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.

Advances in space radiation physics and transport at NASA.

The space radiation environment is a complex mixture of particle types and energies originating from sources inside and outside of the galaxy. These environments may be modified by the heliospheric and geomagnetic conditions as well as planetary bodies and vehicle or habitat mass shielding. In low Earth orbit (LEO), the geomagnetic field deflects a portion of the galactic cosmic rays (GCR) and all but the most intense solar particle events (SPE). There are also dynamic belts of trapped electrons and protons with low to medium energy and intense particle count rates. In deep space, the GCR exposure is more severe than in LEO and varies inversely with solar activity. Unpredictable solar storms also present an acute risk to astronauts if adequate shielding is not provided. Near planetary surfaces such as the Earth, moon or Mars, secondary particles are produced when the ambient deep space radiation environment interacts with these surfaces and/or atmospheres. These secondary particles further complicate the local radiation environment and modify the associated health risks. Characterizing the radiation fields in this vast array of scenarios and environments is a challenging task and is currently accomplished with a combination of computational models and dosimetry. The computational tools include models for the ambient space radiation environment, mass shielding geometry, and atomic and nuclear interaction parameters. These models are then coupled to a radiation transport code to describe the radiation field at the location of interest within a vehicle or habitat. Many new advances in these models have been made in the last decade, and the present review article focuses on the progress and contributions made by workers and collaborators at NASA Langley Research Center in the same time frame. Although great progress has been made, and models continue to improve, significant gaps remain and are discussed in the context of planned future missions. Of particular interest is the juxtaposition of various review committee findings regarding the accuracy and gaps of combined space radiation environment, physics, and transport models with the progress achieved over the past decade. While current models are now fully capable of characterizing radiation environments in the broad range of forecasted mission scenarios, it should be remembered that uncertainties still remain and need to be addressed.

A calculation of the radiation environment on the Martian surface.

In this work, the radiation environment on the Martian surface, as produced by galactic cosmic radiation incident on the atmosphere, is modeled using the Monte Carlo radiation transport code, High Energy Transport Code-Human Exploration and Development in Space (HETC-HEDS). This work is performed in participation of the 2016 Mars Space Radiation Modeling Workshop held in Boulder, CO, and is part of a larger collaborative effort to study the radiation environment on the surface of Mars. Calculated fluxes for neutrons, protons, deuterons, tritons, helions, alpha particles, and heavier ions up to Fe are compared with measurements taken by Radiation Assessment Detector (RAD) instrument aboard the Mars Science Laboratory over a period of 2 months. The degree of agreement between measured and calculated surface flux values over the limited energy range of the measurements is found to vary significantly depending on the particle species or group. However, in many cases the fluxes predicted by HETC-HEDS fall well within the experimental uncertainty. The calculated results for alpha particles and the heavy ion groups Z = 3-5, Z = 6-8, Z = 9-13 and Z > 24 are in the best agreement, each with an average relative difference from measured data of less than 40%. Predictions for neutrons, protons, deuterons, tritons, Helium-3, and the heavy ion group Z = 14-24 have differences from the measurements, in some cases, greater than 50%. Future updates to the secondary light particle production methods in the nuclear model within HETC-HEDS are expected to improve light ion flux predictions.

The radiation environment on the surface of Mars - Summary of model calculations and comparison to RAD data.

The radiation environment at the Martian surface is, apart from occasional solar energetic particle events, dominated by galactic cosmic radiation, secondary particles produced in their interaction with the Martian atmosphere and albedo particles from the Martian regolith. The highly energetic primary cosmic radiation consists mainly of fully ionized nuclei creating a complex radiation field at the Martian surface. This complex field, its formation and its potential health risk posed to astronauts on future manned missions to Mars can only be fully understood using a combination of measurements and model calculations. In this work the outcome of a workshop held in June 2016 in Boulder, CO, USA is presented: experimental results from the Radiation Assessment Detector of the Mars Science Laboratory are compared to model results from GEANT4, HETC-HEDS, HZETRN, MCNP6, and PHITS. Charged and neutral particle spectra and dose rates measured between 15 November 2015 and 15 January 2016 and model results calculated for this time period are investigated.

