A parametric study of space radiation exposures to critical body organs for low earth orbit missions.

The geomagnetically-trapped and galactic cosmic radiation environments are two of the major sources of naturally-occurring space radiation exposure to astronauts in low earth orbit. The exposure is dependent primarily on altitude, spacecraft shielding, crew stay-times, and solar cycle effects for a 28.5 deg orbital inclination. Based on Space Shuttle experience, the calculated results of a parametric study are presented for several mission scenarios using a computerized anatomical man model and are compared with the NASA crew exposure limits for several critical body organs.     

Genetic risks associated with radiation exposures during space flight.

The genetic risks associated with manned space flight are judged to be of little significance to the general population. The risks may be significant to the irradiated individual, particularly if one focuses attention on the incidence of dominant and chromosomal mutations that are expressed in the first generation offspring. Even so, the risk is not increased to a great extent by the low linear energy transfer (LET) component of the space radiations. It is the presumed high LET component, neutrons especially, that would make the major contribution to the risk, because the relative biological effectiveness (RBE) values for this component, relative to low dose-rate photon irradiation, are between 10 and 40, depending upon the particular genetic effect and dose-rate comparison. The appropriate RBE value would probably be 20 or greater, so that even small neutron doses become magnified in their contribution. Under the assumed condition of protracted exposure to 8 rads of low LET radiation and 2 rads of high LET radiation, or from 48 to 88 rem, the individual's risk of transmitting a new dominant mutation that will be expressed in his immediate offspring is estimated to increase by at least 4% and as much as about 40%. The HZE-particle component is not expected to make a significant contribution to the total risk.     

Fluxes of galactic iron nuclei and associated HZE secondaries, and resulting radiation doses, in the brain of an astronaut.

Although galactic iron nuclei constitute only a small percentage of the total flux of radiation in space, they are extremely significant from a biological standpoint, and represent a concern for long-term manned space missions of the future. Dosages resulting from iron nuclei, and the high-charge secondary nuclei subsequently produced in nuclear fragmentation reactions, have been calculated at the centre of a simple model of the human brain, shielded by various thicknesses of aluminium. Three mission scenarios are considered representing different geomagnetic shielding conditions at solar minimum. Without artificial shielding absorbed dose rates outside the magnetosphere, in polar orbit and in the proposed Space Station orbit, are approximately 0.3, 0.1 and 0.03 cGy/year respectively, corresponding to dose equivalent rates of 8.0, 2.5 and 0.8 cSv/year, and decreasing by roughly a factor of two behind 10 g/cm2 of aluminium. In line with new approaches to risk estimation based on particle fluence and track structure, calculations of the number of cell nuclei likely to be struck by these HZE particles are also presented. Behind 10 g/cm2 of aluminium, 3.4%, 1.3% and 0.5% of cell nuclei at the centre of the brain will be traversed at least once by such a particle within three years, for the three mission scenarios respectively.     

Estimates of HZE particle contributions to SPE radiation exposures on interplanetary missions.

Estimates of radiation doses resulting from possible HZE (high energy heavy ion) components of solar particle events (SPEs) are presented for crews of manned interplanetary missions. The calculations assume a model spectrum obtained by folding measured solar flare HZE particle abundances with the measured energy spectra of SPE alpha particles. These hypothetical spectra are then transported through aluminum spacecraft shielding. The results, presented as estimates of absorbed dose and dose equivalent, indicate that HZE components by themselves are not a major concern for crew protection but should be included in any overall risk assessment. The predictions are found to be sensitive to the assumed spectral hardness parameters.     

A comparison of radiation shielding effectiveness of materials for highly elliptical orbits

The Canadian Space Agency (CSA) has proposed a Polar Communications and Weather (PCW) satellite mission, in conjunction with other partners. The PCW will provide essential communications and meteorological services to the Canadian Arctic, as well as space weather observations of in situ ionizing radiation along the orbit. The CSA has identified three potential Highly Elliptical Orbits (HEOs) for a PCW satellite constellation, Molniya, Modified Tundra and Triple Apogee (TAP), each having specific merits, which would directly benefit the performance/longevity of a PCW spacecraft. Radiation shielding effectiveness of various materials was studied for the three PCW orbit options to determine the feasibility of employing materials other than conventional aluminium to achieve a specified spacecraft shielding level with weight savings over aluminium. It was found that, depending on the orbit-specific radiation environment characteristics, the benefits of using polyethylene based materials is significant enough (e.g., 22% in Molniya for PE at 50 krad TID) to merit further investigation.     

A procedure for benchmarking laboratory exposures with 1 A GeV iron ions.

