BY JUSTIN RAY
June 4, 2000 -- Follow the Compton Gamma Ray Observatory's reentry into the Earth's atmosphere to end its nine-year mission. Reload this page for the very latest on the deorbiting.
SUNDAY, JUNE 4, 2000
0620 GMT (2:20 a.m. EDT)
During its time in orbit, the spacecraft explored violent and invisible gamma ray bursts, antimatter fountains, and particle jets streaming millions of miles per hour away from black holes. The telescope detected more than 400 gamma ray sources, 10 times more than were previously known. Compton recorded over 2500 gamma ray bursts; before Compton, only about 300 had been detected, and only one third of those had known positions. Thousands of scientific papers have been published based on Compton observations so far. On average, a Compton-specific publication appears in the scientific literature every other day.
A NASA press conference is scheduled for 6 a.m. EDT (1000 GMT).
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The 17-ton craft will hit the atmosphere at about 2:11 a.m. EDT (0611 GMT) at an angle of two degrees. The telescope will begin to heat up and break apart between 52 and 44 miles above Earth. Impact of about 6-tons of debris from CGRO is expected at 2:20 a.m. EDT (0620 GMT).
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The Descent Burn No. 4 continues to go as expected, with 10 minutes completed and 20 minutes left.
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SUNDAY, JUNE 4, 2000
This engine firing will last 30 minutes to further reduce CGRO's orbital velocity, and lower its orbit altitude into the atmosphere. The craft will hit the top of the atmosphere when it reaches perigee, with reentry expected at 2:11 a.m. EDT (0611 GMT) and impact in the Pacific at 2:20 a.m. EDT (0620 GMT).
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The end of the CGRO mission is now about 62 minutes away with splashdown in the Pacific targeted to occur at 2:20 a.m. EDT (0620 GMT) southeast of Hawaii.
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A few minutes before the maneuver, the observatory will be reoriented to point the onboard Orbit Adjust Thrusters in the direction of the travel of the satellite. The 30-minute firing is done to slow the craft, lowering its orbit into the atmosphere.
0452 GMT (12:52 a.m. EDT)
The craft will now move higher -- away from Earth -- over the next few minutes before the final firing of the onboard thrusters is conducted at 1:22 a.m. EDT to push the craft toward a fiery reentry into the atmosphere. Impact into the Pacific Ocean is expected about one hour later.
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The current burn now underway will last 21 minutes, dropping the low point of CGRO's orbit from 155 miles to 93 miles. The two orbit adjust thrusters being used are engines capable of delivering 100 pounds of thrust each.
Once this maneuver is completed, the observatory will make a final orbit of Earth before the fourth and last descent burn takes place.
0335 GMT (11:35 p.m. EDT)
Clocks are counting down to the upcoming Descent Burn No. 3 that will begin at 11:56 p.m. EDT (0356 GMT). The maneuver will further lower the 17-ton telescope's orbit around Earth. A final nudge will start at 1:22 a.m. EDT (0522 GMT) to drive the craft into the atmosphere to burn up.
SATURDAY, JUNE 3, 2000
The 17-ton telescope was launched in 1991 from space shuttle Atlantis to study the some of the most violent objects in the universe. A sister to the Hubble Space Telescope and Chandra X-ray Observatory, CGRO is being deorbited after NASA became concerned the satellite's control system is no longer reliable.
One of the telescope's three stabilizing gyroscopes malfunctioned in December and NASA managers were worried additional failures could leave the $617 million craft uncontrollable.
Since about six tons of debris from the craft will survive the kamikaze plunge into Earth's atmosphere, the space agency decided in March to bring it down now, while they still have enough control to ensure a harmless breakup over the Pacific Ocean.
The final moments of CGRO's life will unfold this way: At 12:00 a.m. EDT Sunday (0400 GMT), the satellite's thrusters will be fired to further lower the orbit closer to Earth. Two such firings were performed earlier this week. One orbit later, at 1:30 a.m. EDT (0530 GMT), another thruster burn will occur to drive the telescope into the atmosphere, beginning a fiery end to a highly successful 9-year mission.
Ground controllers at NASA's Goddard Space Flight Center expect to indications of the rising temperatures aboard the CGRO as it starts hitting the atmosphere, followed by the satellite beginning to lose attitude control and communications will stop once antennas are broken away.
Spaceflight Now will provide continuous reports throughout the final two burns and reentry in this Mission Status Center. In addition, we shall offer a live streaming video broadcast of NASA Television beginning at 11:30 p.m. EDT tonight (0330 GMT Sunday). Coverage will include science highlights, re-entry animation and commentary on the re-entry activities by Dr. Neil Gehrels, CGRO project scientist.
THURSDAY, JUNE 1, 2000
"The latest prediction has Compton's orbit tracks moving west. This migration now makes the eastern most track the safest re-entry opportunity. This track is now our primary re-entry track," said Mansoor Ahmed, CGRO mission manager at the Goddard Space Flight Center.
