This morning, two astronauts will suit up and head outside the International Space Station for a spacewalk — but the duo isn’t exactly the one the world was hoping for. Originally, NASA had announced that today’s excursion would be performed by the two women currently on board the ISS, Anne McClain and Christina Koch, making it the first all-female spacewalk in history. But due to some last-minute logistics and suit availability on the ISS, a male astronaut, Nick Hague, is taking McClain’s place instead.

When NASA first announced that today’s scheduled spacewalk would be conducted by only women, the news was met with widespread praise and excitement. Since the advent of human spaceflight, only 13 women have ever performed spacewalks, and all of them have done their work with men. It’s no surprise that the news of the astronaut swap was met with widespread disappointment and outrage.

So, what happened?

The decision ultimately came from McClain. A week ago, she performed a spacewalk with Hague, the first in a series of three aimed at swapping out aging batteries on the outside of the ISS. During that walk, she wore a space suit that had a medium-sized torso, a hard fiberglass shell that covers the chest, stomach, and back. “It’s like a piece of armor,” Mark Vande Hei, a NASA astronaut who lived on the ISS in 2017, tells The Verge. Astronauts have different sizing options when they put on a suit, and McClain thought she could operate in either a medium-sized torso or a large-sized one. But when she worked with the medium, she realized that size suited her best and that she did not want to wear a large for her safety.

This realization is what prompted the change. Right now, there are two fully prepared space suits that the crews have been using for their spacewalks — one with a medium torso and one with a large. McClain decided that Koch should wear the medium one while Hague takes the large he used last week. When NASA first announced this, though, the agency made it seem as though there just wasn’t another medium option available. That triggered indignation at the thought that NASA did not have a supply of sizes suitable for women on board.

The reality is much more nuanced

The reality is much more nuanced. NASA has six different-sized torsos available on the ISS right now: two mediums, two larges, and two extra-larges. But wearing these different options isn’t as simple as slipping on a new uniform. The space suits that NASA uses for spacewalks are more a collection of pieces bolted together than they are a single ensemble. Each suit consists of shoulder bearings, a torso, arms, gloves, pants, a life-support pack, and a control module on the front of the suit. “It’s got a lot of flexibility,” says Vande Hei. “You can really adjust the suit to customize it for a particular crew member.”

Putting all of these components together takes time. Getting the second medium-sized torso ready for today’s spacewalk would have taken up to 12 hours, according to NASA. Additionally, the other medium torso is considered a spare, so it doesn’t have the display and control module attached, which is needed for astronauts to control their fans, radio systems, and more. Because of the extra work required to get it ready, the reconfiguration could introduce some risk into the spacewalk — something NASA is fundamentally hesitant to do. “We would have taken a lot of the crew’s time away, and also actually increased the risk that we’re adding in a suit that might have some other issues,” says Vande Hei.

Timing is also important for these spacewalks, as the astronauts have very busy schedules. The crews are working on experiments and research projects. Plus, a Northrop Grumman cargo resupply ship is launching to the ISS in mid-April, and the crews will need to be ready to receive it. Not long after that, some of the current crew members will depart the ISS this summer.

“This decision was based on my recommendation.”

Because of all this, NASA decided to swap the personnel rather than postpone the spacewalk or give the crew extra work. Vande Hei says it was a hard decision. “It was a little challenging when we got the recommendations from the on-board crew to make this change,” he says. “We pushed back hard and made sure they understood that we really believed that they were fully capable of doing it.” But in the end, NASA respected McClain’s decision. “This decision was based on my recommendation,” McClain tweeted. “Leaders must make tough calls, and I am fortunate to work with a team who trusts my judgement.”

But while overt sexism may not have contributed outright to this decision, the news did highlight past decisions NASA has made that have ultimately limited what types of space suits are available on the ISS — and by extension, who can wear them. The space suits on board actually date back to the 1980s. NASA made the decision decades ago to only have three sizes of the space suit torso — medium, large, and extra-large — in part because the pieces are costly to make. This has dictated how spacewalks are assigned ever since. For some smaller women astronauts, this meant spacewalks weren’t even an option.

Meanwhile, the future of NASA’s space suit supply is bleak. A 2017 report by NASA’s inspector general found that of the original 18 suits the space agency created for the ISS, only 11 functioning ones remain. Just four of these suits, with life-support packs, are on the station right now. If anything happens to the current stock, NASA may not have enough space suits to perform activities outside of the space station over the next decade, a shortage that would affect all astronauts, regardless of gender.

That future isn’t here yet, and it’s possible there’s still time for an all-female spacewalk to happen. NASA maintains that it should be on the horizon, since the space agency has more female astronauts than ever. But exactly when that will happen is still unclear. The next two women who will fly to the space station are Suni Williams and Nicole Mann, both of whom are flying on Boeing’s new crew capsule, the CST-100 Starliner. They will each be on separate flights of that vehicle, and the schedule is too fluid to know if they’ll have the chance to do any spacewalks with other women.

Update March 29th 7:25AM ET: This article was updated to clarify upcoming schedules.