ISS Spacewalk Preview.
In a briefing Friday, managers for NASA's International Space Station program previewed a spacewalk to replace a failed backup computer relay system on the space station's truss.
NASA astronauts Rick Mastracchio and Steve Swanson are scheduled to venture outside the space station Wednesday, April 23, to replace a backup multiplexer-demultiplexer (MDM) that failed during routine testing April 11. The box is one of the station's two external MDMs that provide commands to some of the space station's systems, including the external cooling system, solar alpha rotary joints and mobile transporter rail car. If Friday's SpaceX cargo launch to the station is postponed, the two Expedition 39 astronauts will conduct the spacewalk Sunday, April 20.