Going to space is serious business. How serious? Well, let this video from NASA astronaut Drew Feustel tell you.

Feustel spent 197 days - so just over half a year - on board the International Space Station. That's a pretty standard mission length. And all that time is spent in microgravity.

But we humans aren't built for weightlessly floating around. When our bodies aren't constantly straining against the pull of gravity, all sorts of strange things happen. Among those, our muscles atrophy, and we lose bone density at about 10 times the rate of osteoporosis.

Moreover, when astronauts come back to Earth after a duration in space, the return to gravity can induce some pretty severe vertigo as their sense of balance readjusts.

So, moving around is a lot more difficult than when they left. Feustel's video, posted to Twitter in December last year, shows just how much. He stumbles trying to walk just a few steps in a heel-toe straight line.

The ISS is currently equipped with a number of machines to give astronauts a full-body workout. On average, they spend two hours a day using them.

This regimen is designed to try and mitigate the atrophy somewhat, but even with the exercise program in place, it takes at least three to four years for an astronaut to fully recover after a six-month stint.

This is just one of the many challenges we'll need to figure out for the inevitable trip to Mars. The longer the stay, the greater the bone density loss, and the trip to Mars is at least six months each way.

From March 2015 to March 2016, astronauts Scott Kelly of NASA and Mikhail Korniyenko of Roscosmos spent 342 days in space to conduct observations on the health effects of a long space mission.

Kelly, as you might guess, was pretty unsteady on his feet too. Astronauts sure are dedicated!