An Analysis of the Precursor Measurements of Mars Needed to Reduce the Risk of the First Human Mission to Mars
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Table 3
Charged particles arriving at Mars traverse the atmosphere before reaching the surface. The distributions of ion species and energies are modified by energy loss and nuclear interactions. Because of their very different energies and the resulting differences in propagation through the atmosphere, we discuss SEPs and GCR particles separately. Typical SEPs have ranges less than the column depth of the atmosphere, meaning that they stop far short of the surface. Although most or all primary SEPs are stopped in the atmosphere, each incident particle has a non-zero probability of producing a neutron via a nuclear interaction; this is true – though less probable – even for protons stopping high in the atmosphere. Because neutrons do not lose energy through ionization, they can penetrate considerable depths before interacting. Thus one of the main concerns from a large SEP event while the crew is on Mars is the production of large neutron fluxes at the surface. Also, in very rare SEP events, high fluxes of protons in the 100-1000 MeV range have been observed. At these energies, many protons would penetrate to the surface of Mars and pose a hazard of acute exposure to unprotected crewmembers. A high-level decision will be required as to whether mission designers need to account for such rare events, or whether the risk is so remote that it must simply be accepted despite the potentially dire consequences. Unlike SEPs, the great majority of GCR protons have ranges that are much greater than the column depth of the martian atmosphere. Many heavier particles in the GCR also have sufficient energy to reach the surface of Mars, but they also have significant probabilities for undergoing nuclear interactions with nuclei in the atmosphere. The result of such interactions is fragmentation of the incident nucleus into two or more lighter nuclei, which generally emerge from the collision with approximately the same velocity that the incoming nucleus had prior to the collision. Thus the flux of GCR heavy nuclei is significantly smaller on the surface than it is at the top of the atmosphere (though the total flux may be equal to, or greater than, the incident flux). For example, for an iron ion in CO2, the interaction length is approximately 15 g cm-2. This means that, for an atmospheric column depth of 16-22 g cm-2, less than 1/e, or 37% of those incident on the top of the atmosphere reach the surface intact, while 63% will fragment into lighter ions, which typically impart lower dose and dose equivalent than the incident iron ions would have had they reached the surface. Because of nuclear interactions of HZE particles, and because some low-energy GCR particles are stopped before reaching the surface, the atmosphere provides some shielding against the GCR. Nuclear interactions of GCR particles in the atmosphere also produce a continual, small flux of downward high-energy neutrons, but this is orders of magnitude smaller than the neutron flux that can be produced in a large SEP event. Particles are also produced in martian regolith. GCR particles can penetrate the soil and undergo interactions with nuclei in the soil, producing excited “target” nuclei which decay by particle emission to lower-energy states. The emitted particles include gamma rays, protons, neutrons, and alphas. Particles are emitted with no favored direction; some go up and out of the regolith. Charged particles have little chance of exiting the regolith because they are mostly very low energy and ionization energy loss causes most of them to stop after a short distance. Neutrons, however, can and do escape the regolith and will therefore add to dose received on the surface. The neutrons produced in the regolith are often referred to as albedo neutrons. Estimates of GCR Dose and Uncertainties The current state of knowledge is perhaps best illustrated by a discussion of the predicted GCR doses and associated uncertainties for a Mars mission. To make the discussion independent of the chosen mission profile and propulsion method, we begin by putting the results in units of dose equivalent received per day, and then we examine two scenarios. Because SEP events are unpredictable, we defer that part of the discussion. In the absence of information about shielding of the transit vehicle or surface habitat, we assume no shielding for purposes of this discussion. (A likely scenario is that the transit vehicle will be fairly well shielded, the surface habitat modestly shielded, and astronauts working on the surface will have little to no shielding. In this case, all GCR doses are likely to be slightly lower than those given here, and the relative contribution from the surface stay will be slightly larger than shown here. The extent to which the relative contributions will be different from those given here is heavily dependent on the shielding details.) Table 4 shows estimated dose equivalent rates in transit and on the surface, with the two major surface contributions shown separately. The calculations are for solar minimum, when GCR dose is highest. For solar maximum, the numbers can be scaled down by a factor of about 2. The uncertainty in the prediction for any given time in the solar cycle is expected to be about 30%. The surface dose rate from the primary GCR is always lower than the dose rate in transit by at least a factor of two, due to the shielding from a planetary surface. The atmosphere accounts for an additional increment of shielding, the amount of which depends significantly on altitude. At JSC, surface maps color coded by radiation dose equivalent rates have been created [Saganti et al., 2004], using the HZETRN code and MGS MOLA data. Albedo neutrons were not included |