Can people live on Mars? Building a rocket is hard, but living there is even harder. First, there is strong radiation from space, because Mars has almost no protection.

Second, gravity on Mars is only about 38 percent of Earth's. No one knows how the body will change after years there. Third, the soil has a harmful chemical. Also, the air is very thin, so people need spacesuits.

When people talk about living on Mars, they often focus on rockets. However, scientists worry more about what happens after arrival. The first problem is radiation. Earth is protected by its atmosphere and magnetic field, but space and Mars have almost no such shield.

Based on measurements by NASA's Curiosity rover, a round trip with a stay of about 500 days could expose astronauts to about one sievert, hundreds of years' worth of natural radiation in Japan. The second problem is gravity, which is only 38 percent of Earth's. No one knows what years in such low gravity would do to the body. Finally, Martian soil contains perchlorate, which can harm the thyroid.

Discussions of settling Mars tend to center on rockets, yet researchers in space medicine are more concerned with what happens after landing. NASA groups the hazards of human spaceflight into five categories, and three are especially acute on Mars. The first is radiation. Without Earth's thick atmosphere and magnetic field, astronauts would be exposed to galactic cosmic rays.

Instruments on the Curiosity rover recorded about 1.8 millisieverts per day during the cruise to Mars and about 0.64 per day on the surface. Extrapolating to a 180-day journey each way plus a 500-day stay yields roughly one sievert, enough to raise concerns about cancer risk. The second is reduced gravity: at 38 percent of Earth's, it is unclear whether Martian gravity is sufficient to maintain bone and muscle over years, and nobody knows how children would develop.

The third is toxic soil. In 2008, the Phoenix lander found perchlorate making up about 0.4 to 0.6 percent of the soil by weight, a chemical that can disrupt thyroid function, and the fine dust can enter suits and habitats. Ironically, perchlorate might also be broken down to supply oxygen. These estimates involve uncertainty, since actual doses depend on shielding and solar activity.