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.

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.