Abstract/Summary

Recent solar conditions indicate a persistent decline in solar activity—possibly similar to thepast solar grand minima. During such periods of low solar activity, the fluxes of galactic cosmic rays(GCRs) increase remarkably, presenting a hazard for long-term crewed space missions. We used data fromthe Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter(LRO) to examine the correlation between the heliospheric magnetic field, solar wind speed, and solarmodulation potential of the GCRs through Cycle 24. We used this correlation to project observations frompast secular solar minima, including the Dalton minimum (1790–1830) and the Gleissberg minimum(1890–1920), into the next cycle. For the case of conditions similar to the Dalton (or Gleissberg) minimum,the heliospheric magnetic field could drop to 3.61 (or 4.06) nT, leading to a dose rate increase of75% (or34%). We showed that accounting for a floor in the modulation potential, invoked by the Badhwar-O'Neill2014 model, moderates the projected radiation levels in Cycle 25. We used these results to determine themost conservative permissible mission duration (PMD,290.4+37.7−35.9and 204.3+26.6−25.2days for 45-year-old maleand female astronauts, respectively) based on a 3% risk of exposure-induced death (REID) at the upper95% confidence interval in interplanetary space.