compton_reentry_000525 WASHINGTON -- Engineers at NASAs Goddard Space Flight Center call it "crunch time."
After nearly two months of intensive training, a ground-control team is expected this week to decide to send the Compton Gamma Ray Observatory into a suicide dive in the atmosphere over the eastern Pacific Ocean.
The burial at sea maneuver would destroy the bus-sized, $760 million satellite after more than nine years in space.
Compton, at 17 tons (15,422 kilograms), will be the biggest spacecraft to fall from orbit since Skylab more than 20 years ago and the first NASA satellite to be intentionally sent to its death.
The nearly 100-ton (101,600 kilogram) Skylab, which was the upper stage of the Saturn 5 moon rocket, measured 84 feet (26 meters) long and 22 feet (6.5 meters) in diameter. It served as home for nine astronauts from 1973-74 before peak solar activity increased atmospheric drag and pulled Skylab out of its orbit to NASA's dismay.
Much of Skylab burned up over the Indian Ocean during the July 1979 plunge, although some pieces survived and were found over Western Australia.
Specific crash route
Compton, though, will be the first U.S. spacecraft purposely brought out of orbit and for which a specific crash route has been plotted.
Compton is one of NASAs four Great Observatories, which scan the universe in unprecedented detail from their positions in space. The others are the currently orbiting Hubble Space Telescope and the Chandra X-ray Observatory, as well as the Space Infrared Telescope Facility, set for launch in December 2001.
Deployed from Space Shuttle Atlantis in April 1991, Compton has chalked up an impressive roster of science finds, tirelessly operating beyond an expected five-year service lifetime. Built by TRW of Redondo Beach, California, Compton was the first spacecraft designed for reboost in orbit and a successful maneuver in late 1993 added extra years to the satellites life.
Throwing in the towel?
The loss last December of one of Compton's three gyroscopes that are needed to keep safe and steady control of the spacecraft worried NASA officials. Were a second gyroscope to fail, there would be no guarantee that ground operators could accurately guide the heavy satellite to a particular crash site.
So in March, NASA officials approved a step-by-step plan to shut down the still-operating Compton. Engines on the spacecraft will be fired to gradually drop the huge satellite closer and closer to Earth.
Not everyone has been happy with that decision.
"NASA has lost its nerve," said James Ryan, a University of New Hampshire in Durham physicist.
"There once was a time when the agencywould jump at the challenge of keeping a valuable scientific mission afloat," he said. "However, with Compton they are throwing in the towel and giving up without a fight. This is not the same NASA that put a man on the moon."
Putting safety over science, however, is at the heart of NASAs decision.
"Im a gamma-ray astronomer. Like many, I too was certainly disappointed by the decision to bring Compton down," said Don Kniffen, a project scientist at NASA Headquarters. "But I think the decision is right-headed. Its based on a concern for human safety.
"You want to get this done as soon as it can be done safely. We dont want to take any risks," he said.
Taking a tumble
Hurtling through the atmosphere, Compton is expected to break up and be scattered within a rectangular zone 962 miles (1,548 kilometers) long by 16 miles (26 kilometers) wide. The area stretches across the equator west of Mexico and northern South America.
Some spacecraft pieces will vaporize as they high-speed their way through the air. However, large chunks are expected to survive and smack into the desolate area of ocean.
"This will be the first time that NASA will have done a controlled reentry of a science satellite into the Earths atmosphere and into the ocean. This is unique," said Martin Frederick, a consultant with NASA's Goddard Space Flight Center in Greenbelt, Maryland.
Frederick is helping lead a crack team of engineers, navigation and spacecraft controllers to safely bring down Compton.
"Nothing has been easy. From my position, I think the team is ready to go," he said.
This week, ground controllers are to take a final poll to decide if the spacecraft is ready and the team can give a thumbs-up on bringing Compton down.
"If there is doubt, then were going to hold off and take what steps it takes to make it safe again," Frederick said.
If a delay is ordered, the next opportunity to bring Compton down would be 46 days later, when the suns energy would be bathing the satellites solar panels and the splashdown zone would be available.
Learn as you burn
A team of 15 to 20 people at Goddard have put in weeks of work to prepare themselves by running computer simulations of Comptons "D-day" -- deorbit day.
Experts from other NASA centers, the aerospace industry and the Defense Department have also been helping.
"Id say this effort has touched the lives of over 100 individuals," Frederick said. "We have treated this as more critical than a launch."
On May 30, if all goes according to plan, the satellite will fire its engines for 24 minutes, dropping its orbital low point (perigee) from 317 miles (510 kilometers) to 217 miles (350 kilometers) above Earth.
Then, on May 31, an engine burst will shave 62 more miles (100 kilometers) from Comptons perigee. On June 4, a similar firing will reduce its low point by another 62 miles.
A fourth and final engine blast early on June 4 is expected to drop the spacecraft into a watery grave in the east-central Pacific.
"Theres a lot of fuel. And it will take a lot of fuel to bring it down because Comptons so big," Frederick said.
Doing the burns in stages is preferable to one big engine firing, he said, because that way controllers are "able to make sure you nail that last burn perfectly."
It's raining I-beams and six-packs
Compton's leftovers are likely to be many.
Computer studies suggest that a number of spacecraft parts may flutter down from the sky and hit the ocean. Among them: several aluminum I-beams like those used in bridges that form the main spacecraft structure; chunks of science gear; a six-pack of heavy nickel batteries and titanium propulsion tanks.
Still, Compton's reentry shouldn't be a problem, said NASA orbital-debris expert Nick Johnson of the Johnson Space Center in Houston, Texas.
"Were looking for a particular spot in the eastern Pacific to drop it. We have every confidence that well be able to do that," Johnson said.
"From a NASA scientific standpoint, this is unique," he said. "I cant think of when weve ever done this before...and it probably will not be the last either."
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