America's 1st woman astronaut to walk in space explains the history of NASA spacesuit sizing

For the first time in U.S. history, an all-woman team of astronauts — Anne McClain and Christina Koch — were scheduled to step out into space to repair batteries on the International Space Station on Friday.

The spacewalk, or extravehicular activity (EVA) in NASA speak, is now scheduled for Koch and male American astronaut Nick Hague. It turns out that both women need a "medium" sized hard upper torso (HUT) for their space suit, and there's only one that size on the space station that's ready to use. Koch and Hague are scheduled to embark on their space walk on Friday.

Space suits used to be custom made for the individual bodies of astronauts, but with the advent of the space shuttle program, a "Mr. Potato Head" model of mixing and matching space suit parts to allow for the increased numbers of people expected to go to space.

"The shuttle spacesuit was designed to be made of many interchangeable parts, to accommodate the large number of astronauts with widely varying body sizes. These parts (upper and lower torsos, arms, etc.) are made in different sizes," Nasa wrote on its website.

There is currently another medium HUT on the space station, but it would require at least 12 hours of work to adjust it to fit McClain, who practiced space walks in water tanks in both medium and large space suits.

Since announcing that the spacewalk would be undertaken by McClain and Hague instead of McClain and Koch, NASA has endured scrutiny and charges of sexism for not having enough space suits for women.

Unsurprisingly, space suits are intricate.

“The front plate has to be large enough to mount the computer that controls the suit and provides the astronauts all the displays they need to know what's going on and the switches to operate the suit," Kathryn Sullivan, the first American woman to walk in space told ABC News. "The back plate of it needs to be large enough to mount the life support backpack."

The computers and the backpacks are all the same size, so that imposes a limit on how small the HUTs can get, Sullivan said. "The greatest determinant of how productive and mobile you can be in space is how well you fit in the hard upper torso. It is important -- you learn this in zero gravity -- that the HUT fits very snugly around your rib cage…that insures your fingers fit in your gloves and you don't get floppy fingers."

Sullivan completed her history-making spacewalk in 1984, but NASA funding has cut so dramatically since then that today's astronauts are using the same suits. In fact, some of the parts on Koch’s spacesuit were also on Sullivan's suit 35 years ago.

"NASA's space suit inventory is woefully old and woefully paltry," Sullivan said, highlighting the agency’s budget issues.

Despite the interchangeable parts, sizing is not the only issue when it comes to the fit of suits.

Men and women, even of similar height and weights, do not have the same proportions, which was pointed out by Elizabeth Benson and NASA's Sudhakar Rajulu, in a paper titled, "Complexity of Sizing for Space Suit Applications."

“Some groups initially assumed that women could fit in the same sizes as small men — or at worst, that some of the men’s sizes would have to be scaled down proportionately to fit women,” the authors wrote in their paper. "For the same height and weight, women can have significantly wider hips and narrower shoulders than men."

"NASA ordered up small, medium and large for the shuttle program because that will fit a lot of the population," Sullivan said.

But Sullivan remembers that there were differences in how men were accommodated for sizes that fit outside the spectrum of sizes NASA created.

In the mid-80s, one astronaut in her class was a "very tall, hefty guy," Sullivan said. "NASA leadership assigned him a spacewalk, and it was discovered he can't fit the large, so an extra-large was built."

But several women who were in her astronaut class, and who were too small for the petite suits, were not similarly accommodated with extra-small suit sizes. Sullivan said that because of this, they were essentially shut out from participating in spacewalks, even if they met the other qualifications.

"Out of the six women in our group, small was too large to work effectively for people," Sullivan said. "The fact that the small would not fit cut them out of the EVA opportunity. Now you have a guy who was too big for the large, and you made him an extra-large. Surely when you discover four out of six, or six out of the first eight women astronauts, the small doesn't fit them, you make an extra small. And the answer is no, NASA didn't. It was, 'You can't make suits just for women.'"

Sullivan said that’s when the gender issue for space suiting really came in.

“It was the unwillingness, reluctance, refusal to ever practically deal with the reality that the suits failed to fit a large number of women in the astronaut corps,” said Sullivan. “NASA chose never to build an extra-small. So those gals were basically disenfranchised from spacewalks from the get go."

NASA did not immediately respond to a request for comment.