From long experience on the space shuttle and various space stations, we have some knowledge of how mammals, especially people, respond to 0-g. We have even more experience with 1-g on Earth. But we still don’t know what happens in between. What, for example, will happen to humans on Mars where the surface gravity is 0.38-g? Is that enough to keep human explorers functioning properly? And, importantly, how easily will they readapt to 1-g, once they return to Earth? In 2006 a group of mice-astronauts will orbit Earth inside a spinning spacecraft. Their mission: to learn what its like to live on Mars.