enter NASA's Compton Gamma Ray Observatory was initiated at 1:22 a.m. EDT on June 4. Compton's Attitude Control thrusters and Orbit Adjust thrusters were fired for 30 minutes.

After the failure of one of Compton's three gyroscopes, NASA decided to bring the satellite back via a controlled reentry. NASA determined that it was much safer to bring the satellite back now to safe guard against further system failures in the spacecraft that might hinder a controlled reentry.

CGRO de-orbit diagram
The predicted CGRO reentry footprint. Pieces which survived re-entry landed in the Pacific Ocean 3,862 km (2,400 miles) southeast of Hawaii.

"This was a bittersweet day for NASA," said Al Diaz, Director of the Goddard Space Flight Center. "The end of the Compton Gamma Ray Observatory mission marks the end of a remarkable spacecraft. Compton left a legacy of outstanding science and revolutionized our knowledge of the gamma ray sky. And while no one at NASA is ever happy to see the end of a science mission, prolonging this mission would have posed an unacceptable and increasing risk to human life. This was an extraordinarily complex task, involving both operations and engineering proficiency. I'm proud of this team and the job they did. They understood the significance of this task, and they performed it flawlessly."

In a message to the team of engineers and scientists who have worked on Compton, Dr. Neil Gehrels, CGRO project scientist, said "It is with great sadness that I report the re-entry of the CGRO mission. The operation was nominal, with our fine spacecraft performing perfectly to the end. I have tremendously enjoyed working with all of you on this fantastic mission! After a difficult and emotional night, I can not find the right words to tell you ...[sic] The project is not over. We have significant work to do over the next ~2 years to finish the analysis of the data and complete its archiving." Scientists anticipate a continued wealth of information to come from the observations which Compton performed.

Compton spent nine productive years in orbit. Engineers began planning for the Observatory's reentry in April 1999 when gyroscope #3 first began experiencing problems. By the time the gyro actually failed in December 1999, engineers had devised a number of deorbit scenarios. Engineers at Goddard, assisted by their counterparts at the Johnson Space Center in Houston, spent the past five months designing a reentry plan to safely deorbit the CGRO spacecraft.

A total of four burns were used to gradually lower the spacecraft's orbit. The first re-entry burn was conducted on May 30, and a second burn on May 31. At midnight on June 4, controllers fired CGRO's primary thrusters for a third time bringing spacecraft's low point to within 92 miles (148 km) of the Earth's surface.

NASA and international space agencies plan several upcoming missions to continue where Compton left off. The Gamma-ray Large Area Space Telescope (GLAST*) is a proposed new high-energy gamma-ray mission to identify and study nature's highest energy particle accelerators. GLAST will be 30 times more sensitive than the Energetic Gamma Ray Experiment Telescope (EGRET) onboard the Compton Gamma Ray Observatory. Swift will be the first mission to focus on studying the newly-discovered afterglow from gamma ray bursts. Swift's rapid repointing capability will enable high-precision X-ray and optical positions to be determined and relayed to the ground for use by a network of dedicated observers at large telescopes.

The Compton Gamma Ray Observatory was the second of NASA's Great Observatories and the gamma-ray equivalent to the Hubble Space Telescope and the Chandra X-ray Observatory. Compton was launched aboard the Space Shuttle Atlantis in April 1991, and at 17 tons, was the largest astrophysical payload ever flown at that time.

*GLAST launched on June 11, 2008.

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