The next two astronauts to fly the space shuttle Columbia will test a mechanical arm, which is to be used to deploy satellites in space and may have to be left in space if it malfunctions.

"If the arm fails, one of us will have to go outside and reel it in and strap it down so we can close the payload bay doors to get home," astronaut Richard . Truly said yesterday at a news conference at Houston's Johnson Space Center. "If we can't do that, we'll have to jettison the arm and come home without it."

The mechanical arm that Truly talked about is a 50-foot-long robot that was developed for the space shuttle as a means of deploying satellites and then recovering them for maintenance and repair. The arm will be flown and tested for the first time on Sept. 30, when Truly and astronaut Joe Henry Engle will make the second flight of Columbia, a five-day voyage that will double the time of the first one last April by astronauts John Young and Robert Crippen.

The arm was developed by the National Research Council of Canada and built by Spar Aerospace, Ltd., of Toronto at a cast of $100 million. The first of four such arms, it is a gift from the Canadian government. The National Aeronautics and Space Administration bought the three others from Spar for $70 million.

The mechanical arm stretches 25 feet from the shoulder to the elbow, then another 25 feet from the elbow to the wrist where most of the work aboard the shuttle will be done. Though space is weightless, large objects require a great amount of force to move in space.The mechanical arm has the strength to move up to 64,000 pounds in space.

Truly said the arm, which is electrically powered, will be given a full series of tests on next month's shuttle flight, moving it out, up and down and using the grappling device at the end of its wrist to seize on to a mated grappling device fastened to the bulkhead of the shuttle cargo bay.

"We want to see how limber it is," Truly said. "We want to see what its dynamics are in the weightless state."

Should the arm jam -- most likely because of electrical failure -- while fully extended outside the cargo bay doors, Engle will suit up and move into space and inside the open cargo bay to unjam it by hand so the astronauts can close the cargo bay doors. If he cannot unjam it, the crew will have to jettison it and leave it in space. The astronauts cannot land Columbia with its cargo bay doors open, since the speed of rentry would rip the doors from the fuselage.

Reminded that they would be the first astronaut crew in history to fly a used spacecraft, Truly replied: "I wish I could buy a used car that looks as good as this one does." The two rookie astronauts were asked if they'd gotten any advice from Young and Crippen on how to fly Columbia and Truly replied: "Yeah. Don't do nothin' dumb."