Space is not empty but rather is saturated with charged particles. Some are flung from the sun in solar flares and coronal mass ejections. An astronaut protected only by a spacesuit during a spacewalk could become extremely sick if struck by a burst of solar particles.
Those particles pose less of a threat inside a shielded spacecraft. But there are other kinds of particles, called galactic cosmic rays, that are spawned in supernovas around the galaxy and arrive at much higher energies, capable of penetrating thick metal barriers. They are virtually unstoppable.
The effects of interplanetary radiation on the human body are not well understood. Until now, scientists had limited information about how much radiation penetrates a spacecraft during an interplanetary journey. But the Curiosity rover, which bristles with instruments, carried along a Radiation Assessment Detector, and it measured the incoming radiation during its 253-day trip to Mars, which began in November 2011.
Curiosity flew to Mars in a spacecraft that had shielding similar to what astronauts would have on the new crew vehicle being developed by NASA. The detector picked up an average of 1.8 millisieverts of radiation per day. A human being on the surface of the Earth receives only about 3 millisieverts of radiation in an entire year.
“The radiation environment in deep space is several hundred times more intense than it is on Earth, and that’s even inside a shielded spacecraft,” said Cary Zeitlin, a physicist at the Southwest Research Institute in Boulder, Colo., and the lead author on the new study.
In a fast-trajectory journey to Mars using existing propulsion, astronauts would travel for about 180 days to the Red Planet and 180 days home. According to the report, such a trip would expose them to a total of 662 millisieverts of radiation during the round-trip journey.
Some space agencies limit astronauts to 1,000 millisieverts during their entire career. NASA’s standard varies from person to person, influenced by age and gender, and it is designed to permit no more than a 3 percent excess risk of death from cancer over the person’s lifetime.
Astronauts would also be exposed to radiation during their stay on Mars (or in orbit around the planet if the mission did not include a landing). So the total radiation exposure during a mission, particularly one lasting about two years, might exceed the official limits set by space agencies.
That does not mean a Mars trip is impossible. The space agencies could decide, for example, that the importance of a Mars mission would justify the waiving of the radiation exposure limit.
“The radiation exposure on a trip to Mars would — barring a super-huge solar event — not be lethal. The concerns are mostly about cancer induction (a so-called ‘late effect’) and damage to the central nervous system,” Zeitlin said by e-mail.
He said better shielding would help but only to a point. Water and other materials that have a lot of hydrogen are excellent at shielding against cosmic rays. But, Zeitlin said, “Even the best shields will only mitigate the problem to a modest degree, maybe 20-25 percent. While that would be worth pursuing, it would not solve the problem entirely.”
A faster transit is the key. Using chemical propulsion, it takes at least six months to get to Mars. Geoffrey Landis, a researcher at NASA’s Glenn Research Center in Cleveland, said that it might be possible to cut that travel time in half with a nuclear-based propulsion system.
“The difficulty is that you need a very lightweight nuclear reactor to get you enough power for it,” Landis said.