CAPE CANAVERAL, Fla. - Astronauts removed an old pump Saturday, sailing through the first of a series of urgent repair space walks to revive a crippled cooling line on the International Space Station.

The two Americans on the crew, Rick Mastracchio and Michael Hopkins, successfully pulled out the failed ammonia pump well ahead of schedule. That task had been planned for the next space walk on Monday.

"An early Christmas," Mission Control said as Mastracchio tugged the refrigerator-size pump from its nesting spot.

If Mastracchio and Hopkins keep up the quick work, two space walks may be enough to complete the installation of a spare pump, and a third space walk will not be needed on Christmas Day as originally anticipated.

The breakdown 10 days earlier left one of two identical cooling loops too cold and forced the astronauts to turn off all nonessential equipment inside the orbiting lab, bringing scientific research to a near-halt and leaving the station in a vulnerable state.

Mission Control wanted to keep the space-walkers out longer Saturday to get farther ahead, but a cold and uncomfortable Mastracchio asked to go back. The space walk ended after 51/2 hours, an hour short on time but satisfyingly long on content.

Mastracchio managed to unhook all the ammonia fluid and electrical lines from the pump with relative ease, occasionally releasing a flurry of frozen ammonia flakes that brushed against his suit. A small O-ring floated away, but he managed to retrieve it.

"I got it, I got it, I got it. Barely," Mastracchio said as he stretched out his hand.

"Don't let that go; that's a stocking stuffer," Mission Control replied.

"Don't tell my wife," Mastracchio said, chuckling, as he put it in a small pouch for trash.

Adding to the excitement 260 miles up, a smoke alarm went off in the space station as the astronauts toiled outside. It was quickly found to be a false alarm.