A New Evaluation of the Galactic Cosmic Ray Radiation Dose In Interplanetary Space

The radiation dose from galactic cosmic rays (GCRs) during a mission to Mars is expected to be comparable to the allowable limit for astronauts in low Earth orbit. Most of this dose would be due to galactic cosmic rays with energies <1 GeV/nucleon, with major contributions from heavy nuclei in spite of their low abundance relative to H and He. Measurements with the CRIS experiment on ACE have provided the first opportunity to make precise measurements of the energy spectra of cosmic rays from Be to Ni during both solar minimum and solar maximum conditions. We have fit these data, along with HEAO-3 data at higher energy, and balloon-borne and satellite measurements of H and He, with a physics-based model of GCR transport and solar modulation. The results of this model were used to evaluate the radiation dose and dose-equivalent of GCRs under both solar minimum and maximum conditions. We find that the solar-minimum radiation dose is somewhat lower than found in previous studies. We also find a smaller difference between solar minimum and solar maximum, due in part to the fact that heavy cosmic rays vary less over the solar cycle than do H and He. Measurements of Be-10 in polar ice cores and other data show that the near-Earth cosmic-ray intensity was significantly greater within the last century. In view of the fact that these conditions could return, we have evaluated the maximum near-Earth cosmic-ray spectra consistent with our model and the Be-10 data. We estimate the radiation dose under these conditions to be more than twice as great as during recent solar minima. The implications of these results for human missions to Mars will be discussed.