What happens to your brain in space?

For the past 50 years since the end of the Apollo programme, none of this has really been an issue. Astronauts arrive on the ISS and are a bit clumsy for a while, bumping into the walls or moving things around with too much force, but then they adapt. When crew return to Earth, they are helped from the capsule, looked after and rehabilitated back to life in normal gravity. But what about future long-duration missions to the Moon or Mars, that Nasa and China have in their sights. For these, astronauts will need to transition between gravity and no gravity.