CAPE CANAVERAL, Fla. — Two spacewalking astronauts tackled a backlog of outdoor work at the International Space Station on Tuesday.

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.

They made up for lost time as they went through the hodgepodge of chores, removing a bad camera and relocating radiator grapple bars. Some of the work was done to make it easier to swap out bad parts if there’s ever a breakdown.

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.