CRaTER observations and permissible mission duration for human operations in deep space.

Prolonged exposure to the galactic cosmic ray (GCR) environment is a potentially limiting factor for manned missions in deep space. Evaluating the risk associated with the expected GCR environment is an essential step in planning a deep space mission. This requires an understanding of how the local interstellar spectrum is modulated by the heliospheric magnetic field (HMF) and how observed solar activity is manifested in the HMF over time. While current GCR models agree reasonably well with measured observations of GCR flux on the first matter, they must rely on imperfect or loose correlations to describe the latter. It is more accurate to use dose rates directly measured by instruments in deep space to quantify the GCR condition for a given period of time. In this work, dose rates observed by the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument are used to obtain the local GCR intensity and composition as a function of time. A response function is constructed that relates observed dose rates to solar modulation potential using a series of Monte Carlo radiation transport calculations. The record of observed solar modulation potential vs. time is then used to calculate a recent historical record of permissible mission duration (PMD) according to NASA's permissible exposure limits (PEL). Tables are provided for extreme values of PMD. Additional tables include risk of exposure-induced death (at upper 95% confidence interval) accrual rates and NASA effective dose rates as a function of solar modulation potential, astronaut age, sex, and shielding thickness. The significance of the PMD values reported in relation to likely transit duration requirements for future exploration missions is discussed. There is general agreement between CRaTER observations and the prescription of solar modulation vs. time given by the Badhwar-O'Neill 2014 GCR model. However, CRaTER observations do capture the effects of significant heliospheric transients, among other features, that are missing from the prescription of solar modulation potential vs. time.

Precise Detections of Solar Particle Events and a New View of the Moon.

We have invented a new method for detecting solar particle events using data from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter (LRO). Using a simple function of the total particle detection rates from four of CRaTER's six detectors, we can precisely identify solar energetic particle event periods in the CRaTER data archive. During solar-quiet periods we map the distribution of a mare-associated mixture of elements in the lunar regolith using this new method. The new map of the moon probably reflects an as-yet unknown combination of lunar albedo protons, neutrons, and gamma rays, and most closely resembles Lunar Prospector maps of gamma rays characteristic of thorium and iron. This result will lead to multiple follow-up studies of lunar albedo particles and may also contribute to the study of diurnally-varying hydrogenation of the lunar regolith.