Astronauts in space are kind of like royalty: They’re never alone, and someone is always watching. This is, in part, because doing stuff in space is inherently fascinating. It’s also because nearly everything they do is scientifically relevant. Simply existing in a spacecraft is an experiment in itself, and Mission Control is always collecting data. Some of that data comes from monitors aboard the space craft. Some is recorded by the astronauts themselves. And some of that data is pee.

Space travel takes a toll on the human body. Astronauts have to contend with shrinking hearts, squashed eyeballs, stretched spines, and a loss of bone density. It’s that last one that concerns a team of chemists at NASA’s Lyndon B. Johnson Space Center. The space program hopes to eventually send human beings to Mars, but we certainly can’t do it if the trip will wreck our skeletons.

As Smarter Every Day host Destin Sandlin demonstrates in the video above, travelers aboard the International Space Station follow a rigorous exercise program to keep their bones strong. It’s difficult to get in a good workout without gravity, so the space program is working to fine-tune the exercise machines. To do that, they need to know how well the existing machines help preserve bone density. And to do that, they need urine.

Our pee is a pretty good indicator of what’s going on in our bodies. It serves as a repository for all the excess chemicals, nutrients, and minerals we don’t need. Astronauts losing bone mass will have high levels of calcium in their urine. Or, as Destin puts it, “they pee out their bones.”

The urine is collected in space and comes back down with the astronauts. By the time it reaches the laboratory, it’s come a long, long way. The government goes to great lengths to keep the urine safe. As NASA biochemist Scott Smith explains in the video: “It is considered a national treasure.”

Header image via YouTube // SmarterEveryDay

[h/t The Nerdist]