(CNN) — For those of you dreaming of visiting Mars, readings taken during the Curiosity rover's voyage to the Red Planet offer a new measurement to ponder as you weigh the risks.
Mars-bound pioneers will be exposed to radiation levels that could effectively retire astronauts under NASA's current standards, scientists reported Thursday. The radiation astronauts would face on a round trip would be comparable to getting an abdominal CT scan "about once every five days," Cary Zeitlin, principal scientist for the NASA-led Martian Radiation Environment Experiment, told CNN.
The findings were published Thursday in the journal Science, based on data from a device called the Radiation Assessment Detector that took readings during Curiosity's trip to Mars. The spacecraft was similar to one that would carry humans, and scientists were interested in measuring galactic cosmic rays and solar energetic particles on the trip.
Zeitlin and his colleagues found voyagers could be exposed to between 554 and 770 millisieverts of ionizing radiation during the trip, depending on the level of solar activity. By comparison, the typical American receives about 6.2 millisieverts a year from natural and man-made sources, including medical diagnostic procedures.
"It is clear that the exposure from the cruise phases alone is a large fraction of (and in some cases greater than) currently accepted astronaut career limits," they wrote.
In terms of health effects of that exposure, there are a lot of unknowns, Zeitlin said. There's no effective, practical shielding method that would block all of the high-energy particles from getting to the astronauts aboard a mission to Mars, he said. The problem can be mitigated, Zeitlin said, but not eliminated.
A question of physics, a question of biology
The problem has two sides: physics and biology, Zeitlin said. On the physics side, the radiation is in line with expectations.
"As it stands, what we measured is pretty much in line with predictions that people have made based on various models that they put together based on what the deep space radiation environment would be," he said.
Galactic cosmic rays are composed of high-energy, highly penetrating particles that a typical spacecraft's shielding couldn't stop; astronauts would be exposed to them chronically. Solar energetic particles get accelerated close to the sun as a result of solar flares and coronal mass ejections that send solar particles into space.
Solar energetic particle events only contributed about 5% to the overall measured dose equivalent during the laboratory's cruise to Mars. But these events are highly variable in terms of frequency and intensity, the study said, and the current cycle seems to be producing a weak solar maximum.
"The SEP contribution could conceivably be many times larger in a different time frame," the report said.
The health question
On the biology side, there's a lot of uncertainty.
There's a statistical basis linking radiation exposure and cancer. NASA tries to limit the additional lifetime fatal cancer risk to 3% for its astronauts.
Moon missions, while exposing astronauts to a similar rate of radiation accumulation as going to Mars, theoretically delivered a relatively small radiation dose, as the trip was so much shorter. The Apollo 11 moon mission, for instance, launched on July 16, 1969, landing the first humans on the moon before returning to Earth on July 24, 1969.
Curiosity took 253 days to get to Mars, launching November 26, 2011, and landing on August 6, 2012.
According to a 2010 NASA report on space radiation cancer risk projections [PDF], the average American male who is 35 can safely spend between 140 and 186 days in deep space, assuming heavy shielding, and stay below that 3% increase in cancer risk; a 35-year-old female could spend between 88 and 120 days.
For those who have never smoked, however, it's estimated that a 35-year-old man could spend between 180 and 239 days in deep space, and a 35-year-old female could spend between 130 and 173 days. The number of safe days in space increases as men and women age.
A human Mars mission could be designed to get to the Red Planet in 180 days, the study said, according to NASA estimates. But don't forget that if people spend time on the planet, that could add a lot to the accumulated radiation dose, depending on how long they stay and what the shielding conditions are like.
Zeitlin said his team just makes measurements; they don't decide what radiation dose is acceptable.
When can we go?
Interest in going to Mars has been mounting. The private company Mars One has started its recruitment process for a one-way trip for four people to land in 2023. The company received 78,000 applications as of early May.
Even sooner, the Inspiration Mars Foundation wants to send two people, a man and a woman on a 501 day roundtrip journey to Mars and back in 2018, without ever actually touching down.
Buzz Aldrin, the second man to land on the moon, wrote on CNN.com that the United States "should commit itself, within two decades, to commencing American permanence on the planet Mars."
In the meantime, the 2-ton rover Curiosity a $2.5 billion mission has been using its sophisticated tool kit to uncover evidence that Mars was once habitable. Thanks to the rover's efforts, scientists have determined that, yes, conditions that could have supported life once existed there.