Astronaut Rick Mastracchio working on the S0 Truss segment computer.
NASA includes redundancy on many items because when something breaks in space, you can't just run down to the local Apple Store for a fix. Last April 11, during a routine check, the backup computer MDM for the test failed. The main computer still functioned, so the crew was in no risk of losing capabilities, but the backup needed to be replaced. There are 45 MDM computers on the station and this set controlled the Truss systems as well as the mobility of the adjustable solar panels. The Truss is the framework backbone of the ISS, on which hang all the solar panels and modules. This backup computer has been working ever since this S0 section had been delivered to the station by shuttle.
As the station flew into the Earth's shadow, the astronauts had to rely on their suit lights.
No Problem. Back in 2001 the space shuttle Endeavor had brought up a spare MDM which was then stored in the US-built Destiny module. During a short spacewalk yesterday morning astronauts Rick Mastracchio and Steve Swanson floated over to the Truss segment housing the failed computer and replaced it with the spare. All systems are working fine now, and the astronauts returned in what seemed to be a very short time.