MIAMI, Florida - Two US astronauts successfully completed what NASA described as a "critical" spacewalk to repair a failed piece of equipment that helps power the International Space Station.

"We have declared victory," NASA commentator Rob Navias said about two hours into the emergency spacewalk by space station commander Peggy Whitson, 57, and flight engineer Jack Fischer, 43.

The reason for the outing was the sudden breakdown on Saturday of a computer relay box known as a multiplexer-demultiplexer unit.

The MDM - which is about the size of a small microwave oven and would weigh 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.

Since there are two MDMs at the orbiting outpost, the loss of one did not jeopardize the lives of the crew or bring a halt to station operations.

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 as soon as possible.

During the shorter than normal spacewalk - which lasted just two hours and 46 minutes, much less than the typical 6.5 hours - Whitson took the lead and replaced the MDM.

After she took out the malfunctioning unit, she spotted some flakes of debris, and returned briefly to the airlock for equipment to clean the area before installing the new box.

"We now have two healthy MDMs," Navias said afterward.

Fischer, making the second spacewalk of his career, installed a pair of antennas on the US Destiny Laboratory module to enhance wireless communication capability for future spacewalks.

It was unclear what caused the MDM to fail.

The unit had just been installed in March during a spacewalk by Whitson and Expedition 50 commander Shane Kimbrough.

Third place

Nor was it the first time an MDM unit failed.

"A similar MDM replacement spacewalk was conducted in April 2014 by Expedition 39 crew members Steve Swanson and Rick Mastracchio of NASA," the US space agency said.

Whitson is the most experienced woman spacewalker in the world, and during the spacewalk she leapt to third place for the most time spent doing spacewalks.

She now has a total of 60 hours, 21 minutes over 10 career walkabouts in space.

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

Agence France-Presse