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NASA astronauts Peggy Whitson and Jack Fischer are preparing for a hastily-scheduled spacewalk to repair a faulty part on the International Space Station (ISS).

The "contingency spacewalk," which will take place on Tuesday morning, is aimed at removing and replacing a part known as the Multiplexer-Demultiplexer (MDM), which failed on Saturday morning. The cause of the failure is not known, NASA said in a blog post on Sunday.

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While this is not an emergency situation — no lives are at risk on the Space Station as a result of the outage — but having backup systems is critical to the operation of the orbiting laboratory. The MDM is used to keep solar arrays, radiators, and other systems functioning properly.

"The other MDM in the truss is functioning perfectly, providing uninterrupted telemetry routing to the station’s systems. 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 said.

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Whitson, who is the current ISS commander, will replace the failed MDM with a spare unit, while Fischer will install communications antennas on an another station component, which was a leftover to-do item after a May 12 spacewalk.

This will be Commander Whitson’s 10th spacewalk, according to NASA, and will likely move her up into third place for cumulative spacewalking time.

Fischer, by contrast, will be making his second spacewalk when he steps out into the void on Tuesday.

This particular part will be quite familiar to Whitson, since on March 30, she and astronaut Shane Kimbrough conducted a spacewalk to install it, complete with upgraded software, NASA said.

The space agency will air the spacewalk beginning at 6:30 a.m. ET on its website and NASA-TV.

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