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To reach Mars, astronauts would have to hit a moving target that is millions of miles away. They would also have to survive the harsh radiation of open space. According to a new study, that radiation exceeds the annual limit set for workers at nuclear power plants.

But as NPR's Adam Cole reports, people still want to go to Mars.

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UNIDENTIFIED MAN: T-minus 10, nine, eight...

ADAM COLE, BYLINE: In November of 2011, the Mars Science Laboratory lifted off, carrying Curiosity, NASA's Mars rover, into space.

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UNIDENTIFIED MAN: ...one, main engine start, zero and lift off of Curiosity.

COLE: And then in August...

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ALLEN CHEN: Touchdown confirmed. We're safe on Mars.

COLE: During the eight months in between, Curiosity was bombarded with lots of radiation, high-energy protons thrown out by the sun and galactic cosmic rays slicing through the solar system from distant supernovas. Here on Earth, the magnetic field and atmosphere shield us from most of this radiation. But out in the void of space, high-energy particles would rip into an astronaut's DNA, slowly increasing the risk of cancer. That's a problem for NASA and the handful of other private companies who have pledged to send humans to Mars.

For a long time, they've tried to figure out just how much radiation an astronaut would experience on the journey. Enter Curiosity, handily equipped with RAD, the Radiation Assessment Detector. It was designed to gather radiation data on the surface of Mars.

Don Hassler of the Southwest Research Institute leads the RAD team.

DON HASSLER: About two years before launch, we started to realize the fact that RAD was tucked inside the belly of the spacecraft sort of in the same location that a future astronaut might be.

COLE: So they thought: Why not turn it on during space flight? A few days after launch, RAD powered up and started collecting data. Today, in the journal Science, the researchers announced that the bulk of the radiation - 95 percent - was caused by galactic cosmic rays that we don't know how to shield against. Hassler says there's really only one way to minimize the damage caused by these rays.

HASSLER: I think we just want to go quickly.

COLE: The shortest possible trip to Mars with current technology would take about six months. During that time, Hassler says an astronaut would experience about four times more radiation than people on the International Space Station, who would be partially shielded by their proximity to Earth.

HASSLER: An astronaut on ISS for, say, six months, you know, would receive on the order of maybe 80 millisieverts, compared with the 330 that we receive on our way to Mars.

COLE: The average American is only exposed to six millisieverts every year. Three hundred and thirty millisieverts is enough to increase your cancer risk by about one and a half percent or so. Double that number for a round trip visit to Mars, and you quickly approach the limits recommended by the National Council on Radiation Protection. But that doesn't discourage Robert Zubrin.

ROBERT ZUBRIN: Radiation is not a showstopper.

COLE: Zubrin is the president of The Mars Society and author of "The Case for Mars."

ZUBRIN: It's not something that the FDA would recommend that everyone do, but, you know, we're talking about a mission to Mars here.

COLE: Zubrin says there are plenty of other risks involved in a Mars mission, like can they even land, and those are risks that many would be willing to take. The average person already has about a 20 percent chance of dying from cancer. A 40-year-old astronaut, who dreams of setting foot on the red planet, might not care about bumping that number up a few percentage points.

ZUBRIN: What this paper does is place foursquare in front of the NASA leadership scene. There's no cause for delay, at least as far as radiation is concerned, and we should now set our course for Mars.

COLE: Scientists still have questions about radiation on Mars itself. They need more data to better understand the cancer risk for Mars settlers on the surface. But don't worry, Curiosity is working on that too.

Adam Cole, NPR News.

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