§2Radiation
About 1.8 millisieverts a day in cruise, a sievert for the round trip — and a risk figure that comes from a model, not a patient.
The radiation numbers are the best-measured part of the whole Mars problem, which is exactly why they deserve care: one of them was read off an instrument, and the rest were produced by a model. This chapter keeps the two apart.
Two hazards
§2.1 Deep space carries two kinds of radiation. Galactic cosmic rays (GCR) are a chronic background of high-energy particles, including heavy ions, that penetrate typical shielding (~20 g/cm² of aluminum). Solar particle events (SPE) are acute storms. GCR is the settlement problem; SPE is the expedition emergency.
The measured dose
§2.2 The Radiation Assessment Detector on the Mars Science Laboratory measured the cruise environment on the way to Mars. Zeitlin et al. (Science, 2013) reported about 1.8 mSv per day in transit. established — this is a measurement.
From that figure the arithmetic follows: roughly 0.66 Sv for round-trip transit alone, and with a ~500-day surface stay, mission totals of about 1 Sv. ESA caps an astronaut’s career at 1 Sv, a level it associates with ~5% added fatal-cancer risk; NASA’s standard is a 3% risk of exposure-induced death. A Mars mission sits near or above the career limit.
The modeled risk
§2.3 What that dose does to a crew is a different kind of number. Cucinotta-lineage modeling (PLOS One, 2013) projected a central estimate of cancer and circulatory mortality above 5% for a Mars mission, with an upper 95% confidence bound near 10%; morbidity above 10%, upper bound near 20%. model output These are Monte-Carlo projections. No human has needed care beyond low Earth orbit, and the quality factors for heavy ions — how much biological damage a given dose of them causes — are poorly understood. The central-nervous-system risks from heavy ions are an additional, poorly quantified term.
Notice the asymmetry. The dose is solid; the harm is a projection with a wide interval. A reader who wants to say “Mars radiation is survivable” has a measured basis for a single trip. A reader who wants to say “it kills one in ten” is quoting the top of a model’s confidence interval.
Countermeasures
§2.4 The main options are mass and speed. Water, regolith, and hydrogen-rich materials shield better than aluminum; a fast transit shortens the exposure. Neither eliminates GCR, and for a settlement, years of surface life, chronic exposure accumulates in a way no expedition has to face.
Expedition or settlement?
§2.5 The report’s verdict, carried here unchanged: established that radiation is a major but likely survivable risk for a single expedition; uncertain for repeated exposure and CNS effects; a genuine long-term problem for permanent settlement.