NASA Space Radiation Transport Code Development Consortium.

Recently, NASA established a consortium involving the University of Tennessee (lead institution), the University of Houston, Roanoke College and various government and national laboratories, to accelerate the development of a standard set of radiation transport computer codes for NASA human exploration applications. This effort involves further improvements of the Monte Carlo codes HETC and FLUKA and the deterministic code HZETRN, including developing nuclear reaction databases necessary to extend the Monte Carlo codes to carry out heavy ion transport, and extending HZETRN to three dimensions. The improved codes will be validated by comparing predictions with measured laboratory transport data, provided by an experimental measurements consortium, and measurements in the upper atmosphere on the balloon-borne Deep Space Test Bed (DSTB). In this paper, we present an overview of the consortium members and the current status and future plans of consortium efforts to meet the research goals and objectives of this extensive undertaking.

Implications of the space radiation environment for human exploration in deep space.

Human exploration of the solar system beyond Earth's orbit will entail many risks for the crew on these deep space missions. One of the most significant health risks is exposure to the harsh space radiation environment beyond the protection provided by the Earth's intrinsic magnetic field. Crew on exploration missions will be exposed to a complex mixture of very energetic particles. Chronic exposures to the ever-present background galactic cosmic ray (GCR) spectrum consisting of all naturally occurring chemical elements are combined with sporadic, possibly acute exposures to large fluxes of solar energetic particles, mainly protons and alpha particles. The background GCR environment is mainly a matter of concern for stochastic effects, such as the induction of cancer with subsequent mortality in many cases, and late deterministic effects, such as cataracts and possible damage to the central nervous system. Unfortunately, the actual risks of cancer induction and mortality owing to the very important high-energy heavy ion component of the GCR spectrum are essentially unknown. The sporadic occurrence of extremely large solar energetic particle events (SPE), usually associated with intense solar activity, is also a major concern for the possible manifestation of acute effects from the accompanying high doses of such radiations, especially acute radiation syndrome effects such as nausea, emesis, haemorrhaging or, possibly, even death. In this presentation, an overview of the space radiation environment, estimates of the associated body organ doses and equivalent doses and the potential biological effects on crew in deep space are presented. Possible methods of mitigating these radiations, thereby reducing the associated risks to crew are also described.

Prediction of solar particle event proton doses using early dose rate measurements.

A methodology for predicting solar particle event doses using Bayesian inference is being developed. As part of this development, we have tested criteria for categorization of new solar particle events (SPE) using calculated asymptotic doses and dose rates for the 22 SPEs that occurred in 2001. In 9 out of 22 events, our criteria for categorization would have over-predicted the range of asymptotic doses in which the tested events would have fallen. In two cases, our methodology under-predicted the dose range in which the event would have fallen. In order to better predict a new event's group category and thus, to better restrict the Bayesian inference predictive model parameter space, we have reexamined our dose rate criteria for categorization of new events. We report the updating of the grouping criteria using data from the 22 SPEs of 2001, as well as five additional SPEs. Using the revised grouping criteria, we present an analysis of group categorization prediction results for the first ten SPEs of 2002.

Interplanetary crew dose estimates for worst case solar particle events based on historical data for the Carrington flare of 1859.

Over the past two decades, hypothetical models of "worst-case" solar particle event (SPE) spectra have been proposed in order to place an upper bound on radiation doses to critical body organs of interplanetary crews on deep space missions. These event spectra are usually formulated using hypothetical extrapolations of space measurements for previous large events. Here we take a different approach. Recently reported analyses of ice core samples indicate that the Carrington flare of 1859 is the largest event observed in the past 500 years. These ice core data yield estimates of the proton fluence for energies greater than 30 MeV, but provide no other spectrum information. Assuming that the proton energy distribution for such an event is similar to that measured for other recent, large events, interplanetary crew doses are estimated for these hypothetical worst case SPE spectra. These estimated doses are life threatening unless substantial shielding is provided.

