Although a private effort hopes to send some people on a one-way trip to Mars, chances are good that the first people to reach the red planet will be government-supported astronauts who will be taking a round trip. But one of NASA's own instruments has just suggested that there might be an advantage to a one-way journey: a far lower dose of radiation.

The work takes advantage of a bit of hardware that NASA sent to Mars for a completely unrelated project: the radiation detector on the Mars Science Laboratory Curiosity. The Radiation Assessment Detector is actually two sensors; one tracks radiation via the energy it deposits in silicon, and the other watches for flashes of light that occur as radiation travels through a hunk of plastic. Agreement between the two sensors is used to determine the amount of radiation the detector is receiving.

The hardware is meant to sample the radiation environment on Mars (which also has significant implications for future exploration). But a large team of scientists realized that its travel to Mars provided a glimpse of the sorts of exposures crew members might receive during their journey through interplanetary space to Mars.

We have already sent probes to sample the radiation of interplanetary space, but all of those probes have been essentially bolted on to the exterior of the spacecraft. Humans making the journey will undoubtedly be inside a well-shielded capsule, which would (hopefully) reduce their exposure to cosmic rays and ions blasted away from the Sun. The Radiation Assessment Detector was quite different; it was located on the top deck of the rover, so the body of the robot shielded it to some extent. Further coverage was provided by the heat shield that protected the hardware while it entered Mars' atmosphere and the equipment and fuel used to lower the rover to the planet's surface.

Overall, the authors conclude that the shielding was probably much more irregular than humans would probably be given, but it provides some indication of what might get through even the best shielding.

Overall, the Sun isn't much of a concern. It provided a steady background of ions that struck the detector, but these were generally low energy. There were five instances where the Sun sent off bursts of material that passed by the Mars Science Laboratory during its journey, and each of these created noticeable spikes in the amount of radiation received. Overall, though, these turned out to be a relatively small contribution to the total radiation received on the trip.

The same could not be said for cosmic rays, however. The five solar events in total only exposed the radiation detectors to the equivalent of 15 days of cosmic ray exposure. Because cosmic rays are so high-energy, it's highly unlikely that we're going to be putting enough shielding on any mission to keep them from passing through the crew area—it would simply weigh too much.

The rover's cruise to Mars took 253 days, and over that time the exposure of the instrument added up to nearly 500 milli-Sieverts (one Seivert creates a five percent chance of developing cancer). Humans would probably take a shorter trip—180 day flights are possible—but would, of course, turn around and come back, getting another 180 days of exposure. If done with equivalent shielding, the shortest round trip possible would result in 660 milli-Sieverts of exposure, and that doesn't count time spent orbiting Mars or exploring the surface.

Better, more even shielding would improve matters, but only a bit, since 95 percent of the exposure comes from cosmic rays that would pass right through most shields.

"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." the authors conclude. For the ESA and Canada, these limits are 1 Sievert; NASA sets its limit as a three percent chance of developing a fatal cancer as a result of exposure. All of that would seem to indicate that any astronauts lucky enough to complete a round-trip journey to Mars would probably find themselves permanently grounded afterwards.

Science, 2013. DOI: 10.1126/science.1235989 (About DOIs).