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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop 3 Operational Strategies for Space Weather Support Future exploration missions require adequate science-based understanding, appropriate observations, and physics-based models to make it possible to develop operationally robust tools that provide timely forecasts of the space environment. These must contribute to an overall risk mitigation architecture designed to ensure the safety of astronauts throughout all phases of exploration missions. The following elements need to be included: Adequate shelter, Effective radiation monitoring, Reliable communications, and Integrated mission planning and operations concepts. This chapter provides an overview of how radiation monitoring and warning are carried out today to support the space shuttle and the International Space Station (ISS), discusses the general components of a space weather architecture, describes architectures specific to lunar and Mars missions, and discusses the need to transition research to operational support more effectively. CURRENT OPERATIONS SUPPORT (SPACE SHUTTLE AND INTERNATIONAL SPACE STATION MISSIONS IN LOW EARTH ORBIT) The Space Radiation Analysis Group (SRAG) at the NASA Johnson Space Center is responsible for ensuring that the radiation exposure received by astronauts remains below established safety limits. This responsibility includes making preflight and extravehicular activity (EVA) crew exposure projections and carrying out real-time radiation environment monitoring during missions. Factors affecting crew radiation exposures in low Earth orbit (LEO) include the following: The structure and materials of the spacecraft, Mission altitude(s) and inclination,
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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop EVA start time and duration, The status of outer zone electron belts, Interplanetary particle flux, Geomagnetic field conditions, and The phase of the solar cycle. The last four factors are the province of solar and space physics scientists (NRC, 2000). Preflight and Extravehicular Activity Crew Exposure Projections SRAG maintains an extensive set of tools for estimating the exposure received by the crews of manned missions in LEO. This suite of tools includes time-resolved models of Earth’s magnetic field, maps of the radiation fluxes trapped in the geomagnetosphere, and trajectory translator/propagator algorithms. Space environment conditions (interplanetary proton flux, status of the electron belts, geomagnetic field conditions) from the Space Environment Center (SEC) of the National Oceanic and Atmospheric Administration (NOAA) are integrated with mission parameters (altitude and inclination of the spacecraft, location and timing of EVA) in order to project crew exposures. Astronauts in LEO are exposed to radiation trapped in Earth’s magnetosphere and to radiation from the Sun (solar energetic particles [SEPs]) and beyond (galactic cosmic radiation [GCR]). The trapped radiation is most intense in a region off the coast of South America (the South Atlantic Anomaly [SAA]), owing to a slight offset between the magnetic dipole of Earth and Earth’s axis of rotation. The SEPs and GCR are most intense near Earth’s poles, where there are “holes” in the approximately dipolar magnetosphere. The extent of exposure in the polar regions fluctuates during periods of geomagnetic storms. Figures 3.1 and 3.2 demonstrate the radiation environments that the International Space Station is exposed to during its orbit of Earth. Low-inclination, high-altitude flights during solar minimum produce higher dose rates than those with high-inclination, low-altitude flights during solar maximum. At higher altitudes, the area of the SAA is larger and the flux of protons is higher. Although trajectories of high-inclination flights pass through the regions of maximum intensities within the SAA, less time is spent there than during low-inclination flights, and crews on high-inclination flights typically receive less net exposure to trapped radiation for the same altitude. During solar maximum, increases in the Sun’s activity expand the atmosphere; this expansion causes losses of some of the protons in the radiation belts owing to interactions with atmospheric gases. Therefore, trapped radiation doses decrease during solar maximum and increase during solar minimum. The impact of GCR is also lower during solar maximum, because the increased speed and density of the solar wind intensifies the interplanetary magnetic field generated by the Sun, making it more difficult for GCR to penetrate the inner solar system. Radiological Support During Missions The radiation consoles in the Mission Control Center at Johnson Space Center (JSC) are staffed 4 hours daily during nominal space weather conditions and continuously during EVAs and significant space weather activity. SRAG receives data and alerts NOAA’s Space Environment Center in Boulder, Colorado. NOAA continuously monitors data received from its space weather satellites and ground stations to provide current information and forecasts about the space environment. SEC forecasters provide around-the-clock support, providing alerts and warnings about space weather conditions by telephone and pager and by displaying real-time operational space weather data via the Internet.
