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Watch: NASA astronauts performing emergency spacewalk outside ISS

NASA astronauts Peggy Whitson and Jack Fischer are participating in an emergency spacewalk outside the International Space Station on Tuesday to repair a computer relay box that failed on Saturday.

NASA says the box, which is known as a multiplexer-demultiplexer (MDM), is one of two units that regulate the operation of radiators, solar arrays and cooling loops. They also route commands to other vital station systems.

The broken box was installed on March 30 during a spacewalk by Whitson and astronaut Shane Kimbrough.

Whitson prepared the replacement box on Sunday and tested the components before reporting it was ready to be taken outside the ISS and installed.

During the spacewalk, Fischer will also install a pair of antennas on the U.S. Destiny Laboratory module to enhance wireless communication capability for future spacewalks.

The spacewalk is the 201st from the International Space Station. It’s the 10th for Whitson and the 6th for Fischer.