By Irene Klotz

CAPE CANAVERAL, Fla., July 19 (Reuters) - Two crewmen aboard the International Space Station on Thursday prepared for a spacewalk during which more than 1,600 pounds (726 kg) of obsolete gear will be tossed overboard and left to burn up in Earth's atmosphere.

Space station commander Fyodor Yurchikhin and flight engineer Clay Anderson were due to begin a 6-1/2-hour spacewalk on Monday to make room and prepare equipment for the arrival of new research modules built by Europe and Japan.

Their tasks include jettisoning a refrigerator-sized device containing ammonia that was part of the station's first cooling system. NASA upgraded the station's power and cooling systems and no longer needs the 1,400-pound (635-kg) tank.

NASA managers initially had planned to return the ammonia tank to Earth on a space shuttle. But with only 14 flights remaining to the outpost before the U.S. shuttle fleet is retired in three years, there was no room for the carrier rack needed to transport the device in the shuttle's cargo bay.

NASA said ditching the tank was the best option available even though the agency dislikes creating more space junk.

"We agonized over this for a very long time before we came to this decision," said deputy space station program manager Kirk Shireman.

Anderson also will release a 212-pound (96-kg) camera stand that is taking up needed space on a storage platform. NASA's immediate concern is that the jettisoned objects do not fly back into the station's orbit.

CRASH AND BURN

The discarded equipment is expected to remain in space for at least 300 days before friction from crashing into atmospheric particles drags them into the atmosphere.

The camera stand is expected to burn up completely, but chunks of the ammonia tank as heavy as 39 pounds (17.5 kg) could survive re-entry and fall to Earth.

NASA said that while the debris is most likely to land in an ocean, there is about a 1 in 5,000 chance it will hit a populated area.

The agency said tracking radars will follow the objects until they are about two hours away from atmospheric re-entry. Warnings would be issued if the debris seems likely to pose a threat.

Also on Thursday, the shuttle Endeavour astronauts dressed in their bright-orange pressurized flight suits and climbed aboard the spaceship for a countdown dress rehearsal ahead of their planned Aug. 7 liftoff.

NASA managers plan to meet next week to confirm the launch date.

Endeavour's crew, which includes teacher-astronaut Barbara Morgan, plans to spend seven to 10 days at the space station installing a new structural beam, replacing a steering gyroscope and delivering cargo.

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