Radiation Exposure Won't Stop a Manned Mission to Mars

The risk of radiation exposure is not a show-stopper for a long-term manned mission to Mars, new results from NASA's Curiosity rover suggest.
A mission consisting of a 180-day cruise to Mars, a 500-day stay on the Red Planet and a 180-day return flight to Earth would expose astronauts to a cumulative radiation dose of about 1.01 sieverts, measurements by Curiosity's Radiation Assessment Detector (RAD) instrument indicate.
To put that in perspective: The European Space Agency generally limits its astronauts to a total career radiation dose of 1 sievert, which is associated with a 5% increase in lifetime fatal cancer risk.

"It's certainly a manageable number," said RAD principal investigator Don Hassler of the Southwest Research Institute in Boulder, Colo., lead author of a study that reports the results Monday in the journal Science.
A 1-sievert dose from radiation on Mars would violate NASA's current standards, which cap astronauts' excess-cancer risk at 3 percent. But those guidelines were drawn up with missions to low-Earth orbit in mind, and adjustments to accommodate trips farther afield may be in the offing, Hassler said.
"NASA is working with the National Academies' Institute of Medicine to evaluate what appropriate limits would be for a deep-space mission, such as a mission to Mars," Hassler told SPACE.com. "So that's an exciting activity."
The new results represent the most complete picture yet of the radiation environment en route to Mars and on the Red Planet's surface. They incorporate data that RAD gathered during Curiosity's eight-month cruise through space and the rover's first 300 days on Mars, where it touched down in August 2012.
The RAD measurements cover two different types of energetic-particle radiation — galactic cosmic rays (GCRs), which are accelerated to incredible speeds by far-off supernova explosions, and solar energetic particles (SEPs), which are blasted into space by storms on our own sun.

RAD's data show that astronauts exploring the Martian surface would accumulate about 0.64 millisieverts of radiation per day. The dose rate is nearly three times greater during the journey to Mars, at 1.84 millisieverts per day.
But Mars' radiation environment is dynamic, so Curiosity's measurements thus far should not be viewed as the final word, Hassler stressed. For example, RAD's data have been gathered near the peak of the sun's 11-year activity cycle, a time when the GCR flux is relatively low (because solar plasma tends to scatter galactic cosmic rays).
Curiosity's radiation measurements should help NASA plan out a manned mission to Mars, which the space agency hopes to pull off by the mid-2030s, Hassler said. And they should also inform the search for signs of past or present life on the Red Planet — another top NASA priority.
For example, the new RAD results suggest that microbial life is unlikely to exist right at the Martian surface, Hassler said. But future missions may not have to drill too deeply underground to find pockets of Mars life, if it ever existed.
"These measurements do tell us that we think it could be viable to find signs of possible extant or past life as shallow as 1 meter deep," Hassler said.
The new study is one of six papers published in Science Monday that report new results from Curiosity. Most of the other studies present evidence that the rover has found an ancient freshwater lake that could have supported microbial life for tens of thousands, and perhaps millions, of years.

Curiosity Rover Finds Ancient Life-Supporting Lakebed on Mars

The site where NASA’s Mars rover Curiosity landed last year contains at least one lake that would have been perfectly suited for colonies of simple, rock-eating microbes found in caves and hydrothermal vents on Earth.
Analysis of mudstones in an area known as Yellowknife Bay, located inside the rover’s Gale Crater landing site, show that fresh water pooled on the surface for tens of thousands — or even hundreds of thousands — of years.

“The results show that the lake was definitely a habitable environment,” Curiosity lead scientist John Grotzinger, with the California Institute of Technology, told Discovery News.
The really big surprise, however, was that clays drilled out from inside two mudstones and analyzed by the rover are much younger than scientists expected, a finding that extends the window of time for when Mars may have been suitable for life.

These numbers now overlap with the oldest rocks on Earth that contain evidence of a former biosphere on Earth,” Grotzinger said.
The rover explored Yellowknife Bay before heading toward a three-mile high mountain of layered sediments rising from the floor of Gale Crater known at Mount Sharp

Curiosity landed in August 2012 to assess if Gale Crater had the right ingredients and environments to support ancient microbial life. Within six months, scientists had the answer to that question: Yes.
Now, in addition to characterizing specific potential habitats, scientists are coming up with a search strategy to determine which sites hold the most promise for finding organic carbon, a far more difficult and complex challenge and the focus of future studies at Mount Sharp.
“Habitability only requires that the chemicals and minerals in the rock preserve evidence of an ancient environment. To search for organic carbon you’re actually looking for particular material ... that is not really compatible with the present environment of Mars. To find something you need a guidebook, you need some rules,” Grotzinger said.
One of those rules involves how much radiation a rock has been exposed to. Mars today has only has a thin atmosphere and no protective magnetic field, so scientists have been thinking they will need to dig deep to find carbon-laced samples, or they will have to probe craters relatively recently excavated by an impact.
Analysis of the Yellowknife Bay rocks points to another path. Dating the ages of the rocks’ surfaces show they are as young as about 70 million years, the result of being sand-blasted by Martian winds.

“In the future, if we find a rock that looks like a place where organics were accumulating and if it looks like the chemistry would have been favorable for preservation ... we can now very deliberately manage the risk that the rock would have been cooking away in the presence of radiation for hundreds of millions of years by looking for these scarps, these miniature cliffs, and then drilling the rock and date it to see how long it’s been laying around,” Grotzinger said.
In related research, scientists found that the amount of radiation measured by Curiosity during its first year on the Martian surface is about 40% of what the rover experienced during its nine-month cruise.
Most of the reduction, as expected, is due to the planet’s body serving as a buffer, but scientists also found that solar activity is a key factor, said Cary Zeitlin at Southwest Research Institute.
Six papers on Curiosity’s new findings are published in this week’s journal Science and unveiled at the American Geophysical Union conference in San Francisco.