Yet, scientists don't know how gravity and radiation will affect astronauts over such a long time, let alone the psychological impact of being in such a cramped space. Astronauts who've spent time in space suffer bone loss, or osteopenia , from the lack of gravity in space. The cosmonauts have been able to gain back that loss once back on Earth relatively easily, but no one knows exactly how debilitating a longer trip would be. Still, that's the lesser of the two evils, says Leah Braithwaite, project manager of space Life Sciences at the Canadian Space Agency. Finding a way to shield people from cosmic radiation is the biggest obstacle.
Astronauts have cumulative legal limits of radiation that they can be exposed to.
The acceptable risk for radiation is, in practical terms, an increased risk of developing fatal cancer by 3 per cent. Once an astronaut leaves the Earth's atmosphere and magnetic field, they're exposed to a whole new kind of high energy particles called galactic cosmic rays , which originate from the activity of other stars. So, while crewmembers on Mir and the various shuttles have been protected from these more harmful rays, an astronaut on his or her way to Mars would need extremely heavy and as yet uninvented shielding to protect themselves. To visually see how the galactic cosmic rays affect the human body, click on the image above. But first, the problem of estimating the risk of damage to the body's cells and the central nervous system must be solved.