Narrow tubes snaked along the back and legs of their garments, ready to receive water that would help cool down the wearers during the spacewalk rehearsal, a physically challenging exercise that can get pretty sweaty. Beneath the onesies, the candidates wore maximum-absorbency garments, the polite name NASA uses for diapers. A typical spacewalk rehearsal lasts about six and a half hours, with no breaks.

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O’Hara began to suit up near the pool’s edge. A pair of marshmallow-y spacesuit pants lay splayed out on a white mat. O’Hara lay down on her back, reached for the pants, and started shimmying her way in. NASA spacesuits are modular, like Mr. Potato Head parts, and come in different sizes. The upper half of the spacesuit waited for O’Hara on a special platform, propped upright above the ground. O’Hara crouched and wriggled into it from below, until her head poked through the top. Chari did the same with his own suit. NASA staff helped with the rest: sleeves and gloves, a Snoopy-esque cap with a microphone, a tool belt, and finally, the rounded helmet.

The entire process took about 45 minutes, and reminded me of Snow White’s singsongy morning routine, with technicians in T-shirts playing the part of the woodland creatures who help her get ready.

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With a whirring noise, the platform holding the astronauts started moving. A bulky yellow crane carried it over the water, and then lowered it slowly. O’Hara and Chari began to disappear into the pool, boots first. When the water hit their helmets, I inhaled sharply, an involuntary reaction to the strange sight of two people, strapped in, becoming submerged. Then I remembered the astronauts have the air they need, piped into their suits via umbilical-like cords that hang over the pool.

Eight scuba divers, trained to assist during spacewalks, meet the astronauts underwater and guide them toward their starting point, the airlock of the replica station. (When they’re not guiding the spacewalkers, said Brandi Dean, the NASA public-affairs officer who showed me around, they’re allowed to swim laps over the station.) From the deck, the spacewalkers become distorted, white blobs in the aquamarine liquid.

The view can be disorienting, thanks to a combination of the water and the shape of the helmet. “It is a weird sensation as you first go underwater with this bubble around your head,” says Peggy Whitson, a NASA astronaut who has accumulated hundreds of hours of training at the Neutral Buoyancy Laboratory, and conducted 10 spacewalks, a record for women in the astronaut corps. “Some people actually get motion-sick from it until they get used to it.”

In a room upstairs, Mission Control follows the movements of the submerged astronauts, in real-time footage captured by divers with cameras. The astronauts travel glove over glove over the station, using neutrally buoyant versions of tools to conduct tasks that mimic the real work as much as possible. They also practice emergency drills, such as rescuing an incapacitated spacewalking partner in less than 30 minutes, a procedure that requires, perhaps counterintuitively, putting tools away first. In space, astronauts can’t drop everything and run. “Throwing something overboard—you have the risk of potentially running into that later,” Whitson says.