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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop FIGURE 3.1 The radiation environments of the International Space Station: the three regions of space around Earth where penetrating radiation occurs. SOURCE: NRC, 2000, Figure 1.1, p. 8. Crew Exposure Modeling Capability SRAG’s modeling tools include radiation transport codes and computer-aided-design-based geometry evaluation tools. These tools are used as part of an information feedback loop, and real measurements are used to refine the process continuously. This allows SRAG to react to changes in the on-orbit environment and to anticipate and plan for periods of potentially hazardous exposure. Radiation Monitoring Instruments and Dosimeters The present suite of detectors used to monitor the radiation environment during manned missions includes passive dosimeters (crew passive dosimeter [CPD] and radiation area monitor [RAM])1 and active 1 A CPD is a small set of thermoluminescent detectors (TLDs) encased in a Lexan holder. The material responds to radiation via electronic excitation states in the various TLD materials. After exposure, the amount of absorbed energy (dose) is determined by applying heat and measuring the amount of visible light released as these excited states are returned to equilibrium. A CPD is carried by each member of the crew during the entire mission and analyzed upon return to Earth. Identical to the CPD, RAMs are placed throughout the volumes of both the ISS and the space shuttle; the ISS RAMs are swapped out during the periodic shuttle servicing and supply missions.
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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop FIGURE 3.2 International Space Station (ISS) exposure to radiation. Twenty-four hours of ISS ground track overlaid with the magnetic shielding boundaries for quiet and active storm conditions and the South Atlantic Anomaly. The quasi-latitudinal pair of high-latitude lines in each hemisphere indicates the low-latitude borders of areas accessible to radiation from solar energetic particle (SEP) event zones. The higher-latitude line in each hemisphere represents quiet conditions and the lower-latitude line represents disturbed conditions. SOURCE: NRC, 2000, Figure 1.5, p. 14, originally provided by Ronald E. Turner, ANSER Corporation. instruments to measure dose (tissue equivalent proportional counter [TEPC]),2 and record the charge, energy, and directionality of particles (charged particle directional spectrometer [CPDS]).3 Passive dosimeters are mounted throughout the ISS and space shuttle (RAM) and are also worn by crew members throughout the mission (CPD). The TEPC and CPDS display the average dose rate and other parameters in real time for use by the crew, and transmit data to Mission Control so that SRAG personnel can constantly monitor the radiation environment inside the spacecraft. The TEPC is portable, enabling the mapping of the internal spacecraft radiation environment. The TEPC and RAM are flown on both the ISS and the space shuttle; the CPDS is flown only on the ISS. 2 The TEPC measures the dose that a small volume of human tissue would absorb if the tissue were at the detector’s location. The instrument incorporates a spectrometer that performs real-time calculations and a liquid crystal display. 3 The CPDS is a stack of energy-loss and position-sensitive silicon detectors and a Cerenkov counter, designed to measure the charge, energy, and direction of a particle that passes through the instrument. Currently, there are four CPDS instruments in use onboard the ISS. The intra-vehicular charged particle directional spectrometer (IV-CPDS) is designed to be used inside the ISS, with liquid crystal displays and mounting and power options for both the U.S. and Russian segments. The remaining three CPDS instruments are mounted outside the ISS and form the extra-vehicular charged particle directional spectrometer (EV-CPDS). The EV-CPDS instruments are arranged in such a way that one points forward along the velocity vector (EV1), one points aft along the antivelocity vector (EV3), and the third points up along the zenith direction (EV2). The EV-CPDS instruments have no liquid crystal displays and are coated with a special material to allow the instruments to survive the extreme temperatures in space.
