Managing Space Radiation Risk in the New Era of Space Exploration (2008) / Chapter Skim
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2 Current Knowledge of the Radiation Environment
2 Current Knowledge of the Radiation Environment
Pages 19-48
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From page 19... ...
However, these measurements do not provide insight into the surface radiation component from secondary particles created by incident energetic galactic cosmic rays and SPEs. Measurements taken from lunar orbit are closer approximations to the lunar surface environment.
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From page 20... ...
• Major research questions involve the prediction of onset and relevant characteristics and the health risks due to the residual low dose rates when under shielding. Galactic Cosmic Rays • Composed of protons, alpha particles, and heavy ions, up to very high energies exceeding tens of GeV per nucleon.
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From page 21... ...
Galactic cosmic rays also include electrons and positrons, but their intensities are too low to be of practical concern. The GCR flux outside the solar system is presumed to be constant, at least on timescales of tens of millions of years related to the solar system's motion through the Galaxy (Lieberman and Melott, 2007)
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From page 22... ...
The interplanetary magnetic field v aries with the solar activity cycle; the GCR flux near Earth is at a peak near solar minimum (when the inter planetary magnetic field is weak) and at its low point at solar maximum (when the interplanetary magnetic field is strongest)
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From page 23... ...
The count-rates have been averaged over the 27-day solar-rotation period. Dates with significant contributions from solar particle events have been e xcluded from the averages.
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From page 24... ...
Figure 2-4 shows the ratio of GCR dose rates from these two models versus depth of shielding. For solar minimum, the Badhwar-O'Neill results are systematically higher by about 10 percent; at solar maximum, they are systematically lower by about 20 percent.
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From page 25... ...
. SOLAR PARTICLE EVENTS Energetic particles, occasionally with energies exceeding several GeV, are accelerated in sporadic events at the Sun associated with solar activity.
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. SOURCE: NOAA Space Environment Center, "Solar Cycle 24 Prediction," available at http://www.sec.noaa.gov/SolarCycle/SC24/index.html.
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From page 27... ...
In most cases, ESP events are seen only at energies that do not pose a radiation hazard. But a few times per solar cycle, the shock's arrival at Earth also brings very large fluxes at very high energies, extending beyond ~100 MeV.
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From page 28... ...
However, analysis and modeling of observations like these are contributing to an emerging consensus on the way in which flares and CMEs contribute to the production of large SPEs: whereas CME-driven shocks are the ultimate energy source for potentially hazardous solar energetic particles, the reconnection processes associated with flares provide a distinctive contribution to the seed particles that are promoted to high energies by the action of the shock (Mason et al., 1999; Desai et al., 2003, 2006; Tylka et al., 2001, 2005; Lee, 2007)
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From page 29... ...
Because of the higher rate of energy deposition of heavy ions when traversing matter, it is important to assess whether solar heavy ions might pose a significant radiation hazard in themselves. As a starting point for this discussion, it should be remembered that heavier ions must have higher initial energies in order to penetrate a given depth of shielding.
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From page 30... ...
Ions heavier than alphas contributed no more than a few percent to the total dose equivalent. These calculations suggest that solar heavy ions, apart perhaps from alphas, do not make a significant contribution to the SPE radiation hazard for astronauts.
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From page 31... ...
. Both of these empirical models were developed before the central role of fast CMEs in producing large solar energetic particles was widely recognized.
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From page 32... ...
SPACE RADIATION CLIMATOLOGY Nearly all of our knowledge of the space radiation environment comes from the past 50 years of the space age. All of our experience and "rules of thumb" for operating in this environmentfrom the slow, nearly constant rate of single-event upsets caused by GCRs, to lifetimes of solar panels that are continually degraded by the accumulated fluence of solar protons, to the probability and duration of all-clear periodsare based on this era.
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From page 33... ...
It is worthwhile to see how Solar Cycle 23, which is just now drawing to a close, compares with previous experience from the space age. Figure 2-11 shows the cycle-integrated >10 MeV and >30 MeV solar proton fluences for Cycles 19-23.
