Title: What are the Forms of Hazardous Radiation?
Authors: Stanley B. Curtis
Forms of Hazardous Radiation (ppt)
Fred Hutchinson Cancer Research Center, ret.
Department of Environmental Health
University of Washington
The three main radiation environments of concern on manned exploratory missions are the trapped radiation belts, galactic cosmic rays and large solar particle events. The trapped belts must be circumnavigated as the space vehicle escapes the earth's magnetosphere. Galactic cosmic rays (GCR) are always present and annual doses vary by about a factor of 2 or 3 throughout the solar cycle depending on the amount of shielding available. Large solar particle events (SPE's) pose the greatest threat, but can be shielded against more readily than the GCR because of their softer energy spectra. The earth's trapped radiation belts are relatively well characterized, but dynamic models are needed to better predict doses for escape and return trajectories.
For the Apollo missions, the dose was low but measurable from the belts. Risk from GCR doses carry large uncertainties due to the large component of highly ionizing radiation from high energy particles with charge (z) > 2 (denoted HZE particles). On the Apollo missions, no deleterious effects were incurred, but streaks and flashes of light were reported by the astronauts, a consequence of the heavy component of the GCR traversing their retinas. The uncertainty in the biological effects of these particles dominates the cancer risk of long term space missions outside the magnetosphere. A particular concern is risk to the central nervous system (CNS) from these particles, because of the known high density of energy deposition along their trajectories.
A vigorous experimental program is underway at the NASA Space Radiation Laboratory (NSRL) on Long Island using the Brookhaven heavy ion accelerator to provide beams of high energy (up to 1 GeV/amu) heavy ions with z up to iron and beyond. Typical effective doses as estimated by NASA at solar minimum (i.e., excluding SPE's) are about 0.07-0.08 Sv for a 90-day lunar mission behind either 5 or 20 g/cm^2 of aluminum. This would produce a predicted radiation mortality probability from cancer for a 40-year old male of about 0.3% (95% confidence limits: 0.1% 1%) for 5 or 20 g/cm^2 aluminum. The same numbers for a 1000-day mission to Mars with 600 days on the surface are roughly 1 Sv and radiation mortality probabilities of 4.2% (1.3% - 13.6%) and 3.4% (1.1% - 10.8%) for 5 and 20 g/cm^2 aluminum shielding thicknesses, respectively. If a giant SPE like the one that occurred in 1972 is included in these same missions flown near solar maximum, the numbers become effective doses of 0.69 Sv and 0.09 Sv and radiation mortality probabilities of 2.7% (0.95% - 7.6%) and 0.36% (0.12% - 1.2%) for the 90-day lunar mission and 1.24 Sv and 0.6 Sv with radiation mortality probabilities of 5.8% (1.6 14.2%) and 2.4% (0.076% - 7.8%) for 5 and 20 g/cm^2 aluminum.
For a 40-year old female the mortality probabilities are roughly 20% higher due to the increased probability of radiation-induced breast cancer. The order-of-magnitude variation in the uncertainties reflects high uncertainty in the biological effects of the highly ionizing component of the radiation as well as in the effect of protracting the radiation over the mission duration. Finally, the space physics community can help in a number of ways, including improving the temporal and spatial prediction of large SPE's, both in magnitude and spectral shape, increasing our knowledge of Mars atmosphere composition, improvement of dosimeters which include dE/dx and energy (or velocity) measurements.
Title: Understanding and Mitigating Radiation Belt Hazards for Space Exploration
Authors: Geoffrey D. Reeves
Abstract: New observations of the Earth's radiation belts have exposed gaps in our understanding of their formation, dynamics, and even average state that have important consequences for space activities including the human and robotic components of the Exploration Program. Through most of the space era the emphasis on radiation belt observations has been to develop more accurate empirical specifications of the radiation environment and its effects. As societies use of space increases and new technologies are applied to space systems the demands on these empirical specifications have become more exacting and their limitations more apparent. Models that work well at one energy, or for one altitude, or for one class of orbits cannot be extrapolated reliably to other conditions. Under-designing systems to work in the radiation belts has obvious risks but over-designing systems can have equal risks that show up as increased costs and/or decreased capabilities. Because of the dynamic nature of the system, substantial improvements in our ability to specify the radiation environment require a fundamentally different approach that explicitly incorporates the variables of time, space, and energy. New data assimilation-based models combined with new observations from strategic missions such as LWS-RBSP hold the promise for just such a revolutionary change. The development of this new capability now - well in advance of VSE missions to the Moon, Mars and beyond - will enable NASA to implement the VSE program and its supporting architecture with lower risk and better technology selection, ultimately enhancing the science return from these missions.
Title: Solar Energetic Events - An Overview
Authors: Christina M.S. Cohen
California Institute of Technology
At times it appears that each large solar energetic particle (SEP) event is unique, resulting in a zoo of characteristics where commonalities are hard to find. However, similarities do exist and this talk will provide an overview of the general characteristics of large SEP events and the risks they present to human space flight and space-based instrumentation. The variability of SEP intensities, spectral forms, and composition will be examined and what is thought to govern these aspects will be reviewed. Additionally, this talk will present measurements and analysis currently underway in an effort to more accurately understand, and eventually predict, these events and thus evaluate the degree to which a given event poses a space weather risk.
