HOUSTON — In a routine but difficult spacewalk yesterday, 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.

Before fixing the cable reel, Messrs. Fossum and Sellers breezed through the first part of their spacewalk, installing a 1,400-pound spare external pump compartment on the station’s cooling system.

That first part of the spacewalk went so smoothly that the duo — on their second of three spacewalks this mission — exchanged quips, jokes and even gibes at astronauts back in the shuttle.

“Everyone can hear you scream,” said Mr. Fossum, in a twist on the tag line from the movie “Alien.”

Mr. Sellers retorted: “About the time I get outside, I’ll put on my alien costume.”