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Space radiation climate. Ice-core studies indicate that the past ~50 years may have coincided with a comparatively benign space radiation climate, in terms of both GCR modulation levels and the frequency of very large SPE events.
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From page 35... ...
It may be possible to monitor the local arrival of energetic solar electrons as a precursor to large solar proton events, with an advanced warning of tens of minutes (Posner, 2007)
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From page 36... ...
However, operational tools using these codes are currently under development with funding provided by the NASA Living With a Star Targeted Research and Technology program. Significant new capability exists within the space physics community -- observations from ACE, SOHO, S TEREO, and Wind, and the establishment of multiple interdisciplinary space weather modeling consortia such as the Center for Space Environment Monitoring at the University of Michigan or the Community Coordinated Modeling Center, a partnership of government agencies centered at Goddard Space Flight Center -- provide a substantial opportunity to advance the state of the art in CME forecasting and thus better SPE forecasts.
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From page 37... ...
See the text for details. R01155, Figure 2-13 Fixed image, not changeable
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From page 38... ...
. The factor of two is intended to raise the event to the level of a "worst case." CREME96 also includes solar heavy ions, since spacecraft electronics are potentially vulnerable to both solar protons and solar heavy ions.
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From page 39... ...
will be particularly valuable in this regard. TRAPPED RADIATION In addition to galactic cosmic rays and solar energetic particles, particles trapped in Earth's magnetic field comprise the third major component of the near-Earth ionizing radiation environment.
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From page 40... ...
principle. SECONDARY RADIATION Whenever the local space radiation environment in deep space impinges on the shell of the spacecraft, the solar and galactic cosmic rays penetrate the spacecraft structure and shielding and their physical characteristics are altered by atomic and nuclear collisions with the constituent atoms of the structural and shielding materials.
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From page 41... ...
One representative calculation of the percentage variation in organ dose-equivalent contributions from incident galactic cosmic rays and their secondary radiations, produced by projectile and target fragmentation processes, is displayed in Table 2-2. The calculations assumed spacecraft aluminum shielding of the indicated area density, TABLE 2-2 Percentage Contributions to the Annual Total Dose Equivalent, Rounded to the Nearest Whole Percent, from Surviving Incident Galactic Cosmic Rays, Their Projectile Fragmentation Products, and Target Nuclear Fragments for Various Organs and Several Thicknesses of Aluminum Shielding Skin Ocular Lens Bone Marrow Type Contribution (g/cm2)
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From page 42... ...
As the spacecraft shield thickness increases, the increased production of secondary radiations results in an increase in their percentage contributions to the organ dose equivalents. Note that most of the organ dose equivalent behind 30 g/cm2 aluminum shielding results from secondary radiations produced in the spacecraft structure and overlying body tissues of the astronauts.
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From page 43... ...
These lighter particles have ranges and collision mean-free paths that are much larger than the more highly charged parent nuclei that produced them, which accounts for this slowly decreasing trend in the dose-equivalent versus shield-thickness curves. Knowledge Gaps There is a serious concern about the purely one-dimensional nature of the space radiation transport codes used to estimate shielding requirements and potential biological risks to crews on deep-space missions, including planned lunar missions.
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From page 44... ...
will contribute around 0.05 percent of the total dose. Several large studies in the United States, Canada, and Europe have found no evidence of any increase in cancer mortality among people living near nuclear ground power plants.
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From page 45... ...
2004. Intensity variation of large solar energetic particle events associated with coronal mass ejections.
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From page 46... ...
1981. Solar cycle modulation of galactic cosmic rays: Speculation on the role of coronal transients.
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From page 47... ...
2006. Space Radiation Hazards and the Vision for Space Exploration.
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From page 48... ...
2006. A comparative study of ion characteristics in the large gradual solar energetic particle events of 2002 April 21 and 2002 August 24.
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