A new version of the HZETRN code capable of simulating HZE ions with either laboratory or space boundary conditions is under development. The computational model consists of combinations of physical perturbation expansions based on the scales of atomic interaction, multiple scattering, and nuclear reactive processes with use of asymptotic/Neumann expansions with non-perturbative corrections. The code contains energy loss with straggling, nuclear attenuation, nuclear fragmentation with energy dispersion and downshifts, and off-axis dispersion with multiple scattering under preparation. The present benchmark is for a broad directed beam for 1 A GeV iron ion beams with 2 A MeV width and four targets of polyethylene, polymethyl metachrylate, aluminum, and lead of varying thickness from 5 to 30 g/cm2. The benchmark quantities will be dose, track averaged LET, dose averaged LET, fraction of iron ion remaining, and fragment energy spectra after 23 g/cm2 of polymethyl metachrylate.     

The influence of radiation quality on the formation of DNA breaks.

We have aimed to present a comprehensive review of our understanding to date of the formation of DNA strand breaks induced by high LET radiation. We have discussed data obtained from DNA in solution as well as from the formation and "repair" of strand breaks in cell DNA. There is good agreement, qualitatively, between these two systems. Results were evaluated for two parameters: (1) effectivity per particle, the cross section (sigma) in micrometers 2/particle; and (2) the strand break induction frequency as number of breaks per Gy per unit DNA (bp or dalton). A series of biological effects curves (one for each Z-number) is obtained in effectivity versus LET plots. The relationships between induction frequencies of single-strand breaks, or double-strand breaks, or the residual "irrepairable" breaks and LET-values have been evaluated and discussed for a wide spectrum of heavy ions, both for DNA in solution and for DNA in the cell. For radiation induced total breaks in cell DNA, the RBE is less than one, while the RBE for the induction of DSBs can be greater than one in the 100-200 keV/micrometers range. The level of irrepairable strand breaks is highest in this same LET range and may reach 25 percent of the initial break yield. The data presented cover results obtained for helium to uranium particles, covering a particle incident energy range of about 2 to 900 MeV/u with a corresponding LET range of near 16 to 16000 keV/micrometers.     

The role of hydration and radiation quality in the induction of DNA damage--chemical aspects.

The mutagenic and lethal effects of ionising radiation are thought to result from chemical modifications induced within DNA. This DNA damage is significantly influenced by the chemical environment and the radiation quality (LET). Water closely associated with the DNA and its immediate environment is involved in the early chemical pathways which lead to the induction of DNA damage and is reflected in the cellular radiosensitivity. For instance, hydration of DNA influences hole migration leading to its localisation at guanine. Changes in the radiation quality are discussed in terms of the complexity of the radical clusters produced. It is inferred that at higher LET, the influence of the chemical environment (O2 etc) decreases with respect to DNA damage and cellular radiosensitivity. It is therefore important to include these effects of environment of the DNA upon the early chemical pathways in models of radiation action.     

Analysis of secondary electron emission spectra of equal-LET protons and alpha particles for purposes of radiation quality and spaceflight hazard assessment.

Amongst the great variety of heavy particles present in the galactic and solar cosmic ray spectra, hydrogen and helium nuclei are significantly more abundant than all other heavier ions and, as such, represent a major radiation hazard to humans in space. Experimental data have suggested that differences in relative biological effectiveness (RBE) exist between the two species at the same value of linear energy transfer (LET). This has consequences for heavily ionising radiation protection procedures, which currently still assume a simple dependence of radiation quality on LET. By analysing the secondary electron (delta-ray) emission spectra of protons and alpha particles, in terms of the spatial characteristics of energy deposition in cellular targets and the likelihood of complex lesion formation, a numerical quantity representing biological effectiveness is generated. When expressed relative to a reference radiation, this quantity is found to differ for protons and a particles of the same LET, demonstrating not only the ion-specific nature of RBE but also the inadequacy of specifying radiation quality as a function of LET only. Such a method for numerically assessing radiation quality may have implications for procedures for heavy ion protection in space at low doses and for understanding the initial mechanisms of radiation action.     

Radiation exposures during space flight and their measurement.

The paper reviews radiation exposures recorded during space flights of the US and USSR. Most of the data are from manned missions and include discussion of absorbed dose and dose rates as a function of parameters such as altitude, inclination, spacecraft type and shielding. Preliminary data exist on the neutron and HZE-particle component, as well as the LET spectra. For low Earth-orbit missions, the dose encountered is strongly altitude-dependent, with a weaker dependence upon inclination. The doses range from about 6 millirad per day for the Space Transportation System No. 3 flight to about 90 mrad per day for Skylab. The effective quality factor (QF) for the near-Earth orbits and free space has been estimated to be about 1.5 and about 5.5 respectively. Complete shielding from the galactic cosmic rays does not appear practical because of spacecraft weight limitations.     

