Human beings are fragile creatures with enough health issues to worry about just living on the planet where they originated from. Confine them in a spacecraft for a mission to the surface of Mars and back, and new problems arise some which are potentially life-threatening.
One of the most serious hazards to be faced on any long-duration deep-space crewed mission - is radiation exposure. This takes two primary forms: energetic particles from the Sun and galactic cosmic rays. Although the Sun’s heat and light output is pretty steady, the amount of high-energy particles it gives off varies a great deal and is most intense when the Sun is around the peak of its magnetic cycle. But at any time, our neighborhood star can unleash a solar flare or, worse from the point of view of space travelers, a coronal mass ejection (CME) – a huge bubble of plasma that races out into the solar system.
At least solar flares and CMEs can be seen coming. Astronomers constantly monitor the Sun’s activity with a battery of spacecraft and ground-based equipment and can give astronauts plenty of warning of an approaching storm. This would allow the crew, for example, to huddle temporarily in a part of the spacecraft designed to provide better shelter from radiation. Galactic cosmic rays, by contrast, are totally unpredictable. Born mostly in titanic stellar explosions, galactic cosmic rays – the majority of which are fast-moving protons – arrive unannounced from random directions and can be extremely energetic. They account for up to 95 percent of the dangerous radiation to which future crewed Mars missions will be exposed and are very hard to shield against in space.
During its 253-day trip to Mars, NASA’s Curiosity rover monitored its exposure to both solar and cosmic radiation with an instrument called RAD (Radiation Assessment Detector) – and the results were not encouraging for those contemplating human deep-space excursions. Based on Curiosity’s results, assuming a similar level of shielding to that around the rover and the shortest reasonable travel time to and from Mars using present technology, astronauts would receive a dosage of about 660 millisieverts, not include any time spent on the surface, when there would be further exposure.
That figure compares to the mere 3 millisieverts a year that we Earth-bound folk typically experience, the 200 millisieverts a year to which astronauts aboard the International Space Station are subjected, and is a sizeable chunk of the 1,000 millsievert (1 sievert) limit that NASA and other space agencies impose as a lifetime cap on an individual’s exposure. Exposed to 1,000 millisieverts over a period of time, a person’s risk of developing a fatal cancer goes up by about five percent.
Any future settlers on Mars will face a much harsher radiation environment than on Earth, although the Martian atmosphere does provide some protection. Whereas Curiosity’s RAD instrument clocked an average of 1.8 millisieverts a day while in space, this fell to 0.67 millisieverts a day, or about 240 millisieverts a year on the surface of the Red Planet. Of course, any permanent habitation on Mars, such as that envisaged by the Mars One project, would be equipped with substantial radiation shielding, such as deep layers of soil. However, colonists or long-stay visitors to Mars will have to severely restrict how much time they spend outside their shielded bunkers – and that close confinement could lead to its own set of problems.