So the next firing of CGRO's thrusters will occur on Sunday at 12:00 a.m. EDT (0400 GMT). This maneuver will decrease the low point of the observatory's orbit from the current 155 miles to 93 miles. The final burn will begin at 1:30 a.m. EDT (0530 GMT) to send the 9-year old spacecraft plunging into the fiery atmosphere to burn up. Impact of about six tons of debris that will survive reentry is expected at approximately 2:20 a.m. EDT (0620 GMT) in a remote stretch of the Pacific southeast of Hawaii.
NASA successfully completed the second descent burn on Wednesday evening, lowering the orbit's perigee from 226 miles to 155 miles.
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The firing of the craft's thrusters began at 10:36 p.m. EDT (0236 GMT Thursday) and lasted 26 minutes.
"Everything went as planned," said NASA spokeswoman Nancy Neal.
It will take a few hours before controllers are able to determine the 17-ton telescope's new orbit after the maneuver. The burn was designed to sink the orbit's lowest point, called perigee, to around 155 miles or so.
Yesterday's first deorbiting burn lowered the perigee from 316 to 226 miles. That was about 10 miles higher than originally planned, but was within the allowable margin of error, officials said.
The next burn is scheduled for early Sunday at 1:37 a.m. EDT (0537 GMT), reducing the perigee to about 93 miles. A final maneuver will occur an orbit later at 3:05 a.m. EDT (0705 GMT) to drive the telescope into the atmosphere to burn up.
Approximately six tons of debris is expected to survive reentry and fall harmlessly in the Pacific Ocean. The debris field is projected to extend 16 miles wide by 962 miles long.
WEDNESDAY, MAY 31, 2000
Ground controllers at the Goddard Space Flight Center in Greenbelt, Maryland, reported Tuesday's maneuver went according to plan, nudging the 17-ton telescope about 99 miles closer to Earth at the lowest point of its orbit, called perigee.
"Everything performed nominal," said NASA spokeswoman Nancy Neal.
The 23-minute-long engine firing began at 9:51 p.m. EDT (0151 GMT Wednesday).
NASA said it would need about three hours, or two orbits, to track the satellite to determine the new orbit with precision. However, indications are the orbit's perigee was reduced from 316 miles to 217 miles.
"They are pretty sure they are close to where they need to be," Neal said.
The next step in Compton's reentry plan will occur on Wednesday night when the second deorbit burn is scheduled. The firing -- to start at 10:41 p.m. EDT (0241 GMT Thursday) -- is designed to lower the CGRO's perigee to 155 miles.
Two further maneuvers are planned early Sunday to drive the spacecraft back to Earth, scattering wreckage in a remote stretch of the Pacific Ocean, about 2,500 miles southeast of Hawaii.
The observatory is being removed from orbit this week because the 9-year-old craft has suffered a partial failure of its control system. NASA is worried that additional problems with the onboard gyroscopes would mean the massive satellite could make an uncontrolled reentry in the future, threatening lives and property since some 6 tons of debris will survive the fiery trek through the atmosphere.
While the control system is still working, top NASA officials made the controversial decision earlier this spring to end the highly successful CGRO mission with a controlled deorbiting to protect public safety.
You can read our full story on the reentry for more information.
MONDAY, MAY 29, 2000
The procedure began at 3:44 p.m. EDT (1944 GMT), with four separate firings of CGRO's attitude control thrusters and one firing of orbit adjust thruster occurring between 4 and 4:30 p.m EDT (2000-2030 GMT).
NASA reports the thrusters "behaved normally and as expected" during the test, which proved the thrusters were in good health. In addition, the commanding path to the thrusters was verified to be working properly.
Meanwhile, the observatory's four science instruments have been switched to their low power mode, and the craft is currently in a parking attitude.
The next major event in advance of next Sunday's reentry into the atmosphere will be the first orbit lowering maneuver on Tuesday at 9:54 p.m. EDT (0154 GMT Wednesday).
SUNDAY, MAY 28, 2000
The countdown to reentry begins today as ground controllers conduct a test firing of the satellite's thruster engines. This engineering test will be used to gather detailed data on the performance of the propulsion system, and help engineers refine their calculations for the four future descent maneuvers needed to drop CGRO from orbit.
Also, the four science instruments will be shut down.
The deorbiting was ordered earlier this year by top NASA officials because the 33,000-pound probe has experienced a partial failure of its control system. Since chucks of the CGRO will likely survive the reentry, a guided entry into a remote area of the Pacific Ocean, approximately 2,500 miles southeast of Hawaii, is necessary to ensure the public safety.
The first of four 30-minute rocket firings is planned for Tuesday at 9:54 p.m. EDT (0254 GMT Wednesday), which will lower the perigee, or low point, of GRO's orbit from 316 miles to 217 miles.
Later burns are planned on Wednesday and two on entry day -- Sunday, June 4. Impact is expected at around 4:05 a.m. EDT (0805 GMT).
Carried aloft in April 1991 aboard space shuttle Atlantis, CGRO is the second of NASA's four "Great Observatories" along with Hubble, Chandra and the future SIRTF.
Spaceflight Now will provide updates throughout the week and live coverage Sunday morning as the Compton Gamma Ray Observatory hurdles back to Earth. In addition you can read our full story on the reentry.
Animation shows how the Compton Gamma Ray Observatory is believed to have broke apart during its reentry into the Earth's atmosphere.
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CGRO DEORBIT STORY