Two spacewalking astronauts resumed construction on the international space station Tuesday for the first time in more than 3½ years, and NASA pronounced the outing a success, even though a small bolt floated away.

"I felt today like this is what NASA is supposed to do," lead space station flight director John McCullough. "This is what we're here to do."

The spacewalk to attach a new 17½-ton box-like truss section included the connecting of 13 wires or tubes and the tightening or loosening of 167 bolts.

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NASA managers downplayed an astronaut's concern about the 1½-inch bolt that came flying free.

Astronaut Joe Tanner was working with the bolt, which had an attached spring, when the washer holding it in fell off.

The bolt and spring floated over the head of astronaut Heidemarie Stefanyshyn-Piper and skittered across the truss.

While the washer went out into space safely, Tanner worried the bolt and spring could get into the truss's wiring and tubing and causing problems.

"I just hope that bolt is on its way to Mother Earth right now and not on its way" to a crucial joint in the addition, Tanner said.

Even though NASA didn't have any video showing the bolt missing the mechanism, managers at an afternoon news conference said they are certain that the bolt flew off into space harmlessly.

"It's pretty trivial," McCullough said. "It didn't go inside."

Space debris can be dangerous if it punctures space station walls or spacesuits and can jam crucial mechanisms.

However, spacewalkers have a long history of losing material in space. In July, Discovery spacewalkers lost a 14-inch spatula that floated away.

The free-flying bolt marred an otherwise successful and speedy six-hour, 26-minute spacewalk Tuesday morning. Two more spacewalks are planned for later this week.

"You did a phenomenal job and set the bar very high for the rest of the assembly," Mission Control told the crew when the spacewalk ended late Tuesday morning.

Wearing bulky suits and gloves, Tanner and Piper zipped through a jam-packed list of arduous but mundane construction tasks, putting NASA ahead of schedule in connecting the addition.

With extra time, Mission Control assigned them eight extra jobs of bolt removing and cover unlatching that would have been part of a Thursday spacewalk.

Atlantis astronauts Dan Burbank and Steve MacLean will head into space on Wednesday.

The spacewalk was a first for rookie astronaut Piper, who joined an elite club of female spacewalkers.

Only six other women have participated in any of the 159 U.S. spacewalks, and only one has gone on any of the 118 Russian spacewalks.

A major reason: Spacesuits are too big for most women, said Piper, who is 5-foot-10.

"If you fit in a suit then the easier it is to work," she said.

The 45-foot, $372 million addition includes two electricity-generating solar arrays that will be unfurled on Thursday.