MELBOURNE, Fla. — A pair of NASA astronauts will perform a spacewalk 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 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. 

Related: Woman astronaut completes record 8th spacewalk, with a tiny hiccup

“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 Saturday morning, NASA on Sunday gave the go-ahead to proceed quickly with a two-hour spacewalk by Peggy Whitson and Jack Fischer, limiting the crew’s vulnerability to a potential failure by the backup system. It’s unknown why the box failed.

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 that she’ll install Tuesday.

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 May 12. Whitson’s spacewalk Tuesday will tie the record for most spacewalks by a U.S. astronaut.

That excursion was cut short because of a leaky umbilical hose inside the space station. 

Related: Spacewalk opens door for Boeing, SpaceX crews

While Whitson replaces the data relay box, Fischer will install a pair of wireless communications antennas outside the station’s Destiny lab module. That job was left undone during the May 12 spacewalk.

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

The spacewalk is expected to begin around 8 a.m. Tuesday, possibly earlier, and will be broadcast live on NASA TV. 

Contributing: The Associated Press. Follow James Dean on Twitter: @flatoday_jdean