Historical Reconstruction of Astronaut Cancer Risk: Context for Recent Solar Minima

Abstract

Human presence in space is expected to push beyond low Earth orbit (LEO) in the coming years with missions planned to the Moon and Mars (www.nasa.gov/what-is-artemis). In the absence of solar storms, astronaut radiation exposure will come from galactic cosmic rays (GCR) and accumulate proportionally with mission duration. Health effects from prolonged exposure to GCR have been extensively studied over the past several decades with significant uncertainties and knowledge gaps remaining in risk projections primarily due to lack of directly relevant human data and limited ground-based experimental data (Cucinotta et al., 2013; NASA 2013;Simonsen & Slaba, 2021).The GCR environment comprises a complex mixture of fully ionized and highly energetic particles capable of penetrating spacecraft shielding and producing a cascade of secondary particles such as neutrons. Hydrogen and helium ions account for 87% and 12% of the total GCR fluence by mass, respectively. The remaining 1% is covered by heavier ions with charge, Z, extending up to Z = 28 and beyond. The energy spectrum for each of these ions is quite broad with peak fluxes occurring just below 1 GeV/n and energies extending up to 1 TeV/n and beyond. GCR are modulated by the heliospheric magnetic field (HMF) on an approximate 11-year cycle. During solar minimum, the HMF is suppressed, and the flux of GCR ions below ∼5 GeV/n