NASA has aborted its first all-female spacewalk outside the International Space Station because there are not enough spacesuits of the best size to fit both female astronauts, the agency said.

Astronauts Anne McClain and Christina Koch had planned to conclude Women’s History Month with a spacewalk Friday to continue work outside the space station.

But NASA spokeswoman Stephanie Schierholz said in an email Tuesday to The Washington Post that both women need a medium-size hard upper torso — the shirt of the spacesuit — and there is only one medium-size suit on the space station that is ready for use. NASA said astronauts Nick Hague and McClain completed the first spacewalk in the series earlier this month, so Hague and Koch plan to set out on the next one later this week.

Still, Schierholz said, “We believe an all-female spacewalk is inevitable.”

NASA said the two female astronauts had prepared to continue work outside the International Space Station installing lithium-ion batteries for the space station’s solar arrays.

Then there was a spacesuit problem.

Schierholz told The Post that McClain had trained in both large-size and medium-size suits, but, during her first spacewalk last Friday, she realized that the medium was the better fit. Koch also wears a medium, and Schierholz said that although there are two mediums on board, only one of them is in a “readily usable configuration.”

“Koch will wear it,” Schierholz said. “It is more efficient to swap spacewalkers than to reconfigure the elements of the spacesuit.”

Schierholz explained that “spacesuits are not ‘one size fits all.’ ”

“Astronauts typically describe spacewalks as the most physically challenging thing they do,” she said. “Working in a pressurized spacesuit requires physical strength and endurance, and it is essential that the spacesuit fits as well as possible.”

She added that “an optimally fitted spacesuit improves an astronaut’s ability to accomplish the tasks. We do our best to anticipate the spacesuit sizes that each astronaut will need, based on the spacesuit size they wore in training on the ground, and in some cases astronauts train in multiple sizes. However, the sizing needs of crew members may change when they are on orbit, in response to the changes living in microgravity can bring about in a body.

“In addition, no one training environment can fully simulate performing a spacewalk in microgravity, and an individual may find that their sizing preferences change in space.”

Both McClain and Koch were members of NASA’s 2013 astronaut class, 50 percent of which was made up of women, NASA said.

McClain, a U.S. Army officer and a pilot, was chosen in June 2013 to join NASA’s 21st astronaut class.

"I wanted to be an astronaut from the time I was 3 or 4 years old,” McClain said in a 2015 NASA video interview. “I remember telling my mom at that time, and I never deviated from what I wanted to be. Something about exploration has fascinated me from a young age.”

The astronaut said that the “combination of the teamwork, the exploration, being somewhere that nobody else has ever been always fascinated me.”

McClain set out for the International Space Station in December, according to her biography.

Koch, an electrical engineer, also received her astronaut training in 2013, her biography said.

But space is just the latest exciting frontier she has conquered: Her work has taken her on expeditions to the South Pole and the Arctic.

Koch said in an interview last month that there were several spacewalks planned, and she would be “very excited to execute that mission.”

When asked about the importance of conducting her mission during Women’s History Month, she said, “It is a unique opportunity, and I hope that I’m able to inspire folks that might be watching.”

Noting that she did not have many engineers to look up to growing up in Jacksonville, N.C., Koch added, “I hope that I can be an example to people that might not have someone to look at as a mentor … that it doesn’t matter where you come from or what examples there might be around you, you can actually achieve whatever you’re passionate about.”

“If that’s a role that I can serve,” she said, “it would be my honor to do that.”

Koch was sent to the International Space Station earlier this month.

NASA said McClain became “the 13th female spacewalker” earlier this month, and Koch will be the 14th later this week when she and Hague complete their spacewalk.

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