CAPE CANAVERAL, Fla. -

Space station astronauts are taking a spacewalk to replace a dead computer and get their orbiting home back up to full strength.

The two Americans on board, Rick Mastracchio and Steven Swanson, stepped out Wednesday for the urgent repair job.

The computer is a critical backup outside the International Space Station that failed nearly two weeks ago. The prime computer has been working fine, but NASA wanted to install a fresh spare as soon as possible.

Mission Control waited until after the arrival of a capsule full of fresh supplies on Sunday.

The spare computer is a compact 50-pound box. Engineers don't know why the original failed. The astronauts will check for any damage.

It's expected to be a short spacewalk, involving just three bolts.