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CAPE CANAVERAL, Fla. – NASA astronauts will take an unplanned spacewalk Tuesday (May 23) to replace a failed data relay box outside of the International Space Station, restoring critical redundancy to the orbiting outpost, agency officials said Sunday (May 21).

The failed device, known as a multiplexer/demultiplexer, or MDM, was installed on March 30 during a spacewalk by NASA astronauts Peggy Whitson, now the station's commander, and then-flight engineer Shane Kimbrough, who returned to Earth last month. Whitson will perform Tuesday's spacewalk repair with fellow NASA astronaut Jack Fischer  who arrived at the orbiting laboratory in April. [Watch: Whitson and Fischer Play Zero-G Water Ping Pong]

The MDM, one of two located on the outside of the station's S0 segment of the station's backbone-like main truss, controls exterior U.S. systems, including solar arrays, cooling loops, radiators and other equipment.

The upgraded MDM failed on Saturday (May 20), NASA officials said in a statement, adding that the crew was never in any danger.

"The cause of the MDM failure is not known," the statement said.

After the failure, the station switched over to the spare MDM, but NASA wants to restore redundancy as quickly as possible.

Station program managers met on Sunday to assess troubleshooting efforts and decided to replace the failed unit with a spare.

Working inside the station, Whitson spent Sunday assembling and testing a new MDM, which will be installed during a two-hour spacewalk Tuesday. The unit measures 10.5- by 14.9- by 16.4 inches (27- by 38- by 42 centimeters) and weighs 51 lbs. (23 kilograms.)

Whitson and Fischer conducted the station's last spacewalk on May 12. The station’s current crew also includes two Russian cosmonauts and French astronaut Thomas Pesquet of the European Space Agency.

The MDM failure and replacement is not expected to impact next week's launch of a SpaceX Dragon cargo ship to the station, NASA spokesman Dan Huot said.

Launch of a Falcon 9 rocket with the Dragon freighter currently is targeted for June 1.

Tuesday's spacewalk is scheduled to begin at 8 a.m. EDT (1200 GMT). NASA's live webcast coverage will begin at 6:30 a.m. EDT (1030 GMT). You can watch the webcast live here, courtesy of NASA TV.

 Irene Klotz can be reached on Twitter at @free_space. Follow us @Spacedotcom, Facebook and Google+. Original article on