The Martian Chronicles
Issue 8, Winter 2001

Radiation Hazards on a Mars Mission
by Richard Reifsnyder

First Mars Landing, 2005

Art: Yul Tolbert
The sun is currently in a period of increased dynamic activity, with frequent sunspots and flares. As a result, the magnetosphere and atmosphere of Earth have expanded slightly, accelerating the orbital decay of satellites in low Earth orbit. Flares can also cause communications satellites and power grids on Earth to go haywire. Luckily, the Earth’s magnetosphere is good at shielding life against the radiation released during increased solar activity, thereby preventing any noticeable medical effects. But how will the crew of a Mars mission adapt to the problems of radiation?

It is widely known that rapid, heavy doses of radiation cause severe cellular damage or even cancer, so the crew needs to be protected against the occasional solar flare. This can be done with a "storm shelter" about the size of an elevator, with food racks and water tanks packed around the walls to absorb the radiation. Most of a solar flare’s energy is in alpha and beta particles which can be stopped with a few centimeters of shielding.

Cosmic rays are a different story. They are constantly present, coming from all directions. The radiation consists of heavy, slow-moving atomic nuclei that can do far more damage to more cells than alpha and beta particles. This radiation requires several meters of shielding for complete blockage, and since the nuclei come from all directions at all times, unlike the brief solar flares that last only a few hours or days, a storm shelter would be insufficient to protect the crew.

One possible defense involves using a loop of electrically charged wire to create an artificial magnetosphere around the ship. However, the wire would either be ordinary conducting copper wire and need a massive power supply, or superconducting wire and need a massive cooling system. Nevertheless, in just a few years, advances in high-temperature superconducting wire could enable low-mass magnetosphere systems to protect the ship.

But even if such a system proves difficult to engineer, some scientists and doctors believe that the cosmic ray doses can simply be endured. Exposure to a thin, continuous stream of radiation does far less damage than an equal magnitude of radiation delivered in one day. There is still the possibility of cancer, but this probabilitity is rather low.

The astronauts will spend about six months traveling to Mars, eighteen months on the surface, and six months returning to Earth. The permanent habitats of the Mars base can be covered with thick layers of soil to provide full-time radiation protection, so nearly all the crew’s radiation exposure would occur during the year of interplanetary travel. 50 rem per crew member is one estimate for total exposure in that time. This dose leads to a 1% increase in probability of contracting a fatal cancer later in life, compared to an already existing 20% cancer risk for non-smokers on Earth, and would probably be acceptable to the volunteers on this mission. However, since the biological effects of cosmic radiation are poorly understood, the resulting cancer risk may conceivably be off by as much as a factor of 10, and thus jump to 10%, or drop to 0.1%.

Not much research can be done safely on Earth to investigate these radiation effects, since cosmic rays are difficult to generate, and no one would consent to being exposed to a theoretically fatal dosage. The International Space Station could provide a good testing ground, since large numbers of astronauts will be exposed to modest amounts of radiation in their six-month tours of duty, but a full investigation might require waiting decades until these astronauts retire and die either of natural causes or of cancer. Obviously Mars mission advocates have no intention of waiting that long. It actually makes the most sense to accept the radiation risk on the Mars mission, since after all this is a journey into the unknown, and the risk of radiation is mild compared to the dangers that explorers on Earth have faced in the past -- and overcome.