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NASA NTRS2006

Radiation Dose from Lunar Neutron Albedo

The lunar neutron albedo from thermal energies to 8 MeV was measured on the Lunar Prospector Mission in 1998-1999. Using GEANT4 we have calculated the neutron albedo due to cosmic ray bombardment of the moon and found a good-agreement with the measured fast neutron spectra. We then calculated the total effective dose from neutron albedo of all energies, and made comparisons with the effective dose contributions from both galactic cosmic rays and solar particle events to be expected on the lunar surface.

Adams, J. H., Jr.↗

NASA NTRS2007

Evaluating Shielding Effectiveness for Reducing Space Radiation Cancer Risks

We discuss calculations of probability distribution functions (PDF) representing uncertainties in projecting fatal cancer risk from galactic cosmic rays (GCR) and solar particle events (SPE). The PDF s are used in significance tests of the effectiveness of potential radiation shielding approaches. Uncertainties in risk coefficients determined from epidemiology data, dose and dose-rate reduction factors, quality factors, and physics models of radiation environments are considered in models of cancer risk PDF s. Competing mortality risks and functional correlations in radiation quality factor uncertainties are treated in the calculations. We show that the cancer risk uncertainty, defined as the ratio of the 95% confidence level (CL) to the point estimate is about 4-fold for lunar and Mars mission risk projections. For short-stay lunar missions (<180 d), SPE s present the most significant risk, however one that is mitigated effectively by shielding, especially for carbon composites structures with high hydrogen content. In contrast, for long duration lunar (>180 d) or Mars missions, GCR risks may exceed radiation risk limits, with 95% CL s exceeding 10% fatal risk for males and females on a Mars mission. For reducing GCR cancer risks, shielding materials are marginally effective because of the penetrating nature of GCR and secondary radiation produced in tissue by relativistic particles. At the present time, polyethylene or carbon composite shielding can not be shown to significantly reduce risk compared to aluminum shielding based on a significance test that accounts for radiobiology uncertainties in GCR risk projection.

Cucinotta, Francis A.↗

NASA NTRS2007

Anti-radiation vaccine: Immunologically-based Prophylaxis of Acute Toxic Radiation Syndromes Associated with Long-term Space Flight

Protecting crew from ionizing radiation is a key life sciences problem for long-duration space missions. The three major sources/types of radiation are found in space: galactic cosmic rays, trapped Van Allen belt radiation, and solar particle events. All present varying degrees of hazard to crews; however, exposure to high doses of any of these types of radiation ultimately induce both acute and long-term biological effects. High doses of space radiation can lead to the development of toxicity associated with the acute radiation syndrome (ARS) which could have significant mission impact, and even render the crew incapable of performing flight duties. The creation of efficient radiation protection technologies is considered an important target in space radiobiology, immunology, biochemistry and pharmacology. Two major mechanisms of cellular, organelle, and molecular destruction as a result of radiation exposure have been identified: 1) damage induced directly by incident radiation on the macromolecules they encounter and 2) radiolysis of water and generation of secondary free radicals and reactive oxygen species (ROS), which induce chemical bond breakage, molecular substitutions, and damage to biological molecules and membranes. Free-radical scavengers and antioxidants, which neutralize the damaging activities of ROS, are effective in reducing the impact of small to moderate doses of radiation. In the case of high doses of radiation, antioxidants alone may be inadequate as a radioprotective therapy. However, it remains a valuable component of a more holistic strategy of prophylaxis and therapy. High doses of radiation directly damage biological molecules and modify chemical bond, resulting in the main pathological processes that drive the development of acute radiation syndromes (ARS). Which of two types of radiation-induced cellular lethality that ultimately develops, apoptosis or necrosis, depends on the spectrum of incident radiation, dose, dose rate, and functional conditions of impacted cells/organisms. The administration of an experimental anti-radiation vaccine may provide an immunologically based, adjunct method of prevention or prophylaxis against clinical ARS. The administration of experimental anti-radiation serum (ARS) and the use of the blood dialysis methods, such as immune plasma-sorption, may assist in the clearance of radiation-specific toxins and may enhance established strategies for the mitigation of the biological effects leading to ARS, and should be evaluated for use on exploration-class space missions.