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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop SRAG staff at JSC prepare the passive dosimeters prior to flight and analyze them when they return. The staff submits reports containing the results of the analyses to the space radiation health officer and to the crew flight surgeon. These reports are retained as a permanent record of the crew members’ health history and may be used to determine eligibility for additional flights. Solar and Space Physics Support Areas for the Future According to a workshop presentation by the SRAG, operational support in the shuttle and ISS era by the solar and space physics (SSP) scientists could be enhanced by the following: Real-time data from SSP spacecraft, Additional real-time measurements in proton flux (50 to 100 MeV but also 300 to 500 MeV), Integration and transition from research models to configuration-controlled operational tools, SSP spacecraft data being sent directly to vehicles as well as to the ground, Quiet-time forecasts, and Active and electronic personal dosimeters with well-characterized charged particle and neutron sensitivities. The workshop’s breakout group on dosimetry, which included SRAG management and staff, drew up the following list of knowledge requirements for additional support to exploration (lunar and Mars) missions: Dosimetry Using modern techniques, reanalyze data from dosimeters (emulsions and plastic nuclear track detectors) carried on Apollo lunar missions. Fill in high Z energetic gaps for particles with Linear Energy Transfer below 200 keV/µm. Provide reevaluated dose equivalents for Apollo astronauts. Analyze data from the Mars Radiation Environment Satellite instrument on Mars Odyssey to provide estimates of dose equivalent in Mars orbit. SEP event predictive tools for operational decisions Prediction of the next most likely SEP event temporal evolution profile (at selected energies), with associated probabilities. Real-time data from the actual event (at the selected energies). Refining of predictions in real time as the real-time data arrive. Quiet-time (all clear) forecast. Prediction of the differential flux (particles/time/area) at several critical particle energies in real time and 3, 6, 12, and 24 hours before the event at 1 AU in free space for lunar missions and between 0.75 and 1.5 AU for Mars missions. Energies of interest for operational forecast Protons (30, 60, 100, 500, 1000 MeV). Helium (30, 60, 100, 500 MeV per nucleon). Electrons (0.5 to 5.0 MeV). Lead time—real time, and at −3, −6, −12 and −24 hours. Time period and interval—every 5 min. from onset to return to background. GCR and SEP event environment—Determine the limits of the long-term variability in the space radiation environment, including GCR and SEP events, through continued research into the long-term
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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop secular changes in the GCR spectrum, using 10Be, Voyager, and historical data, supporting a goal to predict nominal and worst-case exposure. Robotic missions that allow U.S. access to the necessary data to enable the above. Monitoring at L1 is preferred because it is outside the magnetosphere, is at approximately 1 AU, and gives continuous coverage. SYSTEMS APPROACH TO PROVIDING SPACE WEATHER SUPPORT TO THE VISION FOR SPACE EXPLORATION Meeting all or many of the capabilities identified above will require a future system, or architecture, that incorporates various components: Solar monitoring (monitoring what is going on at the Sun to observe activity that may lead to an SEP event). Heliospheric monitoring (monitoring the state of the solar wind, interplanetary magnetic field, and fluctuations in the nominal solar wind, to be able to predict the propagation of accelerated protons from the source to the astronauts). Energetic particle monitoring (monitoring the solar proton and ion flux in the region near the astronauts). Communications and data fusion (addressing issues that may affect the ability to get the needed data to the appropriate user in a useful format). There are established instruments that can be applied to the task of minimizing the risks that SEP events pose to astronauts. In a few cases, notably observations of the state of the heliosphere, there are some new instruments being investigated that may significantly enhance the current ability to monitor and forecast the evolution of an SEP event. In almost all domains there is a need to develop more effective operational approaches to collect and apply the appropriate data. Components of a Space Weather Risk Mitigation Architecture Any risk mitigation architecture has several components. All of these components have to be utilized to reduce overall risk, and some of them offer greater risk reduction potential than others. Solar monitoring is required in order to place the forecasts and observations of SEP events into a context of ongoing and potential solar activity. Near-real-time observations of solar active regions and emerging coronal mass ejections (CMEs) may provide data useful to project the progress of an SEP event over a period of hours to days. Additional progress in understanding the physics of CMEs may lead to a multiday forecast of the probability of an SEP event, or enhanced, highly reliable forecasts of “all clear” periods. A variety of instruments are needed to support these tasks, from solar surface imagers (observing the Sun in visible, ultraviolet, x-ray, and radio wavelengths) to near-Sun solar coronagraphs. There is an extensive suite of research spacecraft and ground-based facilities providing experience and proof of concept from which it will be possible to select the appropriate operational instruments for an SEP event risk mitigation architecture. Heliospheric observations provide information necessary to model or monitor the propagation of SEPs from the source to the astronauts. Density fluctuations from solar emissions and from boundaries between slow and fast solar wind streams affect the shape of the interplanetary magnetic field along which the
