Radiation transport codes have been an increasingly important tool for studying the space radiation environment, which includes high‐energy and high‐nuclear‐charge particles. The unique advantage of transport models lies in covering a wider range of particles, energies, and angles than would be attainable in a laboratory or measurable by an instrument. However, since there are several transport codes developed by different teams that have contributed heavily to the literature, differences are expected between such codes. In this work, we use five such radiation transport codes (FLUctuating KAscade, GEometry ANd Tracking, High‐Energy Transport Code‐Human Exploration and Development in Space, Monte Carlo N‐Particle, and Particle and Heavy Ion Transport code System) to study the radiation environment near the Moon, specifically the lunar “albedo” radiation, which is the radiation emitted by the lunar surface through interactions with incident galactic cosmic rays and solar energetic particles. The primary goal of this paper is to provide a general characterization of the lunar albedo radiation and to find the areas where the modeled transport codes agree and disagree by using almost identical input parameters. The results of this work show overall good agreement between the codes. However, some areas of discrepancies exist that are reported herein. Thus, this paper equips the space weather and radiation biology communities with a comparison between popular radiation transport models applied to lunar albedo radiation. The overall agreement and, in some cases, discrepancies between these transport codes provide fundamental insight necessary for assessing code reliability and identifying where further study and improvements are needed to advance our understanding of lunar albedo radiation.

Within the past decade, evidence of excess atmospheric ${}^{14}$C production in tree rings, coupled with an increase in annually resolved measurements of ${}^{10}$Be in Arctic and Antarctic ice cores, have indicated that an extremely large solar particle event (SPE) occurred in AD 993/4. The production of cosmogenic nuclei, such as ${}^{36}$Cl in consonance with ${}^{10}$Be, indicate that the event had a very energetic “hard” particle spectrum, comparable to the event of February 1956. Herein, we estimate the potential radiation risk to male and female crew members on a mission to Mars that would occur from such an SPE. Critical organ doses and effective doses are calculated and compared with NASA space radiation limits for an SPE comparable to the AD 993/4 event, occurring during the transit phase to Mars, or while the crew members are operating on the surface of Mars. Aluminum shielding, similar in thickness to a surface lander, a spacecraft, and a storm shelter area within the spacecraft, are assumed for the transit phase. For surface operations, including the shielding provided by the atmosphere of Mars, shielding comparable to a spacesuit, enclosed rover, and a surface habitat are assumed. The results of our simulations indicate that such an event might have severe consequences for astronauts in transit to Mars. However, on the surface of Mars, the atmosphere provides some protection against an event similar to the 993/4 SPE. In general, the results show that additional shielding may be required for some of the assumed shielding scenarios.

Observations of temporary Forbush decreases (FDs) in the Galactic cosmic-ray (GCR) flux due to the passage of solar storms are useful for space-weather studies and alerts. Here, we introduce techniques that use global networks of ground-based neutron monitors and muon detectors to measure variations of GCR rigidity spectra in space during FDs by (1) fitting count rate decreases for power-law rigidity spectra in space with anisotropy up to second order and (2) using the “leader fraction” derived from a single neutron monitor. We demonstrate that both provide consistent results for hourly spectral index variations for five major FDs, and they agree with daily space-based data when available from the Alpha Magnetic Spectrometer. We have also made the neutron monitor leader fraction publicly available in real time. This work verifies that ground-based observations can be used to precisely monitor GCR spectral variation over a wide range of rigidities during space-weather events, with results in real time or from short-term postanalysis.

Understanding the radiation environment near the lunar surface is a key step towards planning for future missions to the Moon. However, the complex variety of energies and particle types constituting the space radiation environment makes the process of replicating such environment very difficult in Earth-based laboratories. Radiation transport codes provide a practical alternative covering a wider range of particle energy, angle, and type than can be experimentally attainable. Comparing actual measurements with simulation results help in validating particle flux input models, and input collision models and databases involving nuclear and electromagnetic interactions. Thus, in this work, we compare the LET spectra simulated using the Monte Carlo transport code PHITS with measurements made by the CRaTER instrument that is currently orbiting the Moon studying its radiation environment. In addition, we utilize a feature in PHITS that allows the user to run the simulations without Vavilov energy straggling to test whether it is the root cause of erroneous phenomena exhibited in similar studies in literature. The results herein show good agreement between the LET spectra of PHITS and the CRaTER instrument. They also confirm that using a Vavilov distribution correction would ultimately provide a better agreement between CRaTER measurements and the previous LET spectra from the transport codes HETC-HEDS and HZETRN.