Popov, Dmitri↗

NASA NTRS2007

Radiation Environments for Lunar Programs

Developing reliable space systems for lunar exploration and infrastructure for extended duration operations on the lunar surface requires analysis and mitigation of potential system vulnerabilities to radiation effects on materials and systems. This paper reviews the characteristics of space radiation environments relevant to lunar programs including the trans-Earth and trans-lunar injection trajectories through the Earth's radiation belts, solar wind surface dose environments, energetic solar particle events, and galactic cosmic rays and discusses the radiation design environments being developed for lunar program requirements to assure that systems operate successfully in the space environment.

Minow, Joseph I.↗

NASA NTRS2005

Thermal, Radiation and Impact Protective Shields (TRIPS) for Robotic and Human Space Exploration Missions

New concepts for protective shields for NASA s Crew Exploration Vehicles (CEVs) and planetary probes offer improved mission safety and affordability. Hazards include radiation from cosmic rays and solar particle events, hypervelocity impacts from orbital debris/ micrometeorites, and the extreme heating environment experienced during entry into planetary atmospheres. The traditional approach for the design of protection systems for these hazards has been to create single-function shields, i.e. ablative and blanket-based heat shields for thermal protection systems (TPS), polymer or other low-molecular-weight materials for radiation shields, and multilayer, Whipple-type shields for protection from hypervelocity impacts. This paper introduces an approach for the development of a single, multifunctional protective shield, employing nanotechnology- based materials, to serve simultaneously as a TPS, an impact shield and as the first line of defense against radiation. The approach is first to choose low molecular weight ablative TPS materials, (existing and planned for development) and add functionalized carbon nanotubes. Together they provide both thermal and radiation (TR) shielding. Next, impact protection (IP) is furnished through a tough skin, consisting of hard, ceramic outer layers (to fracture the impactor) and sublayers of tough, nanostructured fabrics to contain the debris cloud from the impactor before it can penetrate the spacecraft s interior.

Loomis, M. P.↗

NASA NTRS2007

Improvement of Risk Assessment from Space Radiation Exposure for Future Space Exploration Missions

Protecting astronauts from space radiation exposure is an important challenge for mission design and operations for future exploration-class and long-duration missions. Crew members are exposed to sporadic solar particle events (SPEs) as well as to the continuous galactic cosmic radiation (GCR). If sufficient protection is not provided the radiation risk to crew members from SPEs could be significant. To improve exposure risk estimates and radiation protection from SPEs, detailed variations of radiation shielding properties are required. A model using a modern CAD tool ProE (TM), which is the leading engineering design platform at NASA, has been developed for this purpose. For the calculation of radiation exposure at a specific site, the cosine distribution was implemented to replicate the omnidirectional characteristic of the 4 pi particle flux on a surface. Previously, estimates of doses to the blood forming organs (BFO) from SPEs have been made using an average body-shielding distribution for the bone marrow based on the computerized anatomical man model (CAM). The development of an 82-point body-shielding distribution at BFOs made it possible to estimate the mean and variance of SPE doses in the major active marrow regions. Using the detailed distribution of bone marrow sites and implementation of cosine distribution of particle flux is shown to provide improved estimates of acute and cancer risks from SPEs.

Kim, Myung-Hee Y.↗

NASA NTRS2005

Planetary and Interplanetary Environmental Models for Radiation Analysis

The essence of environmental modeling is presented as suited for radiation analysis purposes. The variables of fundamental importance for radiation environmental assessment are discussed. The characterization is performed by dividing modeling into three areas, namely the interplanetary medium, the circumplanetary environment, and the planetary or satellite surface. In the first area, the galactic cosmic rays (GCR) and their modulation by the heliospheric magnetic field as well as and solar particle events (SPE) are considered, in the second area the magnetospheres are taken into account, and in the third area the effect of the planetary environment is also considered. Planetary surfaces and atmospheres are modeled based on results from the most recent targeted spacecraft. The results are coupled with suited visualization techniques and radiation transport models in support of trade studies of health risks for future exploration missions.

DeAngelis, G.↗

NASA NTRS2007

Minimizing Astronauts' Risk from Space Radiation during Future Lunar Missions

This viewgraph presentation reviews the risk factors from space radiation for astronauts on future lunar missions. Two types of radiation are discussed, Galactic Cosmic Radiation (GCR) and Solar Particle events (SPE). Distributions of Dose from 1972 SPE at 4 DLOCs inside Spacecraft are shown. A chart with the organ dose quantities is also given. Designs of the exploration class spacecraft and the planned lunar rover are shown to exhibit radiation protections features of those vehicles.

