CAPE CANAVERAL, Fla. (Reuters) - Two U.S. astronauts completed a hastily planned spacewalk outside the International Space Station on Tuesday to replace a computer that failed on Saturday, NASA said.

Station commander Peggy Whitson assembled a new computer from spare parts aboard the station and installed it during a 2.5-hour spacewalk as the orbiting outpost sailed 250 miles (400 km) over Earth.

The 50-pound (23-kg) computer, which is about the size of a microwave oven, is one of two that control equipment, including solar power panels, cooling loops, radiators and robotics gear, on the U.S. side of the station.

“Good news,” mission commentator Rob Navias said after the new device was tested by ground control teams at the Johnson Space Center in Houston. “It’s up and running and in good shape.”

The loss of one of the computers on Saturday did not affect station operations or pose a risk to the crew, but managers at the National Aeronautics and Space Administration had wanted to restore a backup as quickly as possible, Navias said.

While Whitson replaced the failed computer, flight engineer Jack Fischer installed a pair of wireless communications antennas onto the outside of the U.S Destiny laboratory, one of three research modules aboard the $100 billion station, a project of 15 nations.

The spacewalk moved Whitson, a veteran of 10 previous outings, to third on the list of the world’s most experienced spacewalkers, behind Russian cosmonaut Anatoly Solovyev and NASA’s Mike Lopez-Alegria.

The failed computer was installed on March 30 during a spacewalk by Whitson and former station flight engineer Shane Kimbrough.

“No insight into what the problem is yet,” NASA spokesman Dan Huot wrote in an email. “Likely won’t know more until someone has the chance to do diagnostics on it.”