By Sarah Scoles

On Monday, an astronaut capsule that looks like a giant orange juicer splashed down in the Atlantic Ocean, bringing its four-person crew back under the influence of Earth's gravity. These astronauts have spent six months on the International Space Station, and so the gravity now tugging at their bodies will feel familiar to them, but strange.

This team, called SpaceX Crew-2, spent much of the past half-year in orbit doing spacefaring scientific work, like testing out “tissue chips,” small-scale analogs of human organs. But they also whiled away the hours as gym rats: Six days a week, they had a 2.5-hour exercise block to reduce the damage that living in space can do to the body. Space, as they say, is hard. But it’s particularly hard on humans. Radiation, lack of gravity, and living in confined spaces each take their tolls.

“NASA has always been concerned with the effects of spaceflight on the human body, from the very first space missions,” says Michael Stenger, element scientist for Human Health Countermeasures, the agency’s arm dedicated to understanding how spaceflight affects physiology and mitigating those effects. One big problem is that living on-orbit is physiologically similar to bedrest, even if you’re bouncing around doing experiments all day. “Being in space is a lot like laying around doing nothing,” he says.