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Geoffrey Landis
For any space mission, radiation dosages are an important factor. Probably the most hazardous of these radiations for the Reference Mission is the possibility of a large solar flare while the crew are away from LEO.
Although this will only be an issue for the first mission (the telerobot will have the habitat buried before the next crew arrives), it is nonetheless a consideration. The summer 2003 working launch date will be just after the height of the 11-year sunspot cycle, and the possibility of an "anomalously large" flare event is that much higher.
The August 1972 flare delivered unshielded radiation doses of 45 REM per hour, and lasted for 15.5 hours. Storms with such lethal particle fluxes are expected to appear two days out of every twenty years. (Ref: J.R. Letaw, R. Silberberg and C.H. Tsao, "Natural Radiation Hazards on the Manned Mars Mission," Manned Mars Mission (working papers), NASA M-002, part II, 1986)
Note that NASA's 30-day exposure limit is 25 REM. The terrestrial occupational limit for radiation workers is 5 REM/year. The median lethal dose is 450 REM. According to a Bioastronautics Data book, you have about a 10% chance of death at half this (counting death within 60 days of exposure, that is; there is also delayed death by radiation-induced cancer, about 1E-6 per REM absorbed.) "Vomiting and nausea in about 10-50% of personnel" but "no deaths anticipated" for 100-200 rads. According to that Bioastronautics Data book, "little" flares can give you 50-200 rads of skin dose of over 30 MeV protons. (REMs and rads have about the same danger at this energy level.)
The Solar Cell Radiation Handbook says: "Flares capable of producing large proton events tend to occur when the rate of change in sunspot number becomes greater" -- in other words, the peak flare period is before solar max and: "The size of each event is almost randomly distributed over an 11 year cycle." and, "Statistically, an anomalously large (AL) event will occur sometime during each cycle."
The August 1972 event is an example of an "anomalously large" (AL) event. These are relatively infrequent; the flare of October 1989 was another such very large event, with the same or larger dose of protons below 20 MeV, and about half the dose above 20 MeV, compared to the August 1972 event. Thus, depending on which reference, you get an average of either one such AL flare per cycle, or one every other cycle. Two events in 17 years -- Aug. '72 and Oct. '89 -- is compatable with either number; there just isn't good enough statistics.
Also, it's not clear whether delaying the mission to another part of the cycle will help -- the Aug. '72 event occured very late in the cycle, while the Oct. '89 event occurred very early in the cycle.
The worst of the Apollo missions gave the astronauts about 1 REM, as it happens. That came from the Van Allen belts, not from the sun or from Galactic Cosmic Rays.
A launch in the peak of the sunspot cycle also means atmospheric drag while we're in LEO will be lots higher, and throughout the mission there will be higher solar radiation.
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