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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 4 – GCR Dose, Solar Minimum Conditions, No Shielding





H, mSv/day

Comment


Transit

2.5  0.8




Surface

0.7  0.3

Excludes albedo neutrons

Surface

0.4  0.4

Albedo neutrons

Surface, all contributions

1.1  0.5






in the calculation. The result for solar minimum is shown in Fig. 2, with a range of 0.2 – 0.3 Sv/yr. In the following, we take the middle of the range, 0.25 Sv/yr., as the dose excluding the albedo neutron contribution, which is calculated separately.


To compute the albedo neutron dose in Table 4, the calculated neutron spectrum on Mars (the solid red line in Fig. 3) was crudely convoluted with a fluence-to-dose equivalent curve [Eidelman et al., 2004]. The uncertainty has been set equal to the value to indicate that this is the most likely calculation in which there would be a significant error. The important result of the calculation is that, even with the neutron contribution included and uncertainties folded in, the total dose equivalent rate on the surface is significantly less than in transit.
With these results, we can consider various simplified mission scenarios and determine the contribution of the surface exposure to the mission total. We can write a simple equation for the total: where HGCR is the GCR dose equivalent in mSv and the times ttransit and tsurface are given in days. With current propulsion systems that require about 400 days in transit, the GCR dose equivalent would be in excess of 1 Sv from transit. Surface stays up to 100 days would contribute only about 10% to the overall GCR exposure. If improved propulsion methods can reduce the transit time substantially, and/or if long surface stays are considered, the relative importance of the surface dose equivalent and the associated uncertainty in the neutron component increase significantly. This could also be true if there were a revision (upward) of the neutron fluence-to-dose-equivalent curve.

Estimates of SEP Dose and Uncertainties

Given that we cannot predict the exact nature of future SEP events, perhaps the most instructive way to approach the question of SEP exposures on a Mars mission is to make use of calculations done using spectra measured in previous events [Simonsen et al., 1990; Simonsen et al., 1993]. The calculations use transport models to predict dose and dose equivalent on the surface of Mars, given an input spectrum at the top of the atmosphere. We stress that the cases studied so far have used spectra measured at Earth rather than at Mars, and that is not a particularly good approximation, since (1) particle spectra can be entirely different in the two locations, and (2) even if Earth and Mars are magnetically connected to the same active region on the sun, the radial gradient reduces the flux at Mars by a factor of r-n, where n is likely between 2 and 3. Thus, at 1.5 AU, the radial gradient gives at least a factor of 2 reduction.

Two highly relevant studies were performed by the NASA-Langley radiation group. The calculations, done in the early 1990’s, used the HZETRN and BRYNTRN codes, which have subsequently been revised, most significantly in the heavy-ion transport done by HZETRN. The values calculated for GCR doses on the surface of Mars are therefore not in agreement with current estimates, which are about 50% higher. However, the SEP part of the calculation should still be valid. In 1993, Simonsen and Nealy reported on the three major SEP events in 1989, the largest being the October event. This event also had the hardest spectrum (i.e., more of the flux at high energies). Assuming a pure CO2 atmosphere at Mars, they calculated dose and dose equivalent as a function of depth. At a depth of 16 g cm-2, the large event would have produced a skin dose equivalent of about 0.6 Sv, and about 0.3 Sv at the blood-forming organs, or BFO, which are shielded by tissue and bone and are highly sensitive to radiation. The shielding of the BFO is assumed to be 5 g cm-2 of water-equivalent mass. It is important to note that the radial gradient was not factored into the calculation, so the results likely overestimate the dose equivalent by a factor of 2-3. While the exposure from this event is under 1 Sv, and therefore sub-acute, it nonetheless would contribute significantly to the total for the mission, being equal to that received from the GCR over hundreds of days on the surface of Mars.


There are a few additional features of the SEP event calculations that are noteworthy. First, a more intense event and/or one with a harder spectrum than the October 1989 event could result in acute exposures to crew working outside of shielded areas, particularly if crew is unable to return to a shielded environment within several hours of the event’s onset. Second, the large difference between skin and BFO doses is due to the relatively low energies of the particles in SEP events; in contrast, GCR doses decrease much less in going from skin to BFO. Third, no shielding from a habitat or spacesuit is included in the calculation. In the case of a large SEP event, crew would be advised to return to shelter and stay inside for the duration of the event, further reducing the exposure. Third, because SEP event exposures would be to protons and neutrons, which are low-LET radiation, the risk calculations are likely to be more reliable than are the estimates of risk from GCR exposure, since the latter is strongly dependent on the poorly-understood health effects of HZE particles. (This is because, in a SEP event, nearly all particles have Q=1, and calculation of dose equivalent does not depend on the controversial high-LET portion of the quality factor curve.) Finally, assuming that a particle detector system capable of measuring charged particles and neutrons is operating on Mars during a large SEP event, the data would not be very useful unless there is a simultaneous measurement, in orbit, of the flux of particles at the top of the atmosphere. Simultaneous measurement of the “input” (incident particles) and “output” (flux at the surface) would provide a stringent test of transport models. We therefore recommend that a radiation detector be placed in Mars orbit, on a schedule that would permit simultaneous operation with the landed detector system. The orbiting detector can be a comparatively simple, low-power, low-mass system optimized for SEP events.
Effects of Remanent Martian Magnetism

The MGS magnetometer experiment [Acuña et al., 2001] has shown that, although there is no planetary dipole field at Mars, there are regions of remanent magnetism. Although the fields do interact with the comparatively low-energy particles in the solar wind, they are too weak to have a significant effect on the high-energy GCR particles or on moderately energetic SEPs. Very low energy SEPs will be deflected away from magnetic regions, but these would stop in the upper atmosphere even in the absence of magnetic fields. As a consequence of the deflection of low-energy SEPs, a small reduction in the number of secondary neutrons that reach the surface may be seen in magnetically protected regions. The net shielding effect of the fields, therefore, is expected to be negligible during solar quiet time, and small (possibly negligible) during SEP events.


Other Possible Exposures

Two potential sources of radiation exposure were pointed out by members of the working group. First, there is certainly 238U present in martian regolith, although the concentration is not known. In principle, the radon gas could diffuse into a buried habitat accumulating in sufficient concentrations to pose a hazard. Fortunately this radon hazard is easily mitigated by insuring that ventilation of the gas into the martian atmosphere competes effectively with diffusion into the habitat. Another possible concern is that martian dust may have higher concentrations of alpha-emitting radioactive nuclei than does dust on Earth or it may be retained in the lungs much longer. Since efforts would be made to reduce dust inhalation in any case, there does not seem to be a pressing need for measurements of dust radioactivity.


Radiation Effects on Electronics

In addition to posing direct risks to the health of crew members, radiation also poses a potentially serious risk to crew through the mechanism of damage to electronics essential to life support. As this issue affects the aerospace industry and the military, a well-developed infrastructure exists to study radiation effects and to design hardware capable of withstanding considerable doses of radiation. The success of these efforts is manifested in the continuing successful operation of much hardware in deep space. The most relevant examples of this success are the Mars Exploration Rover vehicles, which have operated on the surface of Mars for over a year at the time of this writing. In their cruise to Mars, the MER’s withstood the very large solar events of Halloween 2003 and have survived on the surface despite several subsequent events, including a significant hard-spectrum event in mid-January 2005. In short, radiation effects on electronics are at present much better understood, and more preventable, than effects on living organisms.




      1. Desired Future State of Knowledge
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