HOUSTON — Two Discovery astronauts stepped into space yesterday, floating, crawling, drifting and dangling from gantries, handholds and tethers all over the space shuttle and the international space station.

At Mission Control, Wayne Hale, the deputy shuttle project manager, announced that NASA planners added a day to Discovery's mission so the crew could transfer equipment, water and other consumables to the station, anticipating that a long time could elapse before the grounded shuttle fleet flies again.

Yesterday's spacewalk, which lasted six hours and 50 minutes, was the first of three for Soichi Noguchi, a Japan Aerospace Exploration Agency astronaut, and Stephen Robinson, a guitar-playing engineer from California. It was also the first by any shuttle crew in the 2 1/2 years since the shuttle Columbia disintegrated on re-entry.

The space walkers used the time to test repair techniques, install part of a storage platform, and replace and fix broken equipment that had crippled the space station for months. They accomplished all of their tasks in the allotted time — and even added extras at the end.

Mark Ferring, space station flight director, was pleased to end the day with a new gyroscope to hold the station in place and a new Global Positioning System antenna. "It's really nice to have the backup," Ferring said.

The crew of the space station, which depends on the shuttle for heavy equipment and large-scale resupply, had been watching the machinery wear out and its resources dwindle ever since the Columbia tragedy.

Discovery's mission, in the first shuttle flight since 2003, was to inaugurate a new era in shuttle travel, but the orbiter's external fuel tank lost a 0.9-pound piece of foam insulation during launch, prompting the National Space and Aeronautics Administration to halt all shuttle flights until the problem is fixed — a decision that put September's scheduled mission in doubt. A larger chunk of debris doomed Columbia.

Despite the new uncertainties about shuttle launches to come, Discovery's mission proceeded virtually without flaw since liftoff Tuesday. The orbiter appeared to have emerged unscathed from the launch, and new imaging devices provided engineers with unprecedented views of the shuttle's heat shielding.

Shuttle astronauts used a new 50-foot boom sensor to conduct laser imaging of several "areas of interest" where scarring might have occurred on the leading edge of Discovery's wings.

At Johnson Space Center, mission operations representative Philip L. Engelauf said the latest survey appeared to have found nothing significant, but he suggested that analysts might look in other areas.

For yesterday's spacewalk, Noguchi cracked open Discovery's airlock hatch and stepped into the void just as the sun was setting over Southeast Asia. "What a view!" he exclaimed.

Attention on the ground focused initially on tests of two experimental techniques that NASA would like to use for the onboard repair of launch-related damage to heat shielding.

The early reviews were somewhat mixed. Robinson used a black, glue-like compound he described as "licorice-flavored pizza dough" to fill cracks and gouges in damage-test panels of reinforced carbon-carbon.

The compound is called nonoxide adhesive experimental, or NOAX. Robinson applied it with a caulk gun, then used a putty knife to force it into the cracks and holes that had been deliberately placed in the panels for the test.

The application process appeared to work well, and the NOAX filled the damaged spots easily and without much bubbling. But it swelled up, even after it started to dry.

Cindy Begley at Mission Control, the official in charge of the spacewalks, said engineers had anticipated the swelling, "and as long as it's not too high, we're doing great."

Noguchi had no trouble with an "emittance wash" used on chipped sections of thermal protection tiles, daubing the damaged areas with a spongy applicator, then spreading the liquid with a stubby paintbrush.

The space station's Ferring might have been the spacewalk's biggest beneficiary.

With his new GPS antenna in place, his gyro running easily again and a second seized-up gyro set to be replaced tomorrow, the station is in better running shape than it had been for a long time.

And with the mission's extra day, Ferring was looking forward to "stealing as many laptops as we can" from the shuttle, maybe picking up an extra spacesuit and getting rid of 2 1/2 years worth of worn-out equipment and trash.

"I'm having a ball," Ferring said.