Kim, Myung-Hee Y.↗

NASA NTRS2007

Heliospheric Physics and NASA's Vision for Space Exploration

The Vision for Space Exploration outlines NASA's development of a new generation of human-rated launch vehicles to replace the Space Shuttle and an architecture for exploring the Moon and Mars. The system--developed by the Constellation Program--includes a near term (approx. 2014) capability to provide crew and cargo service to the International Space Station after the Shuttle is retired in 2010 and a human return to the Moon no later than 2020. Constellation vehicles and systems will necessarily be required to operate efficiently, safely, and reliably in the space plasma and radiation environments of low Earth orbit, the Earth's magnetosphere, interplanetary space, and on the lunar surface. This presentation will provide an overview of the characteristics of space radiation and plasma environments relevant to lunar programs including the trans-lunar injection and trans-Earth injection trajectories through the Earth's radiation belts, solar wind surface dose and plasma wake charging environments in near lunar space, energetic solar particle events, and galactic cosmic rays and discusses the design and operational environments being developed for lunar program requirements to assure that systems operate successfully in the space environment.

Minow, Joseph I.↗

NASA NTRS2007

Can the Equivalent Sphere Model Approximate Organ Doses in Space Radiation Environments?

In space radiation calculations it is often useful to calculate the dose or dose equivalent in blood-forming organs (BFO). the skin or the eye. It has been customary to use a 5cm equivalent sphere to approximate the BFO dose. However previous studies have shown that a 5cm sphere gives conservative dose values for BFO. In this study we use a deterministic radiation transport with the Computerized Anatomical Man model to investigate whether the equivalent sphere model can approximate organ doses in space radiation environments. We find that for galactic cosmic rays environments the equivalent sphere model with an organ-specific constant radius parameter works well for the BFO dose equivalent and marginally well for the BFO dose and the dose equivalent of the eye or the skin. For solar particle events the radius parameters for the organ dose equivalent increase with the shielding thickness, and the model works marginally for BFO but is unacceptable for the eye or the skin The ranges of the radius parameters are also shown and the BFO radius parameters are found to be significantly larger than 5 cm in all eases.

Zi-Wei, Lin↗

NASA NTRS2008

Comparison of Organ Dose and Dose Equivalent Using Ray Tracing of Male and Female Voxel Phantoms to Space Flight Phantom Torso Data

Phantom torso experiments have been flown on the space shuttle and International Space Station (ISS) providing validation data for radiation transport models of organ dose and dose equivalents. We describe results for space radiation organ doses using a new human geometry model based on detailed Voxel phantoms models denoted for males and females as MAX (Male Adult voXel) and Fax (Female Adult voXel), respectively. These models represent the human body with much higher fidelity than the CAMERA model currently used at NASA. The MAX and FAX models were implemented for the evaluation of directional body shielding mass for over 1500 target points of major organs. Radiation exposure to solar particle events (SPE), trapped protons, and galactic cosmic rays (GCR) were assessed at each specific site in the human body by coupling space radiation transport models with the detailed body shielding mass of MAX/FAX phantom. The development of multiple-point body-shielding distributions at each organ site made it possible to estimate the mean and variance of space dose equivalents at the specific organ. For the estimate of doses to the blood forming organs (BFOs), active marrow distributions in adult were accounted at bone marrow sites over the human body. We compared the current model results to space shuttle and ISS phantom torso experiments and to calculations using the CAMERA model.

Kim, Myung-Hee Y.↗

NASA NTRS2007

Using Space Weather Variability in Evaluating the Radiation Environment Design Specifications for NASA's Constellation Program

NASA's Constellation program, initiated to fulfill the Vision for Space Exploration, will create a new generation of vehicles for servicing low Earth orbit, the Moon, and beyond. Space radiation specifications for space system hardware are necessarily conservative to assure system robustness for a wide range of space environments. Spectral models of solar particle events and trapped radiation belt environments are used to develop the design requirements for estimating total ionizing radiation dose, displacement damage, and single event effects for Constellation hardware. We first describe the rationale using the spectra chosen to establish the total dose and single event design environmental specifications for Constellation systems. We then compare variability of the space environment to the spectral design models to evaluate their applicability as conservative design environments and potential vulnerabilities to extreme space weather events

Coffey, Victoria N.↗

NASA NTRS2007

Mars Radiation Risk Assessment and Shielding Design for Long-term Exposure to Ionizing Space Radiation

