NASA officials said a bolt that drifted away from astronauts during a spacewalk Tuesday should not pose a risk, despite concerns from astronaut Joe Tanner.

Tanner and crewmate Heidemarie Stefanyshyn-Piper were attaching a 16-ton truss to the international space station, when a bolt, spring and washer escaped Tanner's hand.

The washer floated completely free, but Tanner worried that the bolt and spring, which slid across the truss structure, might get stuck and jam proper function.

"I just hope that bolt is on its way to Mother Earth right now and not on its way" back to the station, Tanner said.

Meanwhile, NASA engineers are investigating whether the loose pieces pose a risk. But at a news conference, lead space station flight director John McCullough said the escaped bolt is "pretty trivial. It didn't go inside."

It had been 3 ½ years since astronauts had last worked on the station. Canadian astronaut Steve MacLean and his crew member Dan Burbank are scheduled for a spacewalk on Wednesday.

MacLean, a laser physicist known to the crew as "the professor," will then become the second Canadian in history to walk in space. The operation will last about 6.5 hours. During that time, he's expected to install a rotary joint so the station's solar panels can swivel toward the sun.

When astronaut Stefanyshyn-Piper suited up Tuesday to help install an addition to the station, she became the eighth woman to go on a spacewalk.

Of 159 U.S. walks in space, only six women have left the airlock. Only one participated out of 118 Russian walks.

The reason is logistical: spacesuits and their gloves are bulky and don't come in small sizes, so an ill-fitting suit makes it difficult to manoeuvre and manipulate machinery. Piper is 5-foot-10.

Tanner left the airlock first at 5:17 a.m. EDT, and was joined by Piper for the first of three space forays scheduled during the 11-day mission.

Earlier, MacLean and crewmate Jeff Williams had manipulated Canada's robotic arm to move the 45-foot, 17.5-ton addition next to the station's truss segment.

That put it in position for Tanner and Piper to connect the wiring and cables for the truss segment. With the connections made, the crew can set up two solar arrays.

"Welcome to the world of EVAs," Tanner said as Piper exited the lock, using shortened NASA jargon for ExtraVehicular Activity.

"Ah. Wonderful," Piper replied.

Prior to Tuesday's spacewalk, Tanner and Piper had camped all night in the airlock while the air pressure was reduced from 14.7 pounds per square inch to 10.2 pounds per square inch. As for deep-sea divers, the method prevents decompression illness.

Gas bubbles are released in tissue whenever when there is a too-quick decrease in air pressure after being in a compressed atmosphere, causing painful and sometimes fatal neuralgic pain and paralysis. Spacesuits maintain a pressure of five pounds per square inch.

Earlier Tuesday, NASA had resolved a brief communication problem between the orbiting station and Mission Control in Houston.

But news from Mission Control was good: Monday's examination of the shuttle's thermal skin showed it in tip-top shape, and NASA engineers said it doesn't have to be examined again.

The inspections have become part of mission protocols after hard foam broke away from the external fuel tank and damaged the thermal skin of Columbia in 2003. The damage allowed hot gases during re-entry to penetrate a wing and the vessel broke apart as it soared over Texas, killing all seven astronauts on board.

With files from The Associated Press