Radiation Hazards on a Mars Mission|
by Richard Reifsnyder
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
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.