NASA's Compton Orbiter Begins Controlled Re-entry...05/31/00

WASHINGTON, (Reuters) - NASA's ailing, bus-sized Compton Gamma Ray Observatory satellite has started its controlled descent with an expected Pacific Ocean splashdown on Sunday, space agency officials said on Wednesday.

The first of four firings of the big spacecraft's thrusters occurred late on Tuesday, pushing the satellite down from an orbiting altitude of about 317 miles (510 km) to about 218 miles (350 km).

Another thruster burn was scheduled for late on Wednesday, with two more set for Sunday's early hours, bringing the craft down in the eastern Pacific Ocean. Debris from the craft could be strewn along a thin area that could be 2,550 miles (4,100 km) long, stretching from southeast of Hawaii toward the northern coast of South America.

The 33,000-pound (14,970 kg) spacecraft has been operating with only two of its three steadying gyroscopes since December 1999, and NASA officials feared that if it lost another gyroscope and then came down on its own, there would have been a one-in-1,000 chance that someone might be killed by it.

So the National Aeronautics and Space Administration (NASA) opted for controlled reentry, which would be spectacular to see, if anyone were around to see it. Airplanes and ships have been warned to stay out of the expected debris area as a safety measure.

However, NASA is working with the U.S. Defence Department to have some aircraft photographing the descent, according to Neil Gehrels, Compton's chief scientist since its launch in 1991.

TITANIUM BOLTS STREAKING DOWN

What will survive the passage through the atmosphere will range from pebble-sized titanium bolts -- moving at bullet speed -- to chunks of spacecraft weighing hundreds of pounds (kg), Gehrels said in a telephone interview.

"I think it would be a spectacular appearance, with many pieces coming down, each burning up as it's coming in," Gehrels said.

Gehrels and other Compton scientists will monitor the reentry from a control centre at Goddard, where the spacecraft will be providing data until it starts to break up in the atmosphere.

"I absolutely will be saddened tremendously to see it come in," Gehrels said of the spacecraft's impending demise. "It's been a great time. It's a beautiful mission, we hate to see it come in."

But he said he agreed with NASA's decision to end the mission over possible safety concerns.

The Compton observatory studied the universe through gamma rays, which revealed many objects which were not evident in rays of light or radio waves. This meant that Compton discovered celestial objects and phenomena.

Since its launch the spacecraft has peered into the violent world of gamma ray bursts -- powerful daily blasts that last only a matter of seconds -- anti-matter fountains and jets of particles streaming away from black holes.

"The objects in the sky emit electromagnetic radiation, not just in the visible band (of light), but across the entire spectrum," Gehrels said. "Some objects don't even make visible light. You don't even know they exist until you look in gamma rays."

Three new missions looking at the universe in gamma rays -- one European, two from NASA -- are expected to begin operating in the next five years, Gehrels said.

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