Commander Peggy Whitson and her Russian crewmate, Yuri Malenchenko, floated out the hatch an hour early, well before dawn, and began the long job of getting the space station's newest addition ready for its big move.
They should have conducted the spacewalk during Discovery's visit, but the work was put off after a solar wing ripped and demanded immediate attention.
The two had to clear cables from the spot where the Harmony compartment will be relocated next week, and undo a number of electrical and fluid connections. They struggled to loosen some of the bolts.
"You think somebody glued these on?" asked Whitson as she used brute force to free the connections.
"Looks like for the next (spacewalk) we'll need one of those hydraulic jaws-of-life machines," observed astronaut Daniel Tani, who orchestrated the spacewalk from inside.
Harmony, a pressurized chamber the size of a school bus, was delivered by Discovery late last month and installed in a temporary location. Before NASA can launch its next shuttle mission, Harmony must be repositioned at the space station, a job that will require three spacewalks and extensive robotic work over the next two weeks.
There is so much to do that the three space station residents will get only one day off a week for the foreseeable future.
Harmony will serve as the docking port for European and Japanese laboratories. The European lab, Columbus, is supposed to fly up on Atlantis as early as December.
NASA wants to launch Columbus and the Japanese lab Kibo, or Hope, as soon as possible after so many years of delay. The 2003 Columbia disaster put everything on hold for more than two years, and continued problems with insulating foam falling off the fuel tank further stalled the flights.
Adding to the pressure is a looming 2010 deadline for retiring the remaining three space shuttles and finishing space station construction.
If the mangled solar wing had not been fixed, the labs would have faced further delay. But the space station's power system still is not up to par - a joint needed to turn another set of solar wings is clogged with steel grit because of grinding parts.
Whitson is the first woman to serve as the space station's commander. Both she and Malenchenko are one month into a six-month stay.
Discovery undocked from the space station Monday and landed Wednesday. The 15-day mission was highlighted by a dramatic spacewalk to save the torn solar wing, a successful operation that propelled the astronauts into the headlines and space history books. They received a presidential welcome upon their return to Houston on Thursday.