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Sending humans to Mars holds radiation risk, study shows

A radiation-monitoring device carried by NASA's Mars rover Curiosity took measurements during the trip to the red planet. A resulting study appears in Friday's issue of the journal Science.

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RAD took its measurements between Dec. 6, 2011, and July 14, 2012, which was less than a month before Curiosity landed. During that period, it recorded protons from five energetic outbursts from the sun, which collectively delivered a level of radiation that matched 15 days' worth of exposure to galactic cosmic rays and their byproducts.

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Using a unit of radiation exposure known as a sievert, the team estimates that the one-way trip recorded an exposure level from all sources of 466 millisieverts, give or take 84 millisieverts – or about 47 percent of the 1-sievert career limit that Canada, Europe, and Russia have for their astronauts.

For NASA, that level far exceeds the maximum career exposure for astronauts of 198 to 396 millisieverts, depending on gender and age. NASA takes a more conservative stance than its international partners in no small part because of the uncertainties attached to the secondary effects of the heavier ions among galactic cosmic rays.

Scaling the numbers down from Curiosity's trip to a 180-day, one-way excursion, the team calculates that a crew in a CEV-like spacecraft would be exposed to some 331 millisieverts from galactic cosmic rays alone, with additional exposure during solar outbursts. A round trip would yield an exposure level of 662 millisieverts, unless the sun unleashed some hefty burps to increase the level.

Mission planning can tweak these numbers in astronauts' favor, researchers say.

Sending explorers during a low point in the sun's 11-year sunspot cycle could reduce the risk from solar particles. But the sun's magnetic field would be weaker, allowing more galactic cosmic rays to buzz through the solar system. Likewise, a more active sun means a stronger magnetic field and fewer galactic cosmic rays – but greater chances of strong outbursts from the sun.

Since solar particles come from a specific direction, however, it's possible to orient a spacecraft so that it presents the most mass – the stern of a well-stocked service module, for instance – to the oncoming protons. Water, which can stop the protons, could be stored in module walls as added shielding.

Also, astronauts could don suits of radiation armor being developed. One approach uses layers of polyethylene one to two inches thick to develop a suit with a Tokugawa shogunate sort of look.

"I tried on one of those garments once," said Chris Moore, deputy director of advanced exploration systems at NASA headquarters in Washington. "It reminded me of samurai armor."

Such planning also implies a need for substantial improvements in forecasting the sun's behavior – so-called space weather, researchers say.

Looking back at forecasts in 2008 or 2009 for the current solar cycle, predictions for the state of the sun and its magnetic field during Curiosity's cruise phase anticipated "sort of a normal solar maximum" with a similarly beefed-up solar magnetic field, said Cary Zeitlin, a scientist at the Southwest Research Institute in Boulder, Colo., who is a member of the RAD science team and is the lead author on the Science paper describing the results.

"In fact, what we're having is a very weak solar maximum," which exposed RAD to more galactic cosmic rays than would have been envisioned, he said. "In my mind, that really raises the question of how well we can predict the solar cycle" in ways that help reduce the risk of exposure to astronauts operating beyond low-Earth orbit.

Technological approaches involve more powerful rocket propulsion – solar-electric propulsion or nuclear-thermal motors – to reduce the travel time, and hence exposure.

Engineers also are looking at generating magnetic fields around a spaceship. For instance, NASA is exploring the use of high-temperature superconductors, which carry electricity with no resistance when the material is cooled sufficiently. In the process, these superconductors generate enormous magnetic fields compared with typical electrical conductors.

An initial one-year concept study conducted by a team led by Shayne Westover at the Johnson Space Center looked at different exterior magnet designs and found one particularly promising. Results from that study were published last November. The team is undertaking a second, more detailed study to refine overall spaceship-and-magnet design concepts.

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