Title: The Acceleration of Solar-Energetic Particles by Shock Waves
Authors: Joe Giacalone
Department of Planetary Sciences
University of Arizona
Tucson, AZ 85721
I will focus on 4 distinct aspects of the important problem of shock acceleration of solar energetic particles (SEPs): (1) Do shocks accelerate particles? Pervasive power-law energy spectrum provide convincing indirect evidence, but there is also ample direct evidence. (2) Where do shock waves exist? We often observe them directly at 1 AU, but where can we expect them to form near the Sun and how strong are they? (3) What is the mechanism? It will be shown that the magnetic-field and plasma-flow geometry plays an important role (i.e. perpendicular vs. parallel shocks). (4) What is the maximum energy attainable, how fast can the particles be accelerated, and how are they transported to the point of observation? It can be reasonably expected that particles can be accelerated to the highest energies observed for SEPs (beyond a GeV) within the observed time constraints suggested by observations. Recent combined modeling efforts involving MHD/CME calculations and energetic-particle transport are qualitatively reproducing some of the observed features of SEP events. Although this work is just beginning, it may lead to an improved predictive capability and a better understanding of the space-radiation environment near Earth and beyond.
Title: Progress Towards Developing a Self-Consistent Model for the Production and Transport of SEPs by CME-Driven Shocks
Authors: I. I. Roussev, I. V. Sokolov, V. Tenishev, and T. I. Gombosi
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Department of AOSS
University of Michigan
2455 Hayward Street
Ann Arbor, MI 48109-2143, USA
Coronal Mass Ejections (CMEs) play a major role in the Sun-Earth connection. The shock waves driven by the ejecta can accelerate charged particles to ultra-relativistic energies as the result of Fermi acceleration processes.
From the perspective of space weather, CMEs and the solar energetic particle (SEP) events associated with them are of particular importance since they endanger human life in outer space and pose major hazards for spacecraft in the inner solar system. The most efficient particle acceleration takes place near the Sun, and the fastest particles can escape upstream of the shock wave, reaching the Earth shortly after the initiation of the CME.
Our ongoing research efforts target fundamental features of gradual SEP events, such as formation and evolution of realistic CME-driven shocks, particle injection at the shock, excitation of turbulence by the self-generated Alfvén waves, particle diffusion due to the enhanced turbulence, and particle escape upstream of the shock, among other phenomena. Our approach is to integrate the best theories developed for every aspect of the SEP production and transport problem, including a realistic model of turbulence near the shock front and effects of SEP spectrum anisotropy. The strength in this integrated approach is that it enables us to quantify the particle acceleration and scattering by the self-excited Alfvén turbulence, and particle transport along and across the interplanetary magnetic field. The excited turbulence is responsible for both the ion acceleration and the shock wave structure.
In this talk, we present a coupled model that describes the low-frequency plasma turbulence and the SEP acceleration and transport. For simplicity, we use the quasi-linear theory to evaluate the growth rate of the turbulent waves, and the particle motion is described in the diffusive approximation. The effects taken into account are: realistic evolution of the shock wave front, advection of the magnetic field in realistic solar wind model, first-order Fermi acceleration for supra-thermal ions, and a self-consistent description of the self-excited Alfvén turbulence and the particle scattering at this turbulence. In the long run, we are committed to developing a predictive capability of the SEP threat related to robotic and human exploration missions. This talk summarizes the progress made in accomplishing this goal.
Title: CME/Flare Mechanisms
Authors: Spiro K. Antiochos
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Breakout (6 Mb avi file)
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Model (7 Mb avi file)
Flare (2 Mb mpg file)
Naval Research Laboratory
The most spectacular and most energetic manifestations of solar activity are the giant disruptions of the Sun's magnetic field that are observed as a filament eruption/ coronal mass ejection (CME)/ flare event. CMEs/eruptive flares are also the primary drivers of intense SEP storms and of destructive space weather at Earth. By observing the onset of a CME/flare at the Sun, it is possible to obtain one to three days warning before the ejecta and accompanying shock arrive at Earth or Mars, but the energetic particles often arrive within tens of minutes of event onset. Therefore, a useful warning capability for SEPs requires prediction of CME onset from observations of conditions at the Sun. We discuss the current state of the field for theories and modeling of CME initiation, and describe possible predictions schemes for CME onset based on these models. We also discuss how the upcoming NASA missions, STEREO, Solar-B and SDO, and the planned future missions, Solar Probe and Sentinels, may be able to test some of these prediction schemes.
This work was supported, in part, by NASA and ONR.
Title: Operational Aspects of Space Radiation Analysis
Authors: M.D. Weyland, A.S. Johnson, E.J. Semones, T. Shelfer, C.
Dardano, T. Lin 2N.E. Zapp, R. Rutledge, T. George
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1 NASA Johnson Space Center, SF2, Houston, TX 77058 U.S.A.
