The Pacific Ocean impact zone for what NASA tags a "controlled reentry" of the observatory is approximately 16 miles by 962 miles (26 kilometers by 1,548 kilometers). That footprint is some 1,100 statute miles (1,770 kilometers) from any land area, said Martin Frederick, associate chief at Goddard's Navigation and Control Center. "We expect the debris field to be much smaller than that. It's not an exact science, but we think somewhere between 30 to 40 major pieces will come down," he said.

Weiler said debris, from small titanium bolts to flaming chunks tipping the scale at a ton, will make their way down to the ground.

The decision to direct the satellite back to Earth is necessary, Weiler said. Compton is one gyroscope failure away from total loss of spacecraft control, he said. In mid-December of last year one of the observatory's three gyroscopes failed, leaving the spacecraft with only two working ones. A program requirement written nine years ago, prior to the observatory's launch in April 1991, dictates that such a condition would spark immediate plans to deorbit the craft, Weiler said.

If there is no control of Compton due to total gyroscope loss, there is a one in 1,000 chance of a human fatality. With two gyros operating, there is a one in 29 million chance; with one working gyro, there is a one in 4 million chance, NASA officials said.

"That one in 1,000 gets worse every day that passes," Weiler said. "No matter how many science papers are lost due to bringing in the still-operating observatory, compared to increasing risk to human life, [it] is not a trade-off that can be made."

"Safety has to come first," he said.

Not everyone at NASA is happy about the decision to dump one of the space agency's star observatories.

"Astronomers throughout the world will be quite disappointed with this decision. I'm personally, profoundly disappointed," said Neil Gehrels, Goddard project scientist for the observatory. While a loss, scientists must celebrate the great scientific success of the mission, he said.

For the last nine years, Comptons duties have included pioneering look-sees at the violent, but invisible world of gamma-ray bursts. The probe has also relayed important data on black holes, quasars and other mysterious phenomena, far surpassing NASA expectations for a two- to five-year mission.

Gehrels said that Compton has made incredible finds, performing the first survey of the gamma-ray sky. Some 2,000 flashes of gamma rays have been detected by the observatory. "Those are the largest explosions taking place anywhere in the universe since the Big Bang. It's been a remarkable discovery," he said.

Despite Compton's forthcoming swan song, the science satellite will remain on active duty. "We're not done taking science. We have 62 days, 29 hours and 40 minutes left. We're going to make the best use of every one of those minutes," Gehrels said.

Goddard Space Flight Center engineers have already begun practicing observatory deorbit procedures. A team of 15 to 20 people are fine-tuning techniques to bring the spacecraft down to Earth in a controlled way using a set of on-board thrusters, said Thomas Quinn, Goddard's lead engineer working on the reentry plans.

"Everyone should be able to gain from this effort. This doesn't happen all the time. We might as well get as much out of this in terms of planning for future NASA missions," he said.