STORY: NASA Astronauts Steve Swanson and Rick Mastracchio ventured outside for a spacewalk on Wednesday (April 23). The International Space Station (ISS) spacewalk is a planned mission to install a spare backup computer that failed April 11. This is Swanson's 5th spacewalk and Mastracchio's 9th.

The backup computer is known as a Multiplexer/Demultiplexer (MDM) and it is one of two MDMs that help control important functions on the ISS like the external cooling system and mobile transporter rail car, which is part of the docking procedure that attaches spacecraft to the space station. Both MDMs have been at the International Space Station for over 10 years.