Effects of orbit progression on the radiation exposures from solar proton fluxes in low Earth orbit under geomagnetic storm conditions.

The present study examines the effects of orbit progression on the exposures within a Space Station Freedom module in a 51.6-degree inclined orbit at 450 km. The storm evolution is modeled after the November 1960 event, and the solar proton flux evolution is taken from the August 1972 solar proton event. The effects of a strong magnetic shock, such as was observed during the October 1989 event, is also modeled. The statistics on hourly average storm fields for the last forty years reveal that the largest geomagnetic storms approach a Dst value of -500 nanotesla at the storm peak. Similarly, one of the largest satellite-measured proton flux (> 10 MeV) for space exposures is the event of August 1972. The effects of orbit progression (advance of the line of nodes) is examined for the above conditions to study the variation of exposures under differing times of occurrence of the solar proton peak intensity, attainment of geomagnetic storm maximum, and the location of the line of nodes of the last geomagnetically protected orbit. The impact of the inherent inhomogeneity of the space station module is examined as a limiting factor on exposure with regard to the need of additional parasitic shielding.     

A space radiation transport method development.

Improved spacecraft shield design requires early entry of radiation constraints into the design process to maximize performance and minimize costs. As a result, we have been investigating high-speed computational procedures to allow shield analysis from the preliminary design concepts to the final design. In particular, we will discuss the progress towards a full three-dimensional and computationally efficient deterministic code for which the current HZETRN evaluates the lowest-order asymptotic term. HZETRN is the first deterministic solution to the Boltzmann equation allowing field mapping within the International Space Station (ISS) in tens of minutes using standard finite element method (FEM) geometry common to engineering design practice enabling development of integrated multidisciplinary design optimization methods. A single ray trace in ISS FEM geometry requires 14 ms and severely limits application of Monte Carlo methods to such engineering models. A potential means of improving the Monte Carlo efficiency in coupling to spacecraft geometry is given in terms of re-configurable computing and could be utilized in the final design as verification of the deterministic method optimized design.     

Behavioral endpoints for radiation injury.

The relative behavioral effectiveness of heavy particles was evaluated. Using the taste aversion paradigm in rats, the behavioral toxicity of most types of radiation (including 20Ne and 40Ar) was similar to that of 60Co photons. Only 56Fe and 93Nb particles and fission neutrons were significantly more effective. Using emesis in ferrets as the behavioral endpoint, 56Fe particles and neutrons were again the most effective; however, 60Co photons were significantly more effective than 18 MeV electrons. These results suggest that LET does not completely predict behavioral effectiveness. Additionally, exposing rats to 10 cGy of 56Fe particles attenuated amphetamine-induced taste aversion learning. This behavior is one of a broad class of behaviors which depends on the integrity of the dopaminergic system and suggests the possibility of alterations in these behaviors following exposure to heavy particles in a space radiation environment.     

Biochemical mechanisms and clusters of damage for high-LET radiation.

Using mechanisms of indirect and direct radiation, a generalized theory has been developed to account for strand break yields by high-LET particles. The major assumptions of this theory are: (i) damage at deoxyribose sites results primarily in strand break formation and (2) damage to bases leads to a variety of base alterations. Results of the present theory compare well with cellular data without enzymatic repair. As an extension of this theory, we show that damage clusters are formed near each double strand break for high-LET radiation only. For 10 MeV/n (LET = 450 keV/micrometer) neon ions, the results show that on average there are approximately 3 additional breaks and approximately 3 damaged bases formed near each double strand break. For 100 MeV/n helium ions (LET = 3 keV/micrometer), less than 1% of the strand breaks have additional damage within 10 base pairs.     

Uncertainties in radiation effect predictions for the natural radiation environments of space.

Future manned missions beyond low earth orbit require accurate predictions of the risk to astronauts and to critical systems from exposure to ionizing radiation. For low-level exposures, the hazards are dominated by rare single-event phenomena where individual cosmic-ray particles or spallation reactions result in potentially catastrophic changes in critical components. Examples might be a biological lesion leading to cancer in an astronaut or a memory upset leading to an undesired rocket firing. The risks of such events appears to depend on the amount of energy deposited within critical sensitive volumes of biological cells and microelectronic components. The critical environmental information needed to estimate the risks posed by the natural space environments, including solar flares, is the number of times more than a threshold amount of energy for an event will be deposited in the critical microvolumes. These predictions are complicated by uncertainties in the natural environments, particularly the composition of flares, and by the effects of shielding. Microdosimetric data for large numbers of orbits are needed to improve the environmental models and to test the transport codes used to predict event rates.     

