Astronauts Franklin Chang-Diaz and Frenchman Philippe Perrin began repressurizing the international space station's Quest airlock module at 4:20 p.m. today, officially ending a five-hour spacewalk. The astronauts completed their work a full hour and a half ahead of schedule.

On another front, two of the space station's returning crew members - Carl Walz and Daniel Bursch - are closing in on a new U.S. space endurance record. At 10:19:38 p.m. this evening, they will exceed astronaut Shannon Lucid's record of 188 days and four hours off planet, a mark set in 1996 during an extended stay aboard the Russian Mir space station.

Bursch, Walz and Expedition 4 commander Yuri Onufrienko were launched to the international space station Dec. 5. Assuming an on-time touchdown aboard Endeavour next Monday, they will have logged 193 days 17 hours and 20 minutes in space, setting a new U.S. endurance record in the process.

Today's spacewalk by Chang-Diaz and Perrin was a textbook affair with no problems of any significance. The astronauts successfully bolted a $254 million robot arm mounting platform in place atop a motorized cart on the station's unfinished solar array truss; hooked up data and video cables between the platform and the mobile transporter; mounted a video camera in its permanent location; and deployed a cargo grapple fixture.

"It could not have gone any better," said space station flight director Rick LaBrode. "With the permanent mating of the mobile base system to the mobile transporter, we've added another piece to the ISS city in the sky. So things are continuing to go very, very well. The ISS and shuttle systems are in great shape, we're well ahead of the game in transfer ops (and) both crews are really working like a well-oiled machine."

Assuming subsequent testing goes well, the station's on-board crew will move the lab's $600 million Canadarm2 space crane to the mobile transporter platform after the shuttle Endeavour departs. The robot arm currently is attached to the Destiny laboratory module, but it must end up on the mobile transport to continue assembly of the solar array truss. With the completion of today's spacewalk, the mobile transporter and the robot arm interface platform are ready for the arm's long-anticipated walk-off from the lab module.

"I want to say a few words for the Canadians," Perrin radioed as he prepared to re-enter the airlock. "They have put together a wonderful piece of hardware and everything worked fantastic. I'm very proud of the MBS. Thanks to all who worked on that program."

"You guys did a great job today," Canadian astronaut Robert Thirsk replied from mission control. "We couldn't have done it without the team on the ground, but certainly not without the team up there on orbit as well. Your professionalism and skill really showed through. Thank you from Canada."

Ken Lord, deputy director of engineering and operations for the Canadian Space Agency, said today marked "a tremendous first for us."

"The MBS is now firmly located, powered and supported by the space station," he said. "Over the next few days and weeks, the MBS will be checked out, will be fully tested and we hope to see, at the end of June or early July, the formal step off of Canadarm2 from the station to the MBS where it will then be available to support the rest of the assembly of the international station."

Today's spacewalk was the 40th devoted to space station assembly, the 15th staged from the space station and the eighth to use the U.S. Quest airlock module. All told, 31 U.S. astronauts, one Canadian, 1 Frenchman and five Russian cosmonauts have logged 248 hours and 41 minutes of spacewalk time building the international lab complex.

Chang-Diaz and Perrin plan a third and final spacewalk Thursday to replace the wrist-joint of the Canadarm2 space crane.

Between now and then, engineers on the ground will be troubleshooting a problem with the Quest airlock module's hard-wired spacesuit power supply. Earlier today, engineers noticed that the umbilicals used to supply electricity to the spacesuits worn by Chang-Diaz and Perrin suddenly tripped off. When the spacewalkers returned to the airlock today, they were told to keep their suits on battery power throughout the repressurization process.

Spacewalkers normally use the umbilicals to power their suits until just before the leave the airlock. When they return, they hook back up to station power before working through the airlock repressurization procedure.

Lead spacewalk planner Tricia Mack said today if the problem is not resolved by Thursday, the astronauts will take two sets of batteries into the airlock with them. They'll use one set for preliminary work and then switch to a fresh set of batteries just before exiting the airlock. That will ensure a full charge at the beginning of the excursion.