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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.

By Staff writer / May 30, 2013

This July 2007 image provided by NASA shows astronaut Clay Anderson waving during a spacewalk outside the International Space Station. A trip to and from the red planet could expose travelers to an accumulated dose of radiation that would approach – and in some cases exceed – the maximum allowable career limits for a NASA astronaut.

NASA/AP/File

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For any astronaut tapped for a trip to Mars, a journey today would be a once-in-a-lifetime experience – in more ways than one.

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With shielding comparable to the level built into NASA's new crew-exploration vehicle (CEV), just the trip to and from the red planet could expose travelers to an accumulated dose of radiation that would approach – and in some cases exceed – the maximum allowable career limits for a NASA astronaut.

The Europeans, Russians, and Canadians accept a somewhat more liberal exposure limit than the National Aeronautics and Space Administration does. Even at that limit, Mars would probably represent a memorable but one-and-done spaceflight career for any prospective astronaut.

Those are among the implications of measurements taken by a radiation-monitoring device that NASA's Mars rover Curiosity carries. The measurements were taken during Curiosity's 253-day, 347-million-mile trip to the red planet. The data were unveiled Thursday in a study set to appear in Friday's issue of the journal Science.

"NASA is very excited to get this new cruise data," said Eddie Semones, radiation health officer at NASA's Johnson Space Center in Houston during a briefing Thursday to discuss the results. The data also will help shape human-exploration missions other than to Mars, he adds, noting that missions to retrieve asteroids or even missions to the moon will benefit from the information the new results provide.

Scientists and engineers long have recognized radiation in space as perhaps the most significant challenge to exploration beyond low-Earth orbit, where Earth's magnetic field acts as a natural deflector shield.

The hazard has spawned a range of studies on how to deal with the risk, ranging from concepts for spacecraft that can generate protective magnetic fields to individual body armor for astronauts.

To date, researchers had gathered "outdoor" radiation measurements on a wide variety of robotic spacecraft visiting comets, asteroids, and other planets. The detectors on these missions were exposed to space by design, so they could measure radiation levels around a planet or in interplanetary space.

Curiosity's device, the Radiation Assessment Detector, is bolted to the rover's deck to measure surface radiation on Mars. So, like the rover, RAD was cocooned within the Mars Science Laboratory spacecraft during its trip. Thus, its en route measurements provide the first data from within an interplanetary spacecraft and with a level of shielding that the first Mars explorers might receive from their craft.

The study comes at a time when two private groups are aiming to send humans to Mars long before any NASA astronauts are sent – a prospect the Obama administration and the agency envision taking place after 2030.

In March, a group headed by Dennis Tito, the first space tourist to visit the International Space Station, announced the goal of sending two people on a 501-day fly-around mission to Mars, launching in 2018. The mission, Inspiration Mars, is a one-off attempt at inspiring a new generation of space explorers as well as providing NASA with information on technological, physiological, and psychological issues that no Earth-bound Mars analog station can deliver.

Earlier, a nonprofit group based in the Netherlands announced an effort to set up the first permanent human settlement on Mars in 2023.

The radiation that the new study examines – actually speedy, electrically charged particles such as protons and ions – comes from two sources.

The sun sends out a constant stream of protons in a feature called the solar wind. The wind is punctuated by solar storms, ranging from flares to enormous coronal-mass ejections – the most powerful eruptions the sun delivers. The protons from these storms travel with higher energies than the solar wind and so represent the biggest particle-radiation risk from the sun, the RAD researchers say.

The second source is the galaxy itself, which bathes a spacecraft in low but persistent levels of energetic particles known as galactic cosmic rays. These are generated when massive stars explode as supernovae or in other high-energy cosmic events. About 85 percent of the cosmic rays are protons, another 14 percent are helium ions, and the remaining 1 percent consists of heavier ions.

Heavy-ion collisions can have an influence larger than their small proportion would suggest because when these smack into a spaceship, they create a shower of secondary particles that can inflict biological damage. However, estimates of the level of damage carry large uncertainties.

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