Two spacewalking astronauts rode a cart along a rail in Atlantis' open cargo bay Monday, testing techniques that will be needed to build a space station.

It was the second day in open space for Jerry Ross and Jay Apt, who on Sunday took an emergency trip out to fix a stuck antenna on a $617 million observatory."Very nice braking. This thing moves easily," Ross said as he zipped up and down the nearly 47 feet of track on the manually operated cart. "This is the way to travel around the world," said Ross. Said Apt, who joined Ross aboard the cart, "Boy, this thing glides slick."

Monday's spacewalk into the open shuttle bay, which had been on the agenda all along, was to be more leisurely than the unscheduled one Sunday.

The two crew members had to go into space when the 17-ton Gamma Ray Observatory's antenna couldn't be extended by electronic signals from the ground. They shook the antenna loose, then got an early start on Monday's spacewalking activities.

Flight director Phil Engelauf said Monday's spacewalk would go for about six hours unless the astronauts became tired. Only those tests performed Sunday were dropped from Monday's schedule.

"They were pretty pumped up after yesterday's walk. I expect they'll want to go for six hours," Engelauf said.

A battery on a backup spacesuit was installed in Ross' outfit after his failed to recharge properly during the night. There were no problems with Apt's suit.

Ross and Apt went into the shuttle's cargo bay Sunday to extend the observatory's antenna from its 16 1/2-foot boom, and within minutes Ross had shaken it loose.

"It's free, it's free. I can see it move, it's free!" he shouted.

"Far out, good work," said Apt.

It was the first spacewalk by Americans in more than five years.

NASA was quick to point out that the observatory could not have been fixed on the spot if it had been carried aloft by an unmanned rocket."You saw today the value of manned spaceflight," said NASA flight director Chuck Shaw. "We did a lot of work from the ground, and we needed the folks . . . on the scene to finish up the job."

"Talk about a view," Apt exulted during the 41/2-hour spacewalkSunday.

"Feel like you're walking on top of the world? Look what's coming, Jay. Hawaii," Ross said.

The space station, if approved by Congress, will be assembled in space and require considerable hands-on efforts by astronauts.

Sunday's effort was the second unscheduled walk in NASA history. Astronauts went outside Discovery in April 1985 to try to switch on the electrical power to a communications satellite launched from the shuttle, but they couldn't fix it. A crew trained to do the repairs went up four months later and fixed it.

In about a month, the observatory will start recording high-energy gamma rays. It will circle the world for at least two years.



Repair in space\

Astronauts inside the shuttle lift the 17-ton observatory in the shuttle's payload bay using the robot arm.

After the communications antenna and solar-cell arrays extend properly, the satellite was to be released.

But the antenna did not unfold until astronauts during a walk in space shook the balky antenna and freed it.

The repair job made it possible to release the observatory at 4:37 p.m. MDT Sunday.