Humans traveling beyond the protection of Earth's atmosphere and magnetic field risk radiation-caused cancers and other diseases.

Radiation in space takes the form of subatomic particles from the sun as well as from sources in the Milky Way galaxy and beyond. These high-speed particles tear through DNA molecules, splitting them or damaging the instructions they have encoded for cell reproduction. The damaged DNA can lead to cancers or other diseases.

Radiation exposure can be acute (a high dose in a short period of time) or chronic (low levels of radiation over a long time).

The magnetic field generated by electric currents in the Earth’s liquid iron core extends far into space, shielding the planet from 99.9 percent of harmful radiation. The Earth’s atmosphere provides additional protection, equal to a slab of metal about 3 feet (1 meter) thick.

For people outside the protection of Earth's magnetic field, space radiation becomes a serious hazard.

An instrument aboard the Curiosity Mars rover during its 253-day deep-space cruise revealed that the radiation dose received by an astronaut on even the shortest Earth-Mars round trip would be about 0.66 sievert. This amount is like receiving a whole-body CT scan every five or six days.

A dose of 1 sievert is associated with a 5.5 percent increase in the risk of fatal cancers. The normal daily radiation dose received by the average person living on Earth is 10 microsieverts (0.00001 sievert).

The moon has no atmosphere and a very weak magnetic field. Astronauts living there would have to provide their own protection, for example by burying their habitat underground.

The planet Mars has no global magnetic field. Particles from the sun have stripped away most of Mars’ atmosphere, resulting in very poor protection against radiation at the surface. The highest air pressure on Mars is equal to that at an altitude of 22 miles (35 kilometers) above the Earth’s surface. At low altitudes, Mars’ atmosphere provides slightly better protection from space radiation.