Jody K. Wilson

The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) aboard the Lunar Reconnaissance Orbiter spacecraft has been operating in lunar orbit since 2009. CRaTER data provide a long‐term record of the near‐Moon radiation environment that is relevant to human exploration. In considering requirements for operational instruments on crewed lunar missions that are anticipated in the next several years, the question arises whether real‐time measurements by existing assets such as Geostationary Operational Environment Satellite (GOES) are adequate for providing radiation warnings for the safety of astronauts on or near the Moon during Energetic Solar Particle Events (ESPEs). ESPEs are distinguished from more typical Solar Particle Events (SPEs) by their large fluxes of higher‐energy particles. Here we show direct comparisons of contemporaneous ESPE measurements made in the two locations to establish that the radiation environments in lunar space and in geostationary orbit (GEO) are sufficiently similar to allow the use of GOES as a warning system for crew on or near the Moon. The conclusion is based primarily on data from six ESPEs in the October 2021–October 2024 time frame in which the integral flux of protons with energies greater than 100 MeV exceeded 1 at the event's peak as measured by operational GOES satellites, and to a lesser extent on data from two earlier ground‐level events (GLEs) that are of particular interest. We report new methods for analyzing CRaTER data under high‐flux conditions and find good agreement with data reported by GOES‐16 and later satellites.

The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) aboard the Lunar Reconnaissance Orbiter spacecraft has been operating in lunar orbit since 2009. CRaTER data provide a long-term record of the near-Moon radiation environment that is relevant to human exploration. In considering requirements for operational instruments on crewed lunar missions that are anticipated in the next several years, the question arises whether real-time measurements by existing assets such as Geostationary Operational Environment Satellite (GOES) are adequate for providing radiation warnings for the safety of astronauts on or near the Moon during Energetic Solar Particle Events (ESPEs). ESPEs are distinguished from more typical Solar Particle Events (SPEs) by their large fluxes of higher-energy particles. Here we show direct comparisons of contemporaneous ESPE measurements made in the two locations to establish that the radiation environments in lunar space and in geostationary orbit (GEO) are sufficiently similar to allow the use of GOES as a warning system for crew on or near the Moon. The conclusion is based primarily on data from six ESPEs in the October 2021 to October 2024 time frame in which the integral flux of protons with energies greater than 100 MeV exceeded 1 cm$^{-2}$ sr$^{-1}$ s$^{-1}$ at the event’s peak as measured by operational GOES satellites, and to a lesser extent on data from two earlier ground-level events (GLEs) that are of particular interest. We report new methods for analyzing CRaTER data under high-flux conditions and find good agreement with data reported by GOES-16 and later satellites.

Since the beginning of the space age, the Sun has been in a multi‐cycle period of elevated activity (secular maximum). This secular maximum is the longest in the last 9300 years. Since the end of solar cycle 21 (SC21), however, the Sun has shown a decline in overall activity, which has remarkably increased the fluxes of galactic cosmic rays (GCRs). Here, we investigate the correlation between the modulation of GCRs, the heliospheric magnetic field, and solar wind speed for the last 24 solar cycles to find trends that can potentially be used to predict future solar activity. Specifically, we develop a tool for predicting future magnetic field intensity, based on the hysteresis in the GCR variation, during the last phases of the current cycle. This method estimates that SC25 will be as weak as or weaker than SC24. This would mean that the Sun has likely entered a secular minimum, which, according to historical records, should last for another two cycles (SC25 and SC26).

The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) has been orbiting the Moon since 2009 aboard the Lunar Reconnaissance Orbiter (LRO). From this vantage point, it samples the interplanetary energetic particle population outside the shielding of the Earth's magnetosphere. We report the sensor's observations of galactic cosmic rays (GCRs) over a complete solar activity cycle. CRaTER is designed primarily to measure not the spectra of GCR particles outside the sensor but rather their effects on matter, and in particular, it measures the linear energy transfer (LET) or energy‐deposit spectrum in its silicon detectors. We have used the Geant4 radiation‐transport code to devise a background‐rejection algorithm to improve these measurements of LET under 9.9 g/cm² of shielding, and the resulting observations show the changing radiation effects of GCRs as their intensity and spectrum vary with solar modulation. As of 2020 this intensity, after declining during solar maximum activity, has recovered to a level that exceeds by a few percent the historically high values seen during the deep solar minimum at the start of the LRO mission in 2009.

Recent solar conditions indicate a persistent decline in solar activity—possibly similar to the past 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 from the 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 solar modulation potential of the GCRs through Cycle 24. We used this correlation to project observations from past 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% (or 34%). We showed that accounting for a floor in the modulation potential, invoked by the Badhwar‐O'Neill 2014 model, moderates the projected radiation levels in Cycle 25. We used these results to determine the most conservative permissible mission duration (PMD, and days for 45‐year‐old male and female astronauts, respectively) based on a 3% risk of exposure‐induced death (REID) at the upper 95% confidence interval in interplanetary space.