It was the first spacewalk for Italy a major contributor to the orbiting lab as Luca Parmitano floated out. He was accompanied by American Christopher Cassidy, a veteran spacewalker.
Cassidy encountered a stubborn bolt as the maintenance work got underway, eating up precious minutes. A slim gap of just one-eighth of an inch stalled the installation of a new space-to-ground radio transmitter; the old one failed in December.
"Nothing jumps out at me," Cassidy reported to Mission Control. "I can see a little wear on the bolt."
Finally, the former Navy Seal managed to attach the transmitter. Mission Control said it appeared to be a tight fit.
It was smoother going for Parmitano as he collected science experiments for return to Earth later this year aboard a commercial SpaceX capsule.
"Any curve balls over there, Luca?" Cassidy asked. "Nope," came the reply.
Their hodgepodge of chores also included collecting science experiments, removing a bad camera and relocating radiator grapple bars. Some of the work was targeted for potential future breakdowns, making it easier to swap out bad parts when the time comes.
They had some cable work to perform in preparation for a new Russian lab due to arrive in December, and were asked to take pictures of the Alpha Magnetic Spectrometer, a $2 billion cosmic ray detector launched on NASA's next-to-last shuttle mission in 2011. Scientists noticed unusual discoloration on the instrument's radiators and requested photos.
A second spacewalk, next Tuesday, will wrap up the job.
NASA said the tasks had been piling up over the past couple of years. Managers wanted to wait until the to-do list was long before committing to the time-consuming spacewalks.
Parmitano, 36, a major in the Italian Air Force, arrived at the space station at the end of May for a six-month stay.
Cassidy, 43, will wrap up his half-year mission in September.
The rest of the space station crew one American and three Russians assisted the spacewalkers from inside.