On Sunday, a unique space telescope that looks like a giant metallic fly will plunge into the Pacific Ocean, ending a nine-year run of cosmic discoveries.
But behind its fiery descent looms an arcane but important question for the National Aeronautics and Space Administration: When is the right time to retire a spacecraft?
In an age of tight budgets, scientists want to get as much use as they can out of any observatory they send aloft. But NASA administrators also have to bring a craft down at the end of its life cycle before it risks - however remotely - plunging to earth on its own, becoming a safety hazard.
Perhaps at no time in recent years has the tension between safety and continuing discovery been more palpable than over the Compton Gamma Ray Observatory (CGRO), a $700 million spacecraft that will end its laudable run this weekend after months of planning.
NASA administrators believe it is essential to bring the craft down now. But scientists involved with the observatory argue it has several good years left - at a critical time for solar research.
"I think the decision process was fair," says Neil Gehrels, the CGRO project scientist. "But it was a hard, difficult decision that NASA had to make."
To a certain extent, CGRO is a victim of its own success. The 33,000-pound spacecraft with two buglike eyes was designed for a five-year mission. But it has now been in orbit nine years, during which it has contributed to several important discoveries.
One is the origins of mysterious gamma-ray bursts, one of the most violent processes in the universe. Another is the discovery of a new class of celestial objects called "blazars," which have nothing to do with sport coats but are beams of energy from the center of distant galaxies.
Perhaps most important, CGRO is the only spacecraft now in orbit that measures gamma rays, which scientists use to understand "solar flares." These brief energy surges from the sun can cause power outages and communications blackouts.
The sun's activity tends to follow an 11-year cycle, which is expected to peak within the next year. Scientists had hoped to use CGRO to help predict and better understand the flares during this period of heightened activity.
"We think we could have extended the mission for several more years without compromising any safety," says Mark McConnell, an investigator on one of Compton's projects.
Since its inception, engineers had planned to end the mission by directing the satellite down to a specific point on Earth.
In December, however, one of Compton's three gyroscopes - the tools that steer and stabilize the spacecraft - failed.
This contributed to NASA officials' belief that the craft should be brought down now, though many scientists inside and outside the agency believe it could have remained in orbit with minimal risk.
(c) Copyright 2000. The Christian Science Publishing Society