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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop energetic particles move. They also affect the strength, structure, and motion of CMEs and the associated shocks that accelerate the energetic particles. The data that may be necessary for SEP event propagation models include information on the general state of the ambient plasma providing the source particles that are accelerated, the interplanetary magnetic field, and local disturbances moving through the inner heliosphere. Both in situ and remote sensing methods may contribute to the characterization of the heliosphere. The in situ instruments are typically small, low-cost sensors with long heritage. Remote sensing techniques that may provide a measure of CME shock characteristics, including speed, size, and strength, include observations of scattered visible zodiacal light and radio observations of interplanetary scintillations. Direct measurements of in situ solar energetic particles will continue to provide the most important contributions to an SEP event risk management strategy. Measurements at the astronauts’ location will be able to confirm that a solar particle event is under way and to provide information about the flux, rate of change of flux, and total fluence of the event. In addition, instruments may be needed to measure the relative contribution to the total flux from particles with different energies, from 10 MeV through several hundred MeV. Finally, it may be necessary to identify the flux of high-energy, high-mass ions that make up an ongoing SEP event. Additional energetic particle measurements at locations significantly away from the astronauts may also contribute to forecasting the evolution of an ongoing event. Various instruments are available to provide these measurements, including particle telescopes, solid-state detectors, and proportional counters. The natural radiation measured outside a spacecraft generates a shower of secondary particles as the radiation is slowed (and possibly stopped) by shielding surrounding an astronaut. The total radiation exposure to the astronaut is a combination of this secondary radiation and the surviving natural radiation. The complexity of shielding, uncertainties in the flux, and the need to know the crew exposure as well as possible will require real-time dose and dose-rate measurements to substantiate or replace the modeled dose estimates and projections based on measures and forecasts of the external environment. Options include the traditional film-badges, tissue-equivalent proportional counters, solid-state detectors, and possible biodosimetry techniques. The communications infrastructure is an important factor to consider in the construction of a total SEP event warning system. Since the highest-energy particles move with speeds close to the speed of light, techniques are needed to ensure that warnings and support are provided in a timely fashion. Science-based missions can employ cost-saving measures that store data for periods of time and transmit them as opportunities arise. Data from multiple spacecraft observing the same event are stored at various locations and are occasionally pieced together in “campaigns” months after the observations. However, operations data must be routinely delivered promptly from all of the spacecraft that make the observations to the operational center or centers that use that information. Some of these spacecraft may be at locations far from Earth. Once the data are received and processed, to provide real-time alerts and warnings there must be ensured communications access to the astronauts at a lunar base, on a lunar surface expedition, or at or on the way to Mars. On a Mars mission, the communication to the astronauts will take up to 20 minutes to arrive from Earth. Because of the threat posed by SEP events, consideration of radiation safety will be critical to ensure adequate shielding or timely access to a safe haven. Fortunately, awareness of the risk of radiation exposure is widespread, and it is likely that systems will be designed with attention to radiation risk management. It is critical to decide at the outset what the radiation risk mitigation strategy will be and then to integrate this strategy into the mission concept early in the design phase. The generic elements of a radiation risk mitigation strategy include space environment situational awareness, radiation exposure forecasting, and exposure impact and risk analysis. These elements combine to generate recommendations to the mission commander to keep the radiation exposure as low as reasonably achievable. The generic components
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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop of the radiation safety information flow leading to specific recommendations to the person ultimately in charge of the actions to be undertaken, whether delegated to an astronaut on the mission, or reserved for a person in mission control, are shown in Figure 3.3. The return to the Moon will be characterized by longer missions and significant surface excursions compared with the Apollo missions. Risk management architectures are likely to take advantage of a space weather network designed to protect Earth, with additional elements added to measure or estimate astronaut exposure and to provide communication links to the Moon base and astronauts on surface EVA. FIGURE 3.3 Components of the radiation safety information flow leading to specific recommendations to the mission commander. Each element has contributing components: observations, models, rules that must be followed, goals that must be achieved, and so on. For example, space environment situational awareness will be data- and model-driven. The data may come from a variety of sources, including operational and research spacecraft as well as ground-based observatories and instrumentation co-located with the astronauts. These data must be integrated into a useful forecast of the radiation threat level. Radiation exposure forecasts will rely on in situ dosimetry, transport models that start with the forecast particle environment, and models that convert the radiation field into a measure such as dose equivalent. Mission impact and risk analysis will be based on the exposure forecast, considering flight rules, mission manifest, and crew exposure histories. This process leads to recommendations to minimize the radiation risk. These recommendations will be considered in the context of mission objectives and competing risks, and ultimately a course of action will be selected. SOURCE: Courtesy of Ronald E. Turner, ANSER Corporation.
