NASA orders Tuesday spacewalk to repair ISS

A pair of NASA spacewalkers will venture outside the International Space Station on Tuesday morning to replace a data relay box that failed over the weekend.

The box is one of two on a girder in the middle of the football field-length station that controls solar arrays, radiators, cooling loops and other functions, NASA said. 

A backup box, formally called multiplexer-demultiplexer, or MDM, is working and NASA said the station's five-person crew was not in danger. 

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"The crew has never been in any danger, and the MDM failure, believed to be internal to the box itself, has had no impact on station activities," NASA reported. 

However, after the failure on Saturday morning, NASA on Sunday gave the go-ahead to proceed quickly with a spacewalk by Peggy Whitson and Jack Fischer, hoping to limit the crew's vulnerability to a potential failure by the backup system.

The spacewalk, which will be broadcast live on NASA TV, is expected to begin around 8 a.m. Tuesday and last less than three hours. 

Whitson was part of a March 30 spacewalk that upgraded the same data relay box with new software. 

On Sunday she inspected and tested a spare box she'll install Tuesday.

While Whitson does that, Fischer will install a pair of wireless communications antennas outside the station's Destiny lab module.

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The spacewalk will be the 10th for Whitson — NASA's all-time leader for most days in space with more than 560 — and second for Fischer. The pair previously teamed up on May 12.

Tuesday's "extra-vehicular activity," or EVA, as NASA refers to spacewalks, will be the 201st supporting station assembly and maintenance.

Contact Dean at 321-242-3668 or jdean@floridatoday.com.And follow on Twitter at @flatoday_jdean and on Facebook at facebook.com/jamesdeanspace.