CHRIS HADFIELD, Canadian Space Agency:

Yes, it sounds like science fiction, doesn't it, to have an identical twin on a space ship and another one down on the ground?

But it's just luck, but, boy, it sure provides some interesting medical and scientific opportunity. You take two people that are as identical as they can be. You put them in wildly different environments, one of them that is really brand-new for humanity, living in weightlessness off the planet, and then you watch how they change over a year. You measure all of those subtle things, bone density, muscle strength, psychology, vision, blood pressure, blood pressure regulation, all of those, liver function, everything.

And it is really going to help us not only understand spaceflight for long-term flight, for going from here to the moon and Mars and beyond, but also just understand the effects on the body of flight itself, the subtle changes that happen within the body, and teach us inherently about physiology. It's a really cool thing. It's never been done before.