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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop Human expeditions to Mars will be the most ambitious space missions of our time. To execute these missions successfully, the radiation environment must be considered and risks appropriately managed from a systems perspective. Since the spacecraft will be the only source of shelter on a Mars mission for the hundreds of days during which the crew will be in deep space, it is critical that it contain adequate shielding to reduce the radiation dose from exposure to the steady GCR flux and the multiple large SEP events to acceptable levels. Such levels, however, and the shielding required to attain them, have yet to be determined. Particle flux and crew exposure must be measured in near real time throughout the mission. Additional elements must be evaluated for cost benefit and value to the crew. As with the return to the Moon, it is reasonable to assume that the Earth-based solar-monitoring network will support a Mars mission with SEP event forecasts and alerts. However, there are two substantial differences: Earth will not always be in a position to directly monitor critical regions of the Sun, and separation distances of several AU may significantly limit the timeliness of communication between Earth and the astronauts. One method to overcome the limitation of Earth-viewing geometry is to include solar- and heliospheric-monitoring instrumentation on the Mars-bound spacecraft. Instrumentation on additional platforms may be needed to monitor further “blind spots.” For example, solar imagers at widely spaced heliolongitudes can provide situational awareness of the solar active regions at or around the solar limb as viewed by the Mars-bound spacecraft. Additional in situ data points could provide valuable information on the state of the interplanetary magnetic field, solar wind conditions, and propagation conditions for shocks and SEP events. Ensuring that widely dispersed data-collection platforms meet significant timeliness requirements will place severe constraints on a communications architecture. The elements that are ultimately chosen will depend on the strategy employed. Figure 3.3 demonstrates how safety information about radiation flows through a properly constructed system to result in recommendations to the mission commander. Figure 3.4 provides an overview of the “tool kit” that could be constructed to support either a lunar or Mars mission. Figure 3.5 provides one version of a structure that ties these resources together in a system architecture. The final determination of the appropriate elements of the risk mitigation architecture will depend on many things. On the forecast side, the most significant consideration will be the state of the art in predicting the eruption and character of coronal mass ejections and the evolution of associated SEP events. In the past, the NOAA Space Environment Center has been called on to provide data on space weather conditions for missions beyond Earth orbit; in the future it could be called on to extend its space weather forecast and specification services to support the Vision for Space Exploration (VSE). Important considerations on the exposure-reduction side will include the latest understanding of shielding options, pharmacological countermeasures, and an ability to prescreen astronauts for radiation tolerance. Lunar Hardware Elements The phases of a lunar mission from a space weather perspective are described in Box 3.1. Potential hardware elements of an architecture to provide protection from unfavorable space weather during a lunar mission would be drawn from the general elements shown in Figures 3.4 and 3.5. The appropriate cost-effective solution will depend on the lunar mission scenario and on the limits of current understanding of the physics of SEP events. At a minimum, the main lunar base will contain adequate shielding for protection from long-term exposure to GCR and will provide a safe haven from SEP events. Astronauts and surface transportation elements will likely be equipped with real-time and passive tissue-equivalent dosimeters to permit monitoring of dose rate and cumulative dose. Unfortunately, such a system could miss a potentially severe situation if the SEP flux dramatically increased when the CME shock passed the astronaut’s location (an occasional occurrence, known as a “shock-enhanced peak,” due to energetic particles trapped in the
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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop FIGURE 3.4 An overview of a comprehensive “tool kit” from which a solar particle event risk mitigation infrastructure could be constructed to support either a lunar or a Mars mission. The elements are divided between components that could contribute to the detection and forecast of the radiation environment and those that contribute to the reduced impact of an astronaut’s exposure to the radiation environment. The final determination of the appropriate elements of the risk mitigation architecture will depend on many things, including overall mission architecture and the radiation mitigation strategy. SOURCE: Courtesy of Ronald E. Turner, ANSER Corporation. CME shock, such as occurred in October 1989; see Figure 3.6). This experience demonstrates why it is important to understand the interplanetary medium in order to make radiation dose predictions. It is reasonable to assume that an Earth-based solar-monitoring network (including orbital elements such as geosynchronous particle detectors, the planned NOAA soft x-ray imager, and research-quality solar monitors as employed on the Solar and Heliospheric Observatory and planned for future NASA solar missions) will be available to support SEP event monitoring, alerts, and forecasts. A plasma monitor at or beyond the Sun-Earth L1 point, and space-based extreme ultraviolet (EUV) and/or x-ray imagers along with a wide-field coronagraph would be the most important enhancements to the currently projected Earth-based solar observing network. An L1 sentry, stationed 1.5 million km toward the Sun, would provide data on the state of the interplanetary plasmas and fields to be used to detect shock waves, particle flux enhancements, and arrival time of CMEs. The EUV and/or x-ray imager would monitor developing active regions. A wide-field coronagraph could detect emerging CMEs. The combined data could support long-term forecasts by providing information on the state of the Sun and particle propagation conditions.
