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Astronauts face radiation threat on long Mars trip
As plans for a mission to Mars ramp up, radiation is a big concern, as shown by a new study.
The Washington Post
Of all the hazards facing a human mission to Mars — something NASA and countless space buffs would love to see — one of the hardest to solve is the radiation that saturates interplanetary space.
New data, gathered by NASA’s Curiosity rover as it traveled to Mars, have confirmed that interplanetary space is a hostile medium and suggest that engineers need to find a way to speed up space travel significantly if they hope to reduce radiation exposure.
The new research, published online Thursday in the journal Science, is not a game-changer for human spaceflight. But it brings more hard data to a known risk factor and will help NASA and other space agencies come up with strategies for making spaceflight safer.
Space 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, 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 carried a Radiation Assessment Detector, which measured 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 a day.
A human on the surface of the Earth receives about 3 millisieverts of radiation in a 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 lead author of the new study.
In a 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 career. NASA’s standard varies, 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.
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 it might be possible to cut that travel time in half with a nuclear-based propulsion system.