NASA astronauts on Tuesday completed urgent repairs to the cooling system of the International Space Station that should return it to normal operation within a few days.
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In a seven-and-a-half-hour spacewalk, their second in four days, the astronauts, Col. Michael S. Hopkins of the Air Force and Richard A. Mastracchio, installed a new pump module on the outside of the space station.

The module, a 780-pound box about the size of a refrigerator, contains a pump and accompanying apparatus that circulate ammonia coolant through one of two loops on the station.

“It’s like Christmas morning opening up a little present here,” said Mr. Mastracchio, an engineer, as the spacewalk unfolded on NASA Television.

Operations on the space station, including some science experiments, have been curtailed since a valve in the pump module malfunctioned two weeks ago.

Plugging in the last of the electrical connectors on Tuesday afternoon, Colonel Hopkins said, “Houston, you’ve got yourself a new pump module.”

The pump passed a brief check, and was turned on Tuesday evening. “We have a pump that is alive and well,” said Rob Navias, a NASA spokesman who provided commentary during the spacewalk. Nonessential equipment that had been turned off should be switched on by the end of the week or early next week, he said.

The first spacewalk, on Saturday, went quickly and almost flawlessly, and the astronauts were able to get far ahead of schedule and remove the old module, a task that had originally been scheduled for the second spacewalk.

On Tuesday, they ran into trouble when one of the ammonia fluid lines would not detach. With brainstorming help from mission control, they finally succeeded, but then some toxic flakes of frozen ammonia leaked out. The astronauts had to take a few precautions at the end of the spacewalk to ensure their spacesuits were decontaminated.

During a spacewalk in July, the cooling system in an Italian astronaut’s spacesuit malfunctioned and the helmet partly filled with water. There were no such problems this time.

“Fantastic work,” Colonel Hopkins said as he re-entered the airlock. “Merry Christmas to everybody.”

The malfunctioning pump module had been installed just three years ago, and the station has only two more spares available. With the space shuttle fleet retired, NASA has no craft big enough to ferry a replacement up from Earth.

The module will be stored on the outside of the space station, and NASA officials said it might be possible to return it to service in the future, even with the faulty valve.

After a busy few days, the station’s six crew members will have a day off on Christmas.

On Friday, two Russian astronauts are to conduct a previously scheduled seven-hour spacewalk to install two cameras and replenish some science experiments outside the station.