NASA’s Grueling Underwater Test for Astronauts

A giant swimming pool is the best place on Earth for simulating weightlessness.

An astronaut in the pool at NASA’s Neutral Buoyancy Lab

HOUSTON—Before astronauts can launch into space, they have to go for a swim.

The pool sits inside a big, windowless building in Houston, on the grounds of NASA’s Johnson Space Center. It is about 40 feet deep, and holds enough water to fill several Olympic-size pools. Beneath the surface, shrouded in the bluish tint of the water, is a replica of the International Space Station.

This facility, known as the Neutral Buoyancy Laboratory, is where future astronauts train for spacewalks outside the ISS. It’s strange to think that learning to survive in a lifeless void requires bobbing around inside a pool, but water provides the best medium on Earth for simulating the weightlessness of space. Stick an astronaut candidate inside a spacesuit, add some weights, and she becomes “neutrally buoyant”—she neither floats nor sinks. Suspended in this state, candidates wade around in the depths for hours, carrying out repairs and other tasks on the fake space station in preparation for the real thing, 260 miles above Earth.

The morning I visited the lab, two NASA recruits were preparing for a swim. The smell of chlorine was thick in the air, mixed with whiffs of warm popcorn; the staff makes some every Tuesday and Friday in an old-fashioned machine in the front office. All eyes were on Loral O’Hara, an engineer, and Raja Chari, an Air Force pilot, dressed in cream-colored onesies and blue surgical booties.

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.

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.

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.

O’Hara and Chari didn’t look nervous as they went in, but this wasn’t their first run. (Dean said the agency tries to keep reporters and visitors out of the building for those.) The candidates are nearing the end of their two-year training—nine underwater rehearsals are required before they can be considered for flight assignments. Some astronauts do many more runs, especially if years pass between their flights and they need a refresher.

Michael López-Alegría, who, like Whitson, completed 10 spacewalks in his NASA career, says he felt nervous before his first run, in the late 1990s. He wanted to impress the NASA officials who picked which astronauts could spacewalk, known formally as an extravehicular activity, or EVA. López-Alegría and his fellow candidates didn’t know exactly how the selection process worked, and at the time jokingly referred to decision makers as the “the EVA mafia.” “It’s not about, Am I going to drown?” he says. “It’s about, Am I going to do well enough to make it through this opaque filter?

Spacewalking underwater can be hard on the body. Astronauts emerge with sore muscles and tender fingertips, and in some cases, acquire long-term health problems. A 2005 survey of 770 pool rehearsals found that astronauts reported injuries in about 25 percent of tests. Hands suffered the most damage, followed by shoulders and feet. Astronauts are especially vulnerable when they work upside down, an alignment that presses their shoulders against the rigid spacesuit. “You’re kind of pushing down on your shoulder as you’re rotating your arm up, and over time you start wearing out cartilage and tendons,” López-Alegría says. He ended up with torn cartilage, as well as a tendon in his bicep, and later had surgery to address the condition.

Whitson felt it most in her elbows. She attributes the issue to the suit, which, even in the smallest size available—a medium—was too big for her. Technicians added padding, but she says she still found herself flopping around inside of it. The effort to stabilize herself led to overuse in her elbows, including torn tendons and muscles. Whitson says she can’t straighten her arms all the way because of her training in the pool.

Over the years, NASA has made tools lighter and limited the time astronauts spend upside down to reduce discomfort and injury, astronauts say. But real spacewalks still feel easier on the body than pretend ones. “I think sometimes, doing the tasks in the pool is to some degree more challenging than doing them in space,” López-Alegría says.

I didn’t stick around for O’Hara and Chari’s entire spacewalk; from the deck, there’s not much to see. Hours later, the astronaut candidates were hoisted out of the water, undressed, and released. Someday, they will emerge from an airlock, as they have practiced over and over, and see the satiny clouds and oceans instead of a cement pool floor. Until then, Whitson says, their next move is less mesmerizing: “Take aspirin and sleep.”