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

NASA officials and scientists around the world bid a fond farewell to the prized Compton Gamma Ray Observatory early Sunday as the 9-year old craft made a controlled crash back to Earth. Read our full story.

0620 GMT (2:20 a.m. EDT)

After nine years in space, NASA's Compton Gamma Ray Observatory is no more. The $617 million telescope -- the second of NASA's Great Observatories family -- was deorbited this morning because of its crippled control system. The craft reentered the atmosphere where it was destroyed. Debris was projected to land in a remote zone in the Pacific Ocean southeast of Hawaii.

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).

0618 GMT (2:18 a.m. EDT)

NORAD has announced reentry, confirming the end of CGRO.

0616 GMT (2:16 a.m. EDT)

As the Compton Gamma Ray Observatory mission comes to a fiery end in the Earth's atmosphere, the control team at NASA's Goddard Space Flight Center Mission Operations Room just signed off. NORAD will provide exact reentry information later this morning to verify the precision of the observatory's reentry.

0614 GMT (2:14 a.m. EDT)

NASA projects the remains of Compton Gamma Ray Observatory should be at an altitude of 31 miles (50 km), plummeting to the Pacific Ocean.

0612 GMT (2:12 a.m. EDT)

At this point, the Compton Gamma Ray Observatory is beginning to break up, its structures melting while other larger parts fly off. Splashdown of the expected 6 tons of debris that will survive reentry is predicted in about 7 minutes.

0610 GMT (2:10 a.m. EDT)

CGRO IS GONE. Controllers have lost contact with the Compton Gamma Ray Observatory as it falls toward destruction in the Earth's atmosphere after a successful 9-year mission of scientific research.

0609 GMT (2:09 a.m. EDT)

The solar arrays are beginning to heat up, jumping 40 degrees in the last few seconds. The spacecraft is tumbling.

0608 GMT (2:08 a.m. EDT)

The spacecraft is losing attitude control as it starts tumbling.

0606 GMT (2:06 a.m. EDT)

The spacecraft is now at an altitude of 85 miles. Reentry is about six minutes away with splashdown less than 12 minutes from now.

0604 GMT (2:04 a.m. EDT)

"This is a sad time," the CGRO project scientist Neil Gehrels says as the spacecraft is minutes away from destruction.

0602 GMT (2:02 a.m. EDT)

The final commands have been issued to Compton -- communications antenna pointing and orientation of the satellite -- after 9 years in space.

0600 GMT (2:00 a.m. EDT)

The Compton Gamma Ray Observatory is now on its final half-orbit around Earth. Its current orbit has a low point reported to be 17 miles (28 km) with impact predicted to occur at 2:18:50 a.m. EDT (0618:50 GMT).

0552 GMT (1:52 a.m. EDT)

BURN COMPLETED. The Compton Gamma Ray Observatory has completed its final thruster firing designed to deorbit the craft in a controlled fashion to protect public safety.

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).

0547 GMT (1:47 a.m. EDT)

Now five minutes left in the deorbiting burn. Controllers report acceptable pressures inside the hydrazine fuel tanks supplying the thrusters. Also, the predicted perigee of this orbit will be 31 miles, causing the craft to be destroyed in the atmosphere.

0542 GMT (1:42 a.m. EDT)

NASA reports the Descent Burn No. 4 has now lasted long enough to ensure the doomed Compton Gamma Ray Observatory will crash into the intended remote stretch of the Pacific Ocean southeast of Hawaii. This means the debris fromteh 17-ton craft will not overshoot the safety zone, endangering the public. The burn will continue for another 10 minutes.

0537 GMT (1:37 a.m. EDT)

Now passing the half-way point of this deorbiting maneuver to bring down the Compton Gamma Ray Observatory. Officials report activities are going as planned.

0532 GMT (1:32 a.m. EDT)

Controllers are now reporting that an analysis of data received from Compton during its closest passage above Earth at 12:52 a.m. EDT did heat up the solar arrays and high gain antenna. This was caused by friction between the spacecraft and the upper traces of the atmosphere.

The Descent Burn No. 4 continues to go as expected, with 10 minutes completed and 20 minutes left.

0528 GMT (1:28 a.m. EDT)

POINT OF NO RETURN. The Compton Gamma Ray Observatory has passed the point in this 30-minute-long descent maneuver in which it will no longer remain in space. The resulting orbit, if the burn were to stop now, is too low to survive passage through the atmosphere. The thruster firing will continue until about 1:52 a.m. EDT (0552 GMT), pushing the craft toward a predetermined impact zone in the Pacific Ocean.

