Houston ? In a routine but difficult spacewalk Monday, two astronauts fixed a crucial piece of the international space station, allowing it to be added on to later this year.

Space shuttle Discovery astronauts Piers Sellers and Michael Fossum replaced a cable reel to a rail car needed to move large pieces around the giant orbital outpost in a spacewalk dotted with glitches.

The reel, severed accidentally last year by a cable cutter, provides video, data and power to the rail car. The astronauts immobilized the cutter in their first spacewalk Saturday.

NASA managers said fixing the cable reel was vital to space station construction, which will take 15 more shuttle flights.

“Whew, man, do I feel better,” space station flight director Rick LaBrode told journalists after the spacewalk. “I tell you, I’ve spent the better part of the last three years of my life putting together this mission, and this particular (spacewalk) was my main concern.”

The astronauts were just as relieved.

“The job worked out,” Sellers said when he finally finished the difficult task of installing the broken reel in the shuttle’s cargo bay. The reel didn’t fit, so the astronauts twisted harder with a wrench until it was snug.

He was further delayed by a loose piece of spacewalking safety equipment that forced him to stay still until Fossum fixed it.

The two connecting devices of Sellers’ backpack attachment, designed to be used if an astronaut floats free, loosened at different times, but he was never in danger of losing it, NASA officials said.

The astronauts got welcome news Sunday when NASA managers cleared Discovery’s thermal protective skin as safe for a return to Earth on July 17.

Hundreds of images of Discovery were taken during liftoff, during the orbital flight to the space station and before docking with the complex to make sure the shuttle doesn’t have any damage like the kind that doomed Columbia’s seven astronauts in 2003.