The highly anticipated first all-female spacewalk has been canceled, NASA announced Monday, because the International Space Station is not equipped with enough spacesuits in the correct size.

NASA astronauts Anne McClain and Christina Koch had been scheduled to conduct the spacewalk together on March 29, following another spacewalk that featured McClain and NASA astronaut Nick Hague on March 22. Based on discussions with McClain and Koch after that first spacewalk, however, mission managers decided to change the assignments for March 29, NASA explains, "due in part to spacesuit availability on the station."

During the March 22 spacewalk, McClain realized her best fit was a medium-sized hard upper torso — "essentially the shirt of the spacesuit," according to NASA. There are two medium-sized hard upper-torso components aboard the space station, a NASA official tells NBC News, but one is a backup that would need additional work before it could be used in a live spacewalk. That leaves only one available in time for Friday's spacewalk, so Koch will wear it and Hague will fill in for McClain, who is tentatively scheduled to conduct her next spacewalk April 8 with Canadian astronaut David Saint-Jacques.

"It is more efficient to swap spacewalkers than to reconfigure the elements of the spacesuit," NASA spokeswoman Stephanie Schierholz tells NBC News, noting the fit of a spacesuit can affect an astronaut's ability to maneuver and perform tasks.

In the end, the decision was all about safety, and nothing else.

Editor's note: The update above was added March 26. The original story continues below.

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In July 1984, cosmonaut Svetlana Savitskaya became the first woman to walk in space. At the end of March, 35 years after that historic occasion, two women will conduct the first all-female spacewalk.

NASA astronauts Anne McClain and Christina Koch, with support from Canadian Space Agency flight controller Kristen Facciol in Houston, will perform a spacewalk scheduled for March 29. It will be the first time a spacewalk will involve only women.

Facciol broke the news on Twitter on March 1:

In a statement emailed to CNN, NASA spokeswoman Stephanie Schierholz confirmed the news on March 6, saying "As currently scheduled, the March 29 spacewalk will be the first with only women. It is the second in a series of three planned spacewalks. Anne also will join Nick Hague for the March 22 spacewalk. And, of course, assignments and schedules could always change.

"It was not orchestrated to be this way; these spacewalks were originally scheduled to take place in the fall," Schierholz added. "In addition to the two female spacewalkers, the Lead Flight Director is Mary Lawrence, and Jackie Kagey (also a woman), is the lead EVA (spacewalk) flight controller."

Anne McClain at the Gagarin Cosmonaut Training Center in Star City, Russia Anne McClain at the Gagarin Cosmonaut Training Center in Star City, Russia, during her back-up training for Expedition 56 in May 2018. (Photo: Elizabeth Weissinger/NASA/Wikimedia Commons)

The spacewalk is slated to begin at 6:30 a.m. EST, and it should last for about seven hours. It will stream on all of NASA's channels.

McClain and Koch were both selected to be astronauts in 2013. Their astronaut class, Group 21, is comprised of four men and four women, the highest percentage of women in a NASA astronaut class. The other two women from the group, Jessica Meir and Nicole Aunapu Mann, are currently awaiting flight assignments.

"I think it's actually just a reflection of how many really talented women are in science and engineering these days," NASA astronaut Kathleen Rubins told The New York Times in 2013 about Group 21.

McClain arrived aboard the International Space Station in December 2018, a member of the Expedition 58/59 crew. Koch is scheduled to launch into space this week as part of the Expedition 59/60 crew.

Spacewalks are conducted for a range of reasons, from performing experiments outside of the International Space Station to testing and repairing equipment.

NASA cancels first all-female spacewalk due to shortage of spacesuits in correct size
NASA astronauts Anne McClain and Christina Koch will both still perform spacewalks, the agency says, just not at the same time.