Mars is safe from radiation - but the trip there isn't

You needn't fry on Mars. Readings from NASA's Curiosity rover suggest radiation levels on the Red Planet are about the same as those in low Earth orbit, where astronauts hang out for months on the International Space Station. A Mars visit would still be dangerous though, due to the years-long return trip.

Unlike Earth, Mars has no magnetosphere shielding it from solar and galactic radiation. But it does have a thin atmosphere, and readings from two of Curiosity's instruments suggest this provides some protection.

"This is the first ever measurement of the radiation environment on any planet other than Earth," Curiosity team member Don Hassler said at a press briefing on 15 November. "Astronauts can live in this environment."

The rover's weather station recorded evidence of what is known as a thermal tide on Mars. Sunlight heats the planet's atmosphere on the side facing the sun, causing it to expand upwards and triggering a decrease in air pressure. But things chill quickly on the other side, so that the atmosphere deflates and becomes denser.

As Mars rotates, the bulge of heated air travels with the "day" side from east to west. Curiosity feels this effect as changes in air pressure over the course of a Martian day, rover scientist Claire Newman of Ashima Research in California said during the briefing.

Radiation shield

At the same time, the rover's radiation monitor saw daily dips in charged particles that match the increases in air pressure that come with a denser atmosphere. "The atmosphere is acting as a shield to radiation," Hassler said.

The scientists were not ready to put numbers to the daily radiation dose people would experience on Mars. But the overall levels are lower than those the spacecraft carrying Curiosity recorded during its interplanetary flight, and about what astronauts see on the ISS.

"It's roughly what we were expecting," astrobiologist Lewis Dartnell of University College London told New Scientist.

The biggest threat to Mars voyagers would be the cumulative radiation exposure during the long trip. NASA estimates that a return human mission to Mars would take three years. During that time astronauts might receive more than seven times the radiation dose they get during six months on the ISS.

Setting limits

Building up radiation exposure increases the risk of developing various cancers, so NASA has set limits on how much total radiation astronauts can experience over the course of their careers. Figuring out the exact risk on Mars is crucial to understanding the total dose a human mission would face and whether it is within safe limits, Hassler said.

Solar flares would also be a problem. On Earth these eruptions of charged particles from the sun are largely deflected by the magnetosphere. But Mars enjoys no such protection, and since Curiosity has yet to see a flare, it is unclear how much shielding the thin atmosphere would provide. '

Dartnell suggests that a base or colony on Mars could be built underground to avoid surface radiation. Or, with enough advance warning, astronauts could retreat to protective shelters during a flare. But is all that trouble worth it just to send humans where robots already thrive?

"An astronaut or geologist that's trained in science that has a brain and a pair of hands and pair of eyes with a rock hammer can do a lot more on the surface on Mars before breakfast than a robot can do in weeks," says Dartnell.

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