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NASA astronauts Peggy Whitson, seen here on her record-breaking eighth spacewalk in March 2017, and Jack Fischer will venture outside the International Space Station May 23, 2017, to conduct the 201st spacewalk in support of space station assembly and maintenance. Photo: NASA

NASA has ordered up urgent spacewalking repairs at the International Space Station.

On Tuesday, two astronauts will venture out to replace a data relay box that broke over the weekend. The job falls to the two Americans on board: commander Peggy Whitson and Jack Fischer.

The failed unit was installed not quite two months ago. It is one of two that control the station's radiators and solar panels, among other critical systems. NASA said everything is still safe in orbit because one relay box is still working, and no operations have been affected. But officials want the bad one quickly replaced, in case the good one also goes down.

Astronauts performed a similar spacewalk in 2014.

This so-called multiplexer-demultiplexer unit, or MDM, failed Saturday for unknown reasons. It was a refurbished device — containing upgraded software — and was installed at the end of March by Whitson and another astronaut. Station managers decided Sunday to do a spacewalk Tuesday morning to replace it with an on-board spare. The spacewalk should last just two hours.

Whitson and Fischer went spacewalking 1½ weeks ago. That excursion was cut short because of a leaky umbilical hose inside the space station. If Tuesday's replacement work goes well, Fischer also will hook up wireless communications antennas outside the orbiting lab, a job left undone during the May 12 spacewalk.

Six months into a 9 1/2-month mission, Whitson already holds multiple space records, including most accumulated time in space by an American and most spacewalks by a woman. This will be her 10th spacewalk and tie the record for most spacewalks by a U.S. astronaut.

Fischer arrived at the space station last month. Also on board are two men from Russia and one from France.

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