Protocol for the treatment of radiation injuries.

Despite adequate precautionary measures and high-quality safeguard devices, many accidental radiation exposures continue to occur and may pose greater risks in the future, including radiation exposure in the space environment. The medical management of radiation casualties is of major concern to health care providers. Such medical management was addressed at The First Consensus Development Conference on the Treatment of Radiation Injuries, Washington, DC, 1989. The conference addressed the most appropriate treatment for the hematopoietic and infectious complications that accompany radiation injuries and for combined radiation and traumatic/burn injuries. Based on the evidence presented at the conference, a consensus statement was formulated by expert physicians and scientists. The recommended therapies, including a suggested algorithm incorporating these recommendations for the treatment of radiation injuries, will be discussed.     

A new dynamical atmospheric ionizing radiation (AIR) model for epidemiological studies.

A new Atmospheric Ionizing Radiation (AIR) model is currently being developed for use in radiation dose evaluation in epidemiological studies targeted to atmospheric flight personnel such as civilian airlines crewmembers. The model will allow computing values for biologically relevant parameters, e.g. dose equivalent and effective dose, for individual flights from 1945. Each flight is described by its actual three dimensional flight profile, i.e. geographic coordinates and altitudes varying with time. Solar modulated primary particles are filtered with a new analytical fully angular dependent geomagnetic cut off rigidity model, as a function of latitude, longitude, arrival direction, altitude and time. The particle transport results have been obtained with a technique based on the three-dimensional Monte Carlo transport code FLUKA, with a special procedure to deal with HZE particles. Particle fluxes are transformed into dose-related quantities and then integrated all along the flight path to obtain the overall flight dose. Preliminary validations of the particle transport technique using data from the AIR Project ER-2 flight campaign of measurements are encouraging. Future efforts will deal with modeling of the effects of the aircraft structure as well as inclusion of solar particle events.     

A new Mars radiation environment model with visualization.

A new model for the radiation environment to be found on the planet Mars due to Galactic Cosmic Rays (OCR) has been developed at the NASA Langley Research Center. Solar modulated primary particles rescaled for Mars conditions are transported through the Martian atmosphere, with temporal properties modeled with variable timescales, down to the surface, with altitude and backscattering patterns taken into account. The Martian atmosphere has been modeled by using the Mars Global Reference Atmospheric Model--version 2001 (Mars-GRAM 2001). The altitude to compute the atmospheric thickness profile has been determined by using a model for the topography based on the data provided by the Mars Orbiter Laser Altimeter (MOLA) instrument on board the Mars Global Surveyor (MGS) spacecraft. The Mars surface composition has been modeled based on averages over the measurements obtained from orbiting spacecraft and at various landing sites, taking into account the possible volatile inventory (e.g., CO2 ice, H2O ice) along with its time variation throughout the Martian year. Particle transport has been performed with the HZETRN heavy ion code. The Mars Radiation Environment Model has been made available worldwide through the Space Ionizing Radiation Effects and Shielding Tools (SIREST) website, a project of NASA Langley Research Center.     

Approaches to radiation guidelines for space travel.

There are obvious risks in space travel that have loomed larger than any risk from radiation. Nevertheless, NASA has maintained a radiation program that has involved maintenance of records of radiation exposure, and planning so that the astronauts' exposures are kept as low as possible, and not just within the current guidelines. These guidelines are being reexamined currently by NCRP Committee 75 because new information is available, for example, risk estimates for radiation-induced cancer and about the effects of HZE particles. Furthermore, no estimates of risk or recommendations were made for women in 1970 and must now be considered. The current career limit is 400 rem to the blood forming organs. The appropriateness of this limit and its basis are being examined as well as the limits for specific organs. There is now considerably more information about age-dependency for radiation effects and this will be taken into account. In 1973 a committee of the National Research Council made a separate study of HZE particle effects and it was concluded that the attendant risks did not pose a hazard for low inclination near-earth orbit missions. Since that time work has been carried out on the so-called microlesions caused by HZE particles and on the relative carcinogenic effect of heavy ions, including iron. A remaining question is whether the fluence of HZE particles could reach levels of concern in missions under consideration. Finally, it is the intention of the committee to indicate clearly the areas requiring further research.