Two U.S. space shuttle astronauts are performing wrist surgery on the ailing robot arm attached to the international space station. The work requires a spacewalk.

Shuttle Endeavour crewmen Franklin Chang-Diaz and Philippe Perrin have donned their spacesuits and ventured outside the station to replace one of the seven joints on the mechanical arm.

The joint has a short circuit in one of the two redundant power and data channels which control it, causing the arm's brakes to freeze it unexpectedly. Although the joint works flawlessly on the second channel, station controllers wants both operational.

Station flight director Rick LaBrode says the planned seven-hour spacewalk is critical to the success of future station construction chores.

"We are going to go as long as it takes, or as long as the consumables in the suit will allow us to, to get the tasks accomplished," he said.

This is the last of three station maintenance spacewalks for the two astronauts while Endeavour visits the orbiting research outpost.

The shuttle has brought up a replacement U.S. and Russian station crew, who are to remain aloft until October.