In planning manned trips to Mars, when one goes is as important as how one goes, according to the latest research. Based on decades of unmanned Mars missions and manned space stations in low-Earth orbit, many of the technical aspects of a Mars-bound spacecraft are well understood. But astronauts on an interplanetary trip will experience intense particle radiation much more damaging than that experienced by astronauts in the space station, who are shielded by the body of the Earth and its surrounding magnetic fields.
That hazardous radiation comes in two varieties: energetic charged particles (mostly protons) shed directly from the Sun in the “solar wind,” and galactic cosmic rays (including heavier ions) that come from around the Milky Way and bathe us from all directions. Both fluxes of particles rise and fall in step with the roughly eleven-year solar cycle. When solar activity is at maximum, an enhanced solar wind deflects incoming cosmic rays like an invisible force field, reducing the number that make it into the Solar System. Using estimates of the particle fluxes, coupled with computer models of a spacecraft with passengers, a research team based in Russia, Germany, and the United States computed expected doses of radiation for various flight scenarios. The researchers found that the best time to launch a mission is surprisingly when the Sun is most active and cosmic rays are at a minimum, since the latter are more damaging to human tissue than particles from the Sun.
Shrouding the spacecraft in aluminum shielding also helps, but only up to a point, according to calculations. Beyond a certain thickness, not only is shielding unwieldy, but it also increases the radiation dose on the astronauts by producing “secondary” particles, as cosmic rays interact with the aluminum rather than zipping harmlessly through. Optimally shielded, and launched at the optimal time, astronauts would still have to limit the length of their mission to less than four years to avoid exceeding recommended dose accumulations. Since the average flight to Mars takes about nine months, that would leave plenty of time for on-planet exploration. Comparatively short Earth-to-Mars flight trajectories occur near solar maximum in 2030 and 2050, making these the ideal times to go in the near future. “[Although] space radiation imposes strict limitations…for human missions to Mars,” concluded co-author Yuri Shprits of the University of California Los Angeles and the GFZ German Research Centre for Geosciences in Potsdam, “such a mission is viable.” (Space Weather)