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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop FIGURE 3.5 One potential construct of a space radiation warning architecture. SOURCE: Courtesy of Ronald E. Turner, ANSER Corporation. To provide real-time support, any Earth-based elements must have ensured communications access to the lunar base and all surface teams. This communications architecture could be built around the architecture that is used to support routine operational communications between Earth and the Moon. Astronauts engaged in surface exploration or activities outside established habitats will be at additional risk from solar energetic particle events. The many reasons for extended trips away from a main lunar base include mapping, scientific exploration, mining, and construction of facilities. The frequency of expeditions, distance traveled, duration of the stays, and exposure onsite will vary with the purpose of the trip. Each of these factors will affect the resources accessible to the team and the responses available in the event of an SEP event.
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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop BOX 3.1 LUNAR MISSION PHASES Astronauts will be exposed to radiation hazards during all phases of a lunar mission: Low-Earth-orbit assembly and checkout. Solar and galactic radiation protection will be provided by Earth’s magnetic field, but the astronauts will be exposed to the low-Earth-orbit component of the trapped radiation belt, as experienced by the International Space Station. Lunar transfer orbit. Astronauts will experience a brief period of high radiation exposure during trapped radiation belt transit. They will be exposed to free space on the trips to and from the Moon (up to 3 days each way). Lunar orbit. Astronauts will be exposed to a modified version of the free-space radiation environment. At sufficiently low altitude there will be significant shadowing by the Moon, but there will also be some additional exposure from radiation emitted from the lunar surface, created by the interaction of the galactic cosmic radiation (GCR) and lunar surface material. Lunar base habitation. The radiation environment of the lunar surface will be approximately half that of the free-space environment, with additional exposure from the surface radiation created by the GCR impact on the lunar material, particularly from neutrons. Lunar surface exploration. The likelihood of reduced shielding of surface transportation (relative to that of the main base) may expose the crew to solar energetic particle events. Risk mitigation options include aborting to base, embedding a storm shelter in the transport vehicle, carrying the means to construct temporary shelter, and pre-establishing safe shelter outposts. Mars Hardware Elements The phases of a Mars mission from a space weather perspective are described in Box 3.2. The potential elements of an architecture to provide solar radiation protection throughout a human mission to Mars would be drawn from the general elements shown in Figures 3.4 and 3.5. As with the return to the Moon, detailed trade studies will be needed to determine the optimal mix of components from this list. The appropriate solution will depend on the Mars mission scenario and the limits of current understanding of the physics of SEP events. Since the spacecraft will be the only source of shelter on a Mars mission for the hundreds of days that the crew will be in deep space, it is critical that it provide adequate shielding to protect the crew from the steady GCR exposure and the sudden impact of multiple large SEP events. NASA has known for some time that shielding on the order of 100 to 200 g/cm2 or more would be necessary to reduce dose equivalent in space to Earth background levels (Nealy et al., 1989; Simonsen et al., 1990). Beyond about 20 g/cm2, however, there are diminishing returns, and shielding thicknesses increase very quickly for very little reduction in dose equivalent until one is actually at very large shield thicknesses. Much of the bulk needed for this shielding (particularly the shielding for a storm shelter) can be provided by clever configuration of necessary structural components, tanks, fuel, water, equipment bays, and so forth.
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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop FIGURE 3.6 Flux and cumulative fluence of the October 19-20, 1989, solar energetic particle event, as measured by Geostationary Operational Environmental Satellite spacecraft. SOURCE: Turner, 2001. Copyright 2001, American Geophysical Union. Reