WASHINGTON NASA engineers on Sunday test-fired thrusters on board the doomed Compton Gamma Ray Observatory in preparation for maneuvers this week designed to bring the school bus-sized satellite to a flaming end in Earths atmosphere.

Controllers at NASAs Goddard Space Flight Center in Greenbelt, Maryland sent commands Sunday afternoon to fire six of Comptons 12 on-board thrusters for one to two seconds to ensure that the propulsion system worked properly. The thrusters had last been fired three years ago to reboost the spacecraft.

Compton Gamma Ray Observatory

See an interactive graphic on the Compton Observatory. Requires Flash 4 .

"Everything went very smoothly and all the thrusters fired exactly as they should," mission manager Mansoor Ahmed told SPACE.com. "This means we dont have to change anything in our plans."

Comptons four science instruments, which were on low power for Sundays test, will be turned off for good on Tuesday.

The $670 million Compton which was launched into space on April 5, 1991 from Space Shuttle Atlantis is being brought out of Earth orbit because one of the satellites three stabilizing gyroscopes has failed. NASA feared another gyroscope failure might render the satellite uncontrollable, reducing the space agency's ability to manage its fall from orbit -- and determine its impact area on Earth.

"Everything went very smoothly and all the thrusters fired exactly as they should."

As a result, NASA plans to push Compton out of orbit with a series of firings over the next week from the same thrusters tested on Sunday.

At 17 tons, Compton is the heaviest satellite NASA ever has tried to return to Earth in a controlled crash-landing.

Debris is expected to fall into an uninhabited part of the Pacific Ocean about 2,500 miles (4,025 kilometers) southeast of Hawaii. More than 6 tons of metal debris ranging from fragments of just a few ounces to some weighing several hundred pounds are expected to survive the death plunge.

Next up for Compton are a series of 30-minute thruster firings this week.

The first firing comes Tuesday, May 30, starting at 9:54 p.m. Eastern Daylight Time (Wednesday, 01:54 GMT). That will lower Comptons orbit from 316 miles (510 kilometers) above Earth to 217 miles (350 kilometers). Left alone at that orbit, the satellite would have a minimum lifetime of a year before falling somewhere on Earth.

The second comes Wednesday, May 31, starting at 10:41 p.m. EDT. (Thursday, 02:41 GMT) That will lower the orbit to 155 miles (250 kilometers). Minimum lifetime: 80 days.

The third comes Sunday, June 4, starting at 1:37 a.m. EDT (05:37 GMT). That will lower the orbit to 93 miles (150 kilometers). Minimum lifetime: 3 days.

The fourth and final engine firing comes that same day at 3:05 a.m. EDT (07:05 GMT), on Comptons next orbit of Earth. As the satellite enters Earths atmosphere, it will begin to tumble and heat up as it disintegrates. It is expected to crash about one hour after the last firing.