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.