2 Lockheed-Martin Space Operations, Houston, TX 77258 U.S.A.
Minimizing astronaut's short and long-term medical risks arising from exposure
to ionizing radiation during space missions is a major concern for NASA's manned
spaceflight program, particularly exploration missions. For ethical and legal
reasons, NASA follows the "as low as reasonably achievable" (ALARA) principal in
managing astronaut's radiation exposures. One implementation of ALARA is the
response to space weather events. Of particular concern are energetic solar
particle events, and in low Earth orbit (LEO), electron belt enhancements. To
properly respond to these events, NASA's Space Radiation Analysis Group (SRAG),
in partnership with the NOAA Space Environment Center (SEC), provides continuous
flight support during U.S. manned missions. In this partnership, SEC compiles
space weather data from numerous ground and space based assets and makes it
available in near real-time to SRAG (along with alerts and forecasts), who in
turn uses these data as input to models to calculate estimates of the resulting
exposure to astronauts. These calculations and vehicle instrument data form the
basis for real-time recommendations to flight management. It is also important
to implement ALARA during the design phase. In order to appropriately weigh the
risks associated with various shielding and vehicle configuration concepts, the
expected environment must be adequately characterized for nominal and worst case
scenarios for that portion of the solar cycle and point in space. Even with the
best shielding concepts and materials in place (unlikely), there will be
numerous occasions where the crew is at greater risk due to being in a lower
shielded environment (short term transit or lower shielded vehicles, EVAs), so
that accurate space weather forecasts and nowcasts, of particles at the relevant
energies, will be crucial to protecting crew health and safety.
Title: Challenges for Electronics in the Vision for Space Exploration
Authors: Kenneth LaBel
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Abstract: New missions to the Moon and Mars provide grave challenges for the reliable utilization of high-performance modern electronics. Changes in modern integrated circuit (IC) technologies have modified the way we approach and conduct radiation tolerance and reliability testing of electronics in general. These electronics changes include scaling of geometries, new materials, new packaging technologies, and overall speed and device complexity challenges. In this presentation, we will:
- Define briefly the radiation effects on electronics,
- Discuss the specific environment challenges for electronics that relate to the Moon and Mars,
- Provide an overview of the types of electronics that may be required for Exploration as well as discuss a few relevant examples,
- Identify sample radiation and reliability issues for modern electronics that are of import for Exploration,
- Illustrate a four-pronged approach to electronic parts from management to research required for Exploration, and finally,
- Recommend specific research areas that require further exploration to ensure reliability for space electronics utilization.
The focus will be on standard digital technologies, however, other high performance technologies will be discussed where appropriate. The effects of concern will be: Single Event Effects (SEE) and steady state total ionizing dose (TID) response and to a lesser extent, displacement damage.
Title: Understanding and Mitigating the Radiation Hazards of Space Travel: Progress and Future Needs
Authors: Richard B. Setlow
Abstract: A Significant hazard to astronauts, travelling beyond the Earth's orbit for significant periods of time, arises from the presence of High Atomic Number, High Energy (HZE) cosmic ray nuclei. These nuclei are densely ionizing (high linear energy transfer, LET) compared to gamma-rays or X-rays, and could cause significant damages to biological systems. The damages have the potential for mutating cells and inducing cancer, as well as killing cells, inactivating the immune system and affecting the Central Nervous System. The doses from these nuclei may be decreased by space-craft shielding, but their biological effects to astronauts must be extrapolated from experimental data on cells (killing and mutations) and animal models (mutations and cancer induction) exposed to the high LET nuclei. These extrapolations have very high uncertainties compared to those for dosimetry.
Fortunately, NASA has funded the construction of a laboratory at the Brookhaven National Laboratory (the NASA Space Radiation Laboratory, NSRL, completed in 2003) that is connected to a large accelerator. It is set up so that many different nuclei may be used to expose biological sample to appropriate fluxes of the nuclei so as to measure relevant endpoints. I will describe methods of extrapolation, some of the results obtained to date and potential dietary supplements that might minimize the biological effects. Still not clarified are the possibilities that there might be a synergistic interaction between ionizing radiation and stresses from micro-gravity that could affect the repair of DNA damage and the effects on the immune system.
Title: Collaborative Efforts: Agency-wide Radiation Challenges
Authors: Gale J. Allen
Abstract: Radiation impacts much of NASA's mission content including upper atmospheric aeronautics, nuclear power and propulsion systems, Earth and space science, the design of spacecraft, life support, and robotic systems, mission designs and concepts of operations including the Space Shuttle and ISS, as well as the impact of radiation on human biology and future life support requirements. The complex nature of prediction, protection, interaction, mitigation, and adverse effects require that NASA pursue an integrated radiation plan covering both space radiation and radioactive power and propulsion systems. Collaboration among these areas is critical to ensure mission success. Progress on the formation of a Radiation Tiger Team and subsequent agency-wide working group will be presented. A brief update on current radiation health challenges will also be presented.