NASA is now focused on the agency's vision for space exploration encompassing a broad range of human and robotic missions including missions to Moon, Mars and beyond. As a result, there is a focus on long duration space missions. NASA is committed to the safety of the missions and the crew, and there is an overwhelming emphasis on the reliability issues for space missions and the habitat. The cost-effective design of the spacecraft demands a very stringent requirement on the optimization process. Exposure from the hazards of severe space radiation in deep space and/or long duration missions is a critical design constraint and a potential 'show stopper'. Thus, protection from the hazards of severe space radiation is of paramount importance to the agency's vision. It is envisioned to have long duration human presence on the Moon for deep space exploration. The exposures from ionizing radiation - galactic cosmic radiation and solar particle events - and optimized shield design for a swing-by and a long duration Mars mission have been investigated. It is found that the technology of today is inadequate for safe human missions to Mars, and revolutionary technologies need to be developed for long duration and/or deep space missions. The study will provide a guideline for radiation exposure and protection for long duration missions and career astronauts and their safety.

Tripathi, Ram K.↗

NASA NTRS2008

Using Space Weather Variability in Evaluating the Environment Design Specifications for NASA'S Constellation Program

Spectral models of solar particle events and trapped radiation belts are necessary for the design requirements of total ionizing radiation dose, single event effects, and spacecraft charging. Space radiation and plasma environment specifications for hardware design are necessarily conservative to assure system robustness for a wide range of space environments.

Coffey, Victoria N.↗

NASA NTRS2008

The Near-Earth Space Radiation Environment

This viewgraph presentation reviews the effects of the Near-Earth space radiation environment on NASA missions. Included in this presentation is a review of The Earth s Trapped Radiation Environment, Solar Particle Events, Galactic Cosmic Rays and Comparison to Accelerator Facilities.

Xapsos, Michael↗

NASA NTRS2007

RADECS Short Course Session I: The Space Radiation Environment

The presented slides and accompanying paper focus on radiation in the space environment. Since space exploration has begun it has become evident that the space environment is a highly aggressive medium. Beyond the natural protection provided by the Earth's atmosphere, various types of radiation can be encountered. Their characteristics (energy and nature), origins and distributions in space are extremely variable. This environment degrades electronic systems and on-board equipment in particular and creates radiobiological hazards during manned space flights. Based on several years of space exploration, a detailed analysis of the problems on satellites shows that the part due to the space environment is not negligible. It appears that the malfunctions are due to problems linked to the space environment, electronic problems, design problems, quality problems, other issues, and unexplained reasons. The space environment is largely responsible for about 20% of the anomalies occurring on satellites and a better knowledge of that environment could only increase the average lifetime of space vehicles. This naturally leads to a detailed study of the space environment and of the effects that it induces on space vehicles and astronauts. Sources of radiation in the space environment are discussed here and include the solar activity cycle, galactic cosmic rays, solar particle events, and Earth radiation belts. Future challenges for space radiation environment models are briefly addressed.

Xapsos, Michael↗

NASA NTRS2009

Probabilistic Risk Model for Organ Doses and Acute Health Effects of Astronauts on Lunar Missions

Exposure to large solar particle events (SPEs) is a major concern during EVAs on the lunar surface and in Earth-to-Lunar transit. 15% of crew times may be on EVA with minimal radiation shielding. Therefore, an accurate assessment of SPE occurrence probability is required for the mission planning by NASA. We apply probabilistic risk assessment (PRA) for radiation protection of crews and optimization of lunar mission planning.

Kim, Myung-Hee Y.↗

NASA NTRS2009

Comparison of Radiation Transport Codes, HZETRN, HETC and FLUKA, Using the 1956 Webber SPE Spectrum

Protection of astronauts and instrumentation from galactic cosmic rays (GCR) and solar particle events (SPE) in the harsh environment of space is of prime importance in the design of personal shielding, spacec raft, and mission planning. Early entry of radiation constraints into the design process enables optimal shielding strategies, but demands efficient and accurate tools that can be used by design engineers in every phase of an evolving space project. The radiation transport code , HZETRN, is an efficient tool for analyzing the shielding effectiveness of materials exposed to space radiation. In this paper, HZETRN is compared to the Monte Carlo codes HETC-HEDS and FLUKA, for a shield/target configuration comprised of a 20 g/sq cm Aluminum slab in front of a 30 g/cm^2 slab of water exposed to the February 1956 SPE, as mode led by the Webber spectrum. Neutron and proton fluence spectra, as well as dose and dose equivalent values, are compared at various depths in the water target. This study shows that there are many regions where HZETRN agrees with both HETC-HEDS and FLUKA for this shield/target configuration and the SPE environment. However, there are also regions where there are appreciable differences between the three computer c odes.

Heinbockel, John H.↗