SUNDAY, JUNE 4, 2000
0522 GMT (1:22 a.m. EDT)

DESCENT BURN 4 IGNITION! Two Orbit Adjust Thrusters mounted to the Compton Gamma Ray Observatory are now firing again in the final step to deorbit the $617 million telescope, which was launched into space on April 5, 1991 aboard shuttle Atlantis.

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).

0518 GMT (1:18 a.m. EDT)

Contact with Compton Gamma Ray Observatory has switched one of NASA's Tracking and Data Relay Satellites to another. All systems are reported "go" for the final descent burn due to begin at 1:22:21 a.m. EDT (0622:21 GMT) and last 30 minutes.

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.

0500 GMT (1:00 a.m. EDT)

A poll of the flight control team in the Mission Operations Room at Goddard Space Flight Center has been completed and all systems are "go" for Descent No. 4 ignition at 1:22 a.m. EDT (0622 GMT) to deorbit the Compton Gamma Ray Observatory.

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)

PERIGEE REACHED. CGRO has passed the lowest point of its orbit at an altitude of about 93 miles (150 km), the closest the observatory will come to the planet before reentry. This point is called the perigee of the orbit. The perigee was lowered from 155 miles with the descent burn No. 3 that occurred just under an hour ago.

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.

0444 GMT (12:44 a.m. EDT)

Communications with the Compton Gamma Ray Observatory have been lost temporarily as the link is switched between two orbiting Tracking and Data Relay Satellites. The current altitude is 107.5 miles, heading downward toward perigee before the craft loops back higher in preparation for the final descent maneuver.

0434 GMT (12:34 a.m. EDT)

The CRGO spacecraft is cruising closer to Earth in its current orbit. In about 18 minutes, the satellite will reach perigee -- the lowest point of this orbit. The perigee will bring CGRO within 93 miles or so of Earth, low enough for the observatory to possibly experience some heating due to friction with the upper fringes of the atmosphere.

0417 GMT (12:17 a.m. EDT)

BURN COMPLETE! The third of four firings to deorbit the Compton Gamma Ray Observatory has been completed successfully, and controllers are reporting all systems aboard the craft are normal. CGRO will now set up for the final descent maneuver at 1:22 a.m. EDT (0522 GMT) to send the satellite beyond the point return and plummeting into the atmosphere to burn up.

0406 GMT (12:06 a.m. EDT)

Now 10 minutes into this planned 21-minute firing of Compton's two orbit adjust thrusters to slow the craft's velocity slightly, thus lowering the orbital altitude to prepare for this morning's reentry into the Pacific Ocean. No problems have been reported by officials at the Mission Operations Room.

0356 GMT (11:56 p.m. EDT)

DESCENT BURN 3 IGNITION! The two Orbit Adjust Thrusters aboard NASA's Compton Gamma Ray Observatory are now firing to continue the craft's journey back to Earth. Two other descent burns were performed earlier this week to lower the craft's orbit closer to the atmosphere.

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)

Ground controllers are gathered in the Mission Operations Room at NASA's Goddard Space Flight Center in Greenbelt, Maryland, tonight to deorbit the Compton Gamma Ray Observatory after its history-making nine-year science endeavor.

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
0500 GMT (1:00 a.m. EDT)

On the brink of destruction, NASA's Compton Gamma Ray Observatory is circling Earth for the final times today as controllers prepare to send the crippled craft crashing into Pacific Ocean early Sunday.

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
1830 GMT (2:30 p.m. EDT)

Controllers of the doomed Compton Gamma Ray Observatory have altered the timing of the final two descent burns scheduled on Sunday to deorbit the 17-ton telescope. The change essentially moves the two maneuvers 90 minutes earlier than originally planned to adjust the predicted area where debris from the craft will fall in the Pacific Ocean.

"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.

0340 GMT (11:40 p.m. EDT)

NASA completed the second in a series of four descent burns on Wednesday to push the Compton Gamma Ray Observatory back to Earth.

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
0330 GMT (11:30 p.m. EDT)

NASA's Compton Gamma Ray Observatory took the first of four steps towards destruction on Tuesday by firing its thrusters to move into a lower orbit. The craft remains condemned to make a suicidal plunge into the atmosphere early Sunday morning.

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
0100 GMT (9:00 p.m. EDT)

The Compton Gamma Ray Observatory reentry team successfully completed an engineering test firing of the craft's thrusters on Sunday. The thrusters will be used during deorbit maneuvers later in the week to push the bus-sized telescope back into Earth's atmosphere.

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

Launched nine years ago to study the most explosive forces in the universe, NASA's Compton Gamma Ray Observatory will make a controlled plunge into Earth's atmosphere next Sunday, ending a highly successful mission.

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.