Two U.S. astronauts floated in space Wednesday for maintenance chores outside the international space station. One of the spacewalkers took over for a Russian cosmonaut, whom the U.S. space agency NASA disqualified for medical reasons.

Space station commander Kenneth Bowersox and astronaut Donald Pettit had to push extra hard on a sticky external hatch to open it, a struggle that delayed the spacewalk a few minutes. But once outside, they tacked jobs that had to be finished before the next space shuttle visit in March.

During the seven-hour outing, they released locks holding down a large radiator, allowing mission controllers to deploy it. It is one of six that will dissipate heat generated by the station's electronic equipment.

The spacewalkers also used sticky tape to pick up gritty debris on a docking port where cargo containers are attached during shuttle visits. NASA spacewalk supervisor Daryl Schuck says the debris was probably leftover from the last cargo container that was positioned there.

"It's a very small amount," he explained, "but you also have to understand that this is a sealing surface and we want to make sure that we get a good seal. More importantly, we don't want to damage the surface, so that's why we thought it prudent to execute this as soon as possible."

Cosmonaut Nikolai Budarin was supposed to be on this spacewalk, but was replaced by Mr. Pettit when a NASA flight surgeon disqualified the Russian for failing an exercise stress test last month. According to news reports, Russian space agency officials acknowledge that Mr. Budarin has what they term a heart "peculiarity," but they say it has never interfered with his eight previous spacewalks. According to the Itar-Tass news agency, Russian medical experts call NASA's stand an exaggeration.

But station flight director Norman Knight says that because the spacewalk, or EVA as NASA calls it, took place through a U.S. hatch in U.S. spacesuits, the Russians were obliged to comply.

"The Russians as well as the U.S. have policies on who make the decisions for U.S.-based EVA's and Russian-based EVA's," he said. "Since this was a U.S.-based EVA, the U.S. surgeon made the call on whether Nikolai would be going outside or not. The Russians and the U.S. are all in agreement with those protocols."

For Donald Pettit, being on the spacewalk was as unexpected a turn as his being on the station in the first place. He originally trained as a backup to the crew, but Mr. Schuck says he replaced an astronaut whom flight doctors had grounded for health reasons.

"So he wasn't even supposed to fly. So I think he probably just continues to shake his head and wonder what's next," he said.

The three crewmen will remain aboard the station until the March shuttle mission ferries up a replacement team.