Last week, an American and a Russian — Scott Kelly and Mikhail Kornienko — were selected to spend a year living continuously in space, aboard the International Space Station. Only four other people have done this before. All them were Russian, so Scott Kelly is going to break the American record for time spent in space.
The mission won't start until 2015, and it's part of a much longer term goal — sending people to Mars. We know that spending time in space does take a toll on the human body. For instance, hanging out without gravity means you aren't using your muscles, even the ones that you'd use to support your own weight on Earth. Without use, muscles deteriorate over time. Bone density also drops. Basically, after a few months in space, astronauts return to Earth as weak as little kittens. Which is, to say the least, a less than ideal situation for any future Mars explorers.
Having Kelly and Kornienko stay up for a year will give scientists more data on what happens to the human body in space, give them a chance to test out preventative treatments that could keep astronauts stronger, and allows them to see how the amount of time spent in space affects the amount of time it takes to physically recover from the trip. As an extra research bonus, Kelly is the identical twin brother of Mark Kelly, the astronaut married to former congresswoman Gabrielle Giffords. Which means that there will be a built-in control to compare Kelly to when he comes back from his mission.
In honor of that upcoming experiment, here's an old video that will give you an idea of what we knew (and didn't know) back at the dawn of the space age. Science in Action was a TV show produced by the California Academy of Sciences. In this 1956 episode, they explore the then-still-theoretical physiology of space travel … with a special guest appearance by Chuck Yeager!