U.S. Astronauts on 'Critical' Spacewalk to Replace Power Box

Two U.S. astronauts embarked Tuesday on what NASA described as a "critical" spacewalk to repair a failed piece of equipment that helps power the International Space Station.

The spacewalk by NASA's Peggy Whitson and Jack Fischer began at 7:20 am (1120 GMT), and was expected to last just 2.5 hours, far shorter than the typical 6.5-hour outing, the U.S. space agency said.

A computer relay box failed suddenly on Saturday, but the crew was never in danger since there are two of the boxes -- known as multiplexer-demultiplexer (MDM) units -- and one of them continued to work, said NASA.

Still, a space agency spokesman described Tuesday's mission as a "critical contingency spacewalk" and called it a "high priority" to replace the failed box.

The MDM -- which is about the size of a small microwave oven and would weigh 50 pounds (23 kilograms) on Earth -- helps operate solar arrays, electrical power generation, and robotic equipment at the ISS.

It also regulates the operation of radiators and cooling loops. 

"Because each MDM is capable of performing the critical station functions, the crew on the station is in no danger and station operations are not affected," NASA said in a statement.

Whitson, 57, is currently the commander of the space station.

She leapt to third place in the record books during the spacewalk for most time spent in space.

The world record holder for spacewalking is Russian cosmonaut Anatoly Solovyev, who completed 16 spacewalks totaling 78 hours, 21 minutes.

In second place is Spanish-American astronaut Mike Lopez-Alegria, who has done 10 spacewalks totaling 67 hours and 40 minutes.

Whitson, who tackled the main job of replacing the MDM, is the most experienced female spacewalker in the world.

Tuesday marks the 10th spacewalk of her career.

Meanwhile, Fischer, 43, is making his second career spacewalk.

His job is to install a pair of antennas on the U.S. Destiny Laboratory module to enhance wireless communication capability for future spacewalks, NASA said. 

Tuesday's spacewalk is the 201st in support and maintenance of the ISS.

The $100 billion football-field sized orbiting laboratory has been continuously occupied by global astronauts since 2000.