Whitson, the commander of the expedition 51 mission on the ISS, has spent more time in space outside of the ISS than any other woman in history. About an hour into the repair mission, she reached third place for most time spent on spacewalks, with a career spacewalking total of 60 hours and 21 minutes.

The busted box, a device called a multiplexer-demultiplexer data relay, failed without warning Saturday. The box controls radiators, solar arrays and cooling loops from the central truss of the ISS. In a statement Monday, NASA said that “the crew has never been in any danger;” a secondary box took over for the broken relay while the astronauts readied a spare.

Whitson prepared the replacement box within the ISS on Sunday. The last time astronauts conducted an emergency spacewalk was in late 2015, when astronauts had to unstick a brake handle on an ISS external rail car. NASA astronauts Rick Mastracchio and Steve Swanson replaced a relay box that went on the fritz in 2014.

Spacewalks, though, are not exactly a jaunt through the park. NASA plays down the risk, but as The Washington Post noted in 2014, they are “extremely tricky and innately dangerous.” In 2013, a line clogged with aluminum silicate waste caused water to flow into the helmet of Italian astronaut Luca Parmitano, who almost became the first person to drown in space.

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As he recounted in a blog post, the leak first wet the sponges that covered his ears, and then impaired his ability to see. Then, it began to cover his nose. “By now, the upper part of the helmet is full of water and I can’t even be sure that the next time I breathe I will fill my lungs with air and not liquid.” He made it back safely, guided by astronaut Chris Cassidy who also was spacewalking at the time.

“There is nothing about this device that makes it susceptible to being replaced,” NASA spokesman Daniel Huot told The Post in an email. “In fact, when the MDM was replaced in 2014, it had run nonstop for 12 years without issue.” A power card component, which Huot likened to the power supply in a home computer, was responsible for the 2014 failure. Although the device failed with the same lack of symptoms in April 2014, it would not be possible to speculate whether the power supply also broke down Saturday, he said.

Engineers on the ground will have to evaluate some components from inside the relay box to determine what malfunctioned. “We’re currently looking at when we’ll be able to return the hardware,” Huot said, “with the upcoming SpaceX CRS-11 mission being the earliest candidate.” That mission is set to launch June 1.

This post has been updated.