Radiation levels on the International Space Station are as high as they were on the antiquated Russian space station Mir, in spite of NASA's attempts to clad the ISS with better shielding. If NASA can't protect astronauts, its vision of sending a crew into deep space may come to nothing.
Data collected by NASA and a Russian-Austrian collaboration show that astronauts on the ISS are subjected to about 1 millisievert of radiation per day, about the same as someone would get from natural sources on Earth in a whole year. Spending three months in these conditions translates into about one tenth the long-term cancer risk incurred by regular smokers.

While this may be an acceptable risk, sending astronauts beyond the Earth's protective magnetic field will vastly increase their exposure. "If you sent two people to Mars, one of them would die," says Marco Durante of the Frederico II University in Naples, who has studied the health effects of radiation in Mir astronauts for ESA.

Radiation inside the ISS, and the now defunct Mir, is caused when the fast, heavy ions that make up cosmic rays collide with the aluminium hull, releasing a shower of secondary particles into the living quarters. To mitigate this effect, the ISS has been fitted with additional polyethylene shielding that combines lighter atomic nuclei, which are less likely to throw out neutrons when hit by cosmic rays. The dab shows this lowers astronaut exposure by a few per cent, but this is not as much as was hoped, says Thomas Berger of the Austrian Universities' Atom Institute in Vienna.

The shielding could be scaled up to cut out up to 30 per cent of the dose at the Lagrangian point, where NASA envisages setting up a space station. But that is not enough, says Frank Cucinotta, head of NASA's ISS radiation group at the Jet Propulsion Lab in Pasadena, California. "It doesn't get rid of the whole exposure unless you put in so much it's incredibly heavy." Entirely new shielding technologies will have to be developed, he says, and no one knows how long that might take.

The effects of this kind of radiation on the body are not well understood. NASA works with the same exposure limits as those set by the US Environmental Protection Agency for radiation workers on Earth - no one should receive a dose that increases their relative risk of dying of cancer by over 15 per cent. Cucinotta says the radiation risk on the ISS is 5 per cent.

But Durante disagrees. In a study of eight astronauts who had spent 70 days or longer on Mir, he found three with chromosomal abnormalities that might be pre-cancerous. From this he calculates that there is a 20 per cent higher risk of dying from cancer above NASA's limits.

There is even less of a consensus on how to convert radiation dose into cancer risk for the three to fourfold increase in radiation levels that exist beyond low-Earth orbit. That makes it impossible for authorities or the astronauts to make an informed decision about what they are letting themselves in for. And while it might be worth taking a higher risk for a one-off mission, it's not acceptable to send dozens of astronauts routinely into high-risk situations, like those that could exist at the Lagrangian points.