Neutron yields and effective doses produced by Galactic Cosmic Ray interactions in shielded environments in space.

In order to define the ranges of relevant neutron energies for the purposes of measurement and dosimetry in space, we have performed a series of Monte Carlo transport model calculations that predict the neutron field created by Galactic Cosmic Ray interactions inside a variety of simple shielding configurations. These predictions indicate that a significant fraction of the neutron fluence and neutron effective dose lies in the region above 20 MeV up to several hundred MeV. These results are consistent over thicknesses of shielding that range from very thin (2.7 g/cm(2)) to thick (54 g/cm(2)), and over both shielding materials considered (aluminum and water). In addition to these results, we have also investigated whether simplified Galactic Cosmic Ray source terms can yield predictions that are equivalent to simulations run with a full GCR source term. We found that a source using a GCR proton and helium spectrum together with a scaled oxygen spectrum yielded nearly identical results to a full GCR spectrum, and that the scaling factor used for the oxygen spectrum was independent of shielding material and thickness. Good results were also obtained using a GCR proton spectrum together with a scaled helium spectrum, with the helium scaling factor also independent of shielding material and thickness. Using a proton spectrum alone was unable to reproduce the full GCR results.

Space radiation protection: comparison of effective dose to bone marrow dose equivalent.

In many instances, bone marrow dose equivalents averaged over the entire body have been used as a surrogate for whole-body dose equivalents in space radiation protection studies. However, career radiation limits for space missions are expressed as effective doses. This study compares calculations of effective doses to average bone marrow dose equivalents for several large solar particle events (SPEs) and annual galactic cosmic ray (GCR) spectra, in order to examine the suitability of substituting bone marrow dose equivalents for effective doses. Organ dose equivalents are computed for all radiosensitive organs listed in NCRP Report 116 using the BRYNTRN and HZETRN space radiation transport codes and the Computerized Anatomical Man (CAM) model. These organ dose equivalents are then weighted with the appropriate tissue weighting factors to obtain effective doses. Various thicknesses of aluminum shielding, which are representative of nominal spacecraft and SPE storm shelter configurations, are used in the analyses. For all SPE configurations, the average bone marrow dose equivalent is considerably less than the calculated effective dose. For comparisons of the GCR, there is less than a ten percent difference between the two methods. In all cases, the gonads made up the largest percentage of the effective dose.

Quality factors for space radiation: A new approach.

NASA has derived new models for radiological risk assessment based on epidemiological data and radiation biology including differences in Relative Biological Effectiveness for leukemia and solid tumors. Comprehensive approaches were used to develop new risk cross sections and the extension of these into recommendations for risk assessment during space missions. The methodology relies on published data generated and the extensive research initiative managed by the NASA Human Research Program (HRP) and reviewed by the National Academy of Sciences. This resulted in recommendations for revised specifications of quality factors, QNASA(Z,ß) in terms of track structure concepts that extend beyond LET alone. The new paradigm for quality factors placed demands on radiation monitoring procedures that are not satisfied by existing dosimetry systems or particle spectrometers that are practical for space exploration where mass, volume, band width and power consumption are highly constrained. We have proposed a new definition of quality factors that relaxes the requirements for identifying charge, Z, and velocity, ß, of the incident radiation while still preserving the functional form of the inherent risk functions. The departure from the exact description of QNASA(Z,ß) is that the revised values are new functions of LET for solid cancers and leukemia. We present the motivation and process for developing the revised quality factors. We describe results of extensive simulations using GCR distributions in free space as well as the resulting spectra of primary and secondary particles behind aluminum shields and penetration through water. In all cases the revised dose averaged quality factors agreed with those based on the values obtained using QNASA(Z,ß). This provides confidence that emerging technologies for space radiation dosimetry can provide real time measurements of dose and dose equivalent while satisfying constraints on size, mass, power and bandwidth. The revised quality factors are sufficiently generalized to be applicable to radiation protection practices beyond space exploration.