CNN  — 

NASA astronauts Anne McClain and Christina Koch were set to make some history at the end of this week with the first all-female spacewalk.

But the moment was put on hold – Koch will now do the spacewalk Friday with a male astronaut – because there weren’t enough spacesuits at the International Space Station that fit the two women.

NASA’s budget is $21.5 billion. Um, what exactly is going on up there?

For an explanation, we talked to two folks from NASA’s public affairs office: Brandi Dean (via email) and Stephanie Schierholz (by phone). Their answers have been edited for length.

CNN: Why weren’t there enough spacesuits that fit?

Brandi Dean: We do our best to anticipate the spacesuit sizes that each astronaut will need, based on the spacesuit size they wore in training on the ground, and in some cases (including Anne McClain’s) astronauts train in multiple sizes. However, individuals’ sizing needs may change when they are on orbit, in response to the changes living in microgravity can bring about in a body. In addition, no one training environment can fully simulate performing a spacewalk in microgravity, and an individual may find that their sizing preferences change in space.

This is all fascinating. So body sizes change in space?

Stephanie Schierholz: When you’re in space, your body changes because of the lack of gravity, right? So if you’re standing on Earth, gravity is pulling you down. The two most notable changes are fluid shifts in your body and most people’s spine tends to elongate. And that just creates conditions where your body behaves differently in space. So combine that then with spacewalking and training for spacewalks.

[Part of the astronauts’ training is at the Neutral Buoyancy Lab near the Johnson Space Center in Texas, which is essentially a giant swimming pool.]

When they’re in the neutral buoyancy lab they’re in a suit that’s pretty similar to the suit that they’d wear in space, but obviously there’s some changes made since you’re in water. And that’s the best analog we have … astronauts can have a situation in which they have a different experience when they’re actually in space than they do when they’re in the pool.

And so Ann (McClain) had trained in both a medium- and a large-torso spacesuit and thought that she was fine in either one. But when she actually got out in space wearing a spacesuit – she was wearing a medium last Friday – then she … really felt like that she should still be in a medium for future spacewalks. A large would be too big.

Dean: We have two medium hard upper torsos in space, two larges and two extra larges. However, one of the mediums and one of the extra larges are spares that would require additional time for configuration.

Schierholz: You don’t want the astronauts fighting against their spacesuit … because it’s already a physically demanding task to put them in a spacesuit and have them do work in it. And the suit itself is pressurized when they’re in space, so they’re already to some degree fighting against the spacesuit.

You want it to be as optimally fitted as possible. So the feedback from Anne was that she felt she should really be in a medium suit. There’s another medium torso but it’s not currently configured into a spacesuit. So it’s easier to swap out the spacewalkers than to swap out parts of the spacesuit.

It also helps them keep the schedule. If they reconfigure the suit, that would take significantly more time. They wouldn’t be able to do the spacewalk by Friday. And they want to do the spacewalk Friday because they have another spacewalk coming up April 8 and then there’s a rocket launch that’s bringing cargo to the space station on April 17. So they have all of these things sort of lining up that are keeping them busy.

Yeah, I can imagine.

Schierholz: It’s like if you have to reschedule an appointment, right? And you’re like, ‘Oh crap, where did that go? It doesn’t fit in the schedule anymore.’

We all can relate to that.

Schierholz: (Laughs)

How much does the body change in microgravity?

Schierholz: Most astronauts will gain up to two inches in length when their spine elongates. But it’s different for every astronaut. And it’s completely based on the lack of gravity, so the minute that they get back into a gravity environment they go back to a more compressed spine. And some astronauts report that they know that they’re longer but they don’t really feel any difference and others report, you know, they feel different as a result.

Tell me more about the spacesuits.

Schierholz: I’m pretty sure that they’re the spacesuits we had for the shuttle program, too. I know that they’re suits we’ve had a long time. And there aren’t a lot of them and that they were made to be modular so that you can configure them to be the right shape and size for as many astronauts as possible.

Anything you’d like to add before I let you go?

Schierholz: I think it’s just inevitable that there will be an all-female spacewalk. It just won’t be this Friday.