What we must know about solar particle events to reduce the risk to astronauts

Abstract

Solar particle events (SPEs) pose a health risk to astronauts on deep space missions and in high-inclination, low Earth orbit (LEO), as with the International Space Station. Risk mitigation strategies to minimize this threat will require a combination of in situ radiation monitoring with forecasts and observations of coronal mass ejections (CMEs) and the state of the heliosphere. Since only a small fraction of CMEs are sufficiently fast to produce SPEs, a very high confidence in CME prediction will be needed to reduce false alarms to an acceptable level. However, an achievable predictive capability for "no CME" could be very helpful for mission scheduling. Combining published data on dose equivalent per particle at energies from 10 MeV to several GeV with fitted SPE fluence data from cycle 22, one can see that the most important energy range for astronaut safety is from 20 to about 200 MeV. To avoid exposing astronauts to an excessive dose rate, it will be important to accurately characterize the ambient solar wind and to forecast or detect shock-enhanced flux. This is demonstrated by comparing the fluence of the October 1989 event with and without the prominent shock-enhanced peak.