Cape Canaveral, Fla. ? Astronauts completed a nearly seven-hour spacewalk Tuesday, the first of three difficult forays outside during the shuttle Discovery’s stay at the international space station.

U.S. astronaut Robert Curbeam, a veteran spacewalker, and the European Space Agency’s Christer Fuglesang, who was making his first spacewalk, installed an addition to the orbiting space lab. They stepped back inside the spacecraft at 9:07 p.m. CST after 6 hours and 36 minutes of toiling outside.

Just before doing that, though, the pair sent a cosmic shout-out.

“I just want to say congratulations to all the Nobel Prize winners this year and especially to John Mather from Goddard Space Center,” Curbeam said of this year’s physics honoree. “We’re proud of you.”

Mission Control astronaut Steve Robinson radioed back: “From here on the ground, we’d like to send our congratulations for a 100 percent successful first EVA,” or extravehicular activity, the technical name for a spacewalk.

Curbeam and Fuglesang guided mission specialists Joan Higginbotham and Sunita “Suni” Williams as they used a robotic arm from inside the station to install a 2-ton, $11 million addition along the space lab’s truss in a space where clearance at times was less than 3 inches.

“We don’t want to scream over the loop,” said Higginbotham, referring to the communications system that links the spacewalkers, Mission Control and the space station, after a difficult maneuver. “But we’re very happy.”

The spacewalkers, who are tethered at all times, later connected utility cables between the addition and an existing solar array; replaced a field camera outside the station; and moved a handle that was used to grip the new segment.

The spacewalk started several hours after NASA engineers studying Discovery’s heat shield for damage recommended against any extra inspection of the spacecraft’s belly and wings.

The engineers’ recommendation doesn’t mean the shuttle was cleared of damage, since experts were still reviewing data, but it was an optimistic sign. Later in the day, mission managers approved the recommendation.