Mars Odyssey

An artist's depiction of the 2001 Mars Odyssey spacecraft firing its engine upon reaching Mars. In addition to mapping the Martian landscape, onboard is the Mars Radiation Environment Experiment.

Courtesy NASA/JPL.

NASA's Mars Odyssey spacecraft has given scientists their first detailed look at the radiation hazards facing future Mars explorers, and the results are not as dire as portrayed in some recent news stories. Results from Odyssey's Mars Radiation Environment Experiment (MARIE) were presented at the 34th Lunar and Planetary Science Conference in Houston, Texas.

Mars voyagers, or for that matter anyone venturing beyond the sheltering effects of the Earth's magnetic field, face two kinds of radiation threats. Low-energy protons emitted by active regions on the Sun can be lethal to an unprotected astronaut, but such outbursts are an intermittent hazard, mostly a concern around the time of solar maximum. Galactic cosmic rays, which consist of high-energy atomic nuclei, are constantly streaming through interplanetary space. While not immediately lethal, they present a long-term health hazard; the damage they cause to DNA increases an astronaut's risk of cancer. Unlike solar flare particles, which can be effectively screened in a solar "storm shelter" lined with shielding material such as water or soil, there is no practical way to shield astronauts from cosmic rays.

Since February 2002, when Odyssey's scientific operations began, MARIE has documented the fluxes of both cosmic rays and solar-flare particles at Mars. According to MARIE scientist Cary Zeitlin (Baylor College of Medicine), the data show that a Mars crew would receive a dose of about 1.2 milliseiverts (120 millirem) of cosmic radiation per day during their voyage through interplanetary space. That figure, Zeitlin says, agrees closely with pre-mission model predictions. While on the Martian surface, he notes, shielding by the planet itself would cut that rate in half.

MARIE also documented about a dozen solar-particle outbursts, including one intense flare last October that produced a radiation dose of about 10 milliseiverts — the equivalent of about a week's cosmic-ray exposure. To give astronauts advance warning of such events, and time to retreat to their storm shelter, NASA's Tim Cleghorn (Johnson Space Center) says a network of solar particle detectors should be placed at strategic locations in the inner solar system.

All told, says Cleghorn, the radiation exposure for a Mars crew during a 3-year mission is well within the recommended career limits set by NASA, which are calculated based on an increase of 3 percent in an astronaut's lifetime risk of cancer. However, as Zeitlin notes, scientists know relatively little about the biological effects of space radiation, especially heavy-nucleus cosmic rays. For now, he says, "radiation is not a showstopper" for human missions to Mars, "but more research could change that conclusion."


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