| ACE News Archives | ACE News #58 - January 3, 2002 |
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Radiation exposure limits for manned interplanetary missions have not yet been defined. However, it has been estimated that for a manned mission to Mars ~11cm of aluminum shielding would be needed to bring the dose equivalent to the internal organs from solar-minimum cosmic rays below the current exposure limit for astronauts in low-Earth-orbit. Most of this dose would be from galactic cosmic rays (GCRs) with energies < 1 GeV/nucleon, with important contributions from heavy nuclei (~48%) in spite of their relatively low abundances, due to the fact that they are more heavily ionizing.
Given the heavy shielding requirements during solar minimum (when GCR intensities are at their maximum level), the feasibility of conducting interplanetary missions during other phases of the solar cycle is under study. These studies make use of radiation transport codes and models of the GCR radiation environment to estimate the radiation hazard behind a given shield configuration during a given time period. Accurate spectra of key GCR species spanning at least an 11-year solar cycle are needed for these estimates.
The spectra of elements from Be to Ni (4 <= Z <= 28) with energies from ~40 to ~500 MeV/nucleon (depending on species) are being continuously measured at 1 AU by the CRIS and SIS instruments on ACE. The collecting power of these instruments allows statistically precise spectra to be measured every few months for most elements. The figure on the left shows GCR element spectra measured by ACE during the 1997 solar minimum, compared with the predictions of two models of the GCR radiation environment commonly used for radiation shielding and dose calculations (the Badhwar & ONeill and CREME96 models). The solar-minimum predictions of these models (based on balloon and spacecraft data of varying precision from the past three decades) are in disagreement with the ACE measurements by as much as 15-20%. Both models need revision in light of the new data from ACE.
The reliability of models of the GCR radiation environment (and of dose estimates based on these models) could be further improved by making use of cosmic-ray transport models that incorporate knowledge of the astrophysical processes that determine cosmic ray composition and spectra. The figure on the right shows that a conventional model for cosmic ray transport in the galaxy, coupled with a model for transport in the heliosphere, produces cosmic ray spectra that fit the ACE measurements significantly better than the predictions of the Badhwar & O'Neill and CREME96 models. This model can also match the observed changes in intensity and shape of the spectra from solar minimum to solar maximum (the spectra labeled E represent the lowest intensities measured during the period from 1997 to the end of 2001). This suggests that an improved predictive model of the near-Earth cosmic-ray environment over the solar cycle should incorporate our understanding of the physics of cosmic rays in the heliosphere and the Galaxy.
Contributed by Andrew Davis and Richard Mewaldt of Caltech.
Last modified 3 January 2002, by
Andrew Davis