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They fired the braking engines off the coast of southern Africa, and kept them on as the satellite streaked northeast across the Indian Ocean, Malaysia and the Philippine island of Luzon.
Out over the western and central Pacific, the brakes fired on NASA's doomed Compton Gamma Ray Observatory, for 30 minutes over thousands of miles, as if on a 17-ton truck slowing down a mountain road.
Finally, just before 2 a.m. yesterday, the engines stopped, and the venerable spacecraft that NASA has been steering toward its end all week began its plunge to a fiery dismemberment in the atmosphere.
After nine years in space and 51,658 faithful orbits of Earth, Compton, one of NASA's three great flying observatories, plunged to the ocean, showering about six tons of charred and broken debris.
Its end came as it "hit the wall" of the upper atmosphere, started to tumble, and controllers at the Goddard Space Flight Center in Greenbelt announced that its temperature was soaring and they had "lost lock," or communication.
One NASA scientist long associated with the satellite said it was hard to listen to its "death screams."
When it was over, and a special Air Force tracking plane had confirmed its demise, weary members of the Goddard reentry team stood at their computers, pulled off their headsets and shook hands.
"It's been real," remarked one.
"It's been surreal," said another.
Compton, which carried four special instruments that revolutionized observation of the mysterious energy of invisible gamma rays, became endangered in December when one of its controlling gyroscopes failed.
NASA, which had launched the $670 million satellite in 1991 aboard the space shuttle Atlantis, became concerned that if another gyro failed, the agency could lose control of Compton and it might crash to Earth anywhere.
So the space agency decided, over the strong objections of some of its own scientists and many outside experts, to kill Compton before it veered out of control. Critics argued that Compton could be reentered safely later even if more gyros went out and that the satellite was too precious to lose.
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But NASA stuck to its decision. In a series of engine firings--two last week and two early yesterday--Goddard's "deorbit" team gradually lowered the satellite until it reached the atmosphere, where it would be destroyed.
Yesterday's first engine "burn" began about 11:56 p.m. Saturday and continued for 21 minutes and 28 seconds, lowering the satellite's orbit perigee, or closest passage to Earth, from about 186 miles to about 96 miles.
The crucial firing, though, was the last, and the longest, which was to lower the perigee to where the craft would make its dive into the atmosphere. At 1:22 a.m., reentry conductor Tom Quinn counted down to "mark," and the order to fire Compton's orbit adjust thrusters was sent.
From Africa to the mid-Pacific, the engines fired, until they shut down at 1:52 a.m. Things then grew even more tense as the controllers, having fired the last burn, waited to see whether it worked.
Quinn began calling out the satellite's descending altitude in kilometers--"240 . . . 225 . . . 210 . . . 151 . . . 138 . . . 124." He also began nervously polling other controllers for signs, in temperature or alignment, that the spacecraft was undergoing reentry: "Hey, see anything yet?"
"I think it's going to start to go down pretty quickly here," he said at one point. "We ramp down very fast. . . . From 138 to 80 takes about five minutes. We're about to hit the wall."
Suddenly, another controller in the room radioed that the spacecraft's temperature had "jumped up about 40 degrees in the last 10 seconds." A second quickly reported that Compton had started tumbling. Then a third: "We've lost lock."
And that was it. The spacecraft's final moments were played out in silence, its communication with the ground gone for good. The controllers began to push their chairs back from their consoles and shake hands.
Quinn went around the room and asked everyone for a final comment. "We've pretty much completed our side of the mission," he said, " . . . over and out."
Afterward, Neil Gehrels, the Compton project scientist who narrated the reentry event, said it had been agony to watch.
"Different emotions in different rooms," he said. Whereas the controllers were glad the reentry had gone well, "I was shedding a tear in my room, as a scientist having lost the spacecraft."
"I'm disappointed that we reentered it," he said. "I think NASA headquarters folks had a difficult decision to make. I'm not going to second-guess that they made the right decision. They certainly had a lot more things than just science to think about.
"We had a lot of tough debates between the management and the scientists and the engineers, to try to figure out what the right course of action was. There were certainly differences of opinion."
Mansoor Ahmed, the reentry mission manager, noted: "Some of our engineers who are part of this team were actually involved in building this thing and designing this thing. For them, it's like putting their own child to rest.
"But they were proud of that child," he said. "The spacecraft delivered over and above anything that anyone ever expected."