NASA was set to reach a milestone this Friday with the first ever all-female spacewalk. The historic moment was postponed, however, because only one suit in the correct size for both women was safe-to-use and at-the-ready on the International Space Station (ISS), report Jacey Fortin and Karen Zraick at The New York Times.

Astronaut Anne McClain and fellow astronaut Christina Koch were set to exit the ISS to conduct a six-hour mission to replace two large lithium-ion batteries outside the space station on March 29. Women would have also performed crucial work on the ground; Mary Lawrence and Kristen Facciol were set to serve as lead flight director and lead spacewalk flight controller, respectively, supporting McClain and Koch from NASA’s Johnson Space Center in Houston.

McClain had noticed a week prior that the top half of the suit, or “shirt,” she had been using was too big to use outside the ISS and decided it be safer for someone else to take her place. (There are two medium-size suits available, but one is not currently suitable for work outside the spacecraft.) Ill-fitting suits can affect visibility and mobility while conducting tasks outside the space station, as well as create difficulty changing the settings within the suit itself. Spacewalks are considered one of the most physically challenging parts of an astronaut’s job.

In addition, it can take close to an hour just to put a suit on, and that’s without making adjustments. It’s much easier to substitute an astronaut who will properly fit the available suit, so NASA astronaut Nick Hague will tap in.

“When you have the option of just switching the people, the mission becomes more important than a cool milestone,” NASA spokesperson Stephanie Schierholz said.

The mission would have been a fitting landmark moment during Women’s History Month, though NASA spokeswoman Stephanie Schierholz tells CNN’s Gianluca Mezzofiore that it “was not orchestrated to be this way” and “assignments and schedules could always change.”

A spacewalk, also known as extravehicular activity (EVA), is a broad term that applies to any time an astronaut exits a vehicle while in space. According to NASA, astronauts conduct spacewalks for a number of reasons: to carry out experiments in space, to test new equipment, or to repair satellites and spacecraft.

The first ever spacewalk was carried out by Alexei Leonov in March 1965, and the first woman to take part in a spacewalk was Russian cosmonaut Svetlana Savitskaya, who performed welding experiments outside the Salyut 7 space station on July 25, 1984. Though women make up a growing number of NASA astronauts, women continue to be underrepresented in space exploration. And for the most part, “spacewalks have been ... conducted by male astronauts, with the help of some female crewmembers,” as Shaiann Frazier previously reported for NBC News.

The two women set to make space history on March 29 were both part of NASA’s 2013 astronaut class. McClain, an aerospace engineer and a senior army aviator, has been on board the International Space Station since December 2018. Koch, who has a background in electrical engineering and physics, will launch into space on March 14 and join McClain at the ISS.

Earlier this month, Facciol, the lead flight controller, tweeted her support for an all-female spacewalk. But, she added in another tweet, “here’s hoping this will be the norm one day!”

Because in this case, two of the six astronauts trained to complete this specific task happened to be women, the opportunity for another all-female spacewalk could come around soon, report Lindsey Bever, Kayla Epstein and Allyson Chiu for the Washington Post.

“We believe an all-female spacewalk is inevitable,” Schierholz told The Post.

Editor's Note, March 27, 2019: This story was updated to reflect the cancellation of the first all-female spacewalk.