Floating in the near-vacuum of space, NASA astronauts Christina Koch and Jessica Meir are making history today as they embark on the first all-women space walk, spending more than five hours outside the International Space Station on a mission to replace a failed power controller for the orbiting laboratory.

The moment comes after much hubbub; Koch and Anne McClain were supposed to make the historic all-woman walk seven months back, on March 29. But in a space walk a few days earlier, McClain got her first in-flight experience working in the type of spacesuit designed for activities outside the station, known as an extravehicular mobility unit, or EMU.

Live: Watch the first all-woman spacewalk

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While she had trained on Earth in both a medium and large version of the EMU, McClain realized after her space walk that the medium version of the hard upper torso fit best. Koch required the same size, and the other available medium components could not be correctly configured for that spacewalk in time, so McClain switched places with fellow astronaut Nick Hague.

The swap caused an uproar, but the decision—recommended by McClain herself—was logical for the situation at hand. Fit is paramount for spacesuits, preventing astronauts from bodily harm and excessive fatigue.

“When you have the option of just switching the people, the mission becomes more important than a cool milestone,” Stephanie Schierholz, a spokesperson for NASA, told the New York Times in March.

View Images Kristine Davis, a spacesuit engineer at NASA’s Johnson Space Center, shows off NASA's new spacesuit prototype intended for use in the Artemis mission to the moon. Photograph by Joel Kowsky, NASA

The recent suit debacle is more complicated than straightforward sexism. Still, the day’s events raised a very real issue for women in all fields traditionally dominated by men: The tools weren’t initially designed with women in mind. And there’s no denying that spaceflight has had a difficult past welcoming women into the astronaut ranks, from the decision to exclude women from the early U.S. astronaut corps to unfounded fears that menstruating in space might cause physical harm.

So, how are suits for space walking designed in the first place, and will new models for getting humans to the moon and beyond take women into account? We’ve got you covered.

From tailored suits to many sizes fit most

Spacesuits are vital to the success of long-duration ventures beyond our planet’s gravity, providing an appropriately pressurized, oxygenated capsule for our frail human bodies. Without them, rapid decompression in the near-vacuum of space could not only cause gases to bubble out of your bloodstream, but could also trigger rapid expansion of air in your lungs, leading to possible rupture.

Each of the early NASA spacesuits was specially designed for the individual wearers, who were entirely men until the first class of women joined NASA’s astronaut ranks in 1978. (Here’s why women may be best suited for deep-space exploration.)

The first American spacesuit designed for space walking was successfully tested in June 1965 by astronaut Ed White on a historic walk outside the Gemini spacecraft. The 21-layer outfit was adapted from thinner flight suits, which are one-piece garments worn during launch and re-entry. The primary oxygen supply to the suit remained on board, tethering White to the spacecraft. (See how flight suits evolved to keep astronauts safe.)

But with all eyes set on exploring the moon, that setup had to evolve quickly.

“The NASA Apollo program really changed the game,” says Cathleen Lewis, curator of spacesuits and international space programs at the Smithsonian’s National Air and Space Museum. “They had to design a suit that would not only operate in the vacuum of space, but would allow astronauts to explore another world—they could get up, walk around, and be autonomous.”

The Apollo spacesuits were like “mini spacecraft,” Lewis says, with each one equipped with a pressure control system, oxygen supply, urine-collection capabilities, puncture-resistant materials, and more. But designing these individualized capsules for each astronaut was costly, and the suits only got more complex through the Apollo years, Lewis explains. Even things that would seem easy to change for Earth-bound getups posed costly design hurdles for space, such as modifying the suits so that astronauts could sit down in a lunar rover.

This eventually led NASA to request suits that could be reused, based on a modular design in which components, including the arms, legs, and torso, could be swapped out. Around this time, the first American women were accepted into the astronaut training program. That’s when fit became especially challenging—and the differences between men’s and women’s bodies became an important factor.

The goal was to have components that could fit people ranging in size from a petite five-foot-tall women to a moderately sized six-foot, four-inch man—“not a linebacker, probably not even today’s quarterback,” Lewis says.

Tests revealed unexpected challenges even for the men, Lewis notes. For example, one tester initially had such limited range of motion, he couldn’t touch his elbows together out in front of him. But no one realized it was an issue until women with narrow torsos had the same challenge. Shifting the arm holes closer together allowed both small-chested men and petite women to have greater reach.

“It took trial and error to get there,” Lewis says. “When you have this range, it’s not as clear cut as male versus female; it’s trying to get the widest range of human factors.”

Initially, the sizes for the various components ran from extra-small to extra-large. Over the years, however, NASA cut the small and extra-small suits, and because women tend to be smaller than men, the change mostly impacted women astronauts.

These modular suits were part of the scrapped all-women spacewalk debacle in March, and they will again be used in today’s event. They have been updated more over the years, Lewis notes, but NASA originally intended for each suit to get plenty of use, with the initial contract requiring a 15-year lifespan for non-glove components.

Spacesuits, the next generation

The situation may soon make a small step forward—at least for spacewalkers headed beyond Earth’s orbit. On October 8, NASA unveiled its next-generation spacesuit, the Exploration Extravehicular Mobility Unit, or xEMU, to be worn on the planned Artemis missions to the moon.

While the design looks quite similar to the current suits—now with jaunty red and blue stripes—each is fitted with a host of new features. According to a NASA release, for instance, the xEMU will allow spacefarers to be more nimble, reducing the comical but energy-intensive astronaut bunny hops on the lunar surface. The astronauts should even be able to lift their arms over their heads, an impossible move in the current EMU.

And since the goal of Artemis is to put a man and a woman on the moon, the suits promise to have more of a custom fit than previous iterations. Each astronaut will undergo a full-body 3D scan as they move and hold various postures. This will allow NASA to match each man or women with the “spacesuit components that will provide the most comfort and the broadest range of motion,” according to the release.

While still a mix-and-match design, the new suits will be assembled with more components made in a wider range of sizes. Adjustable shoulders should further help customize the fit, according to The Verge.

Before the Artemis suits are moon-bound, they will have to undergo a slew of tests on Earth and possibly on the International Space Station. Even then, the new suits will likely be far from perfect. No matter who wears them, the reality of traditional pressurized spacesuits is that they are uncomfortable and difficult to maneuver. Astronauts are constantly working against the internal air pressure, so wearing one is a bit like trying to move around inside a balloon.

“Everything you’re doing seems to be pushing back against you,” Lewis says. Spacewalking astronauts must spend many grueling hours conducting experiments and executing repairs, and even in a well-fitted suit, spacewalks commonly leave astronauts suffering from various soft tissue injuries and tendinitis. (Find out why spacesuit gloves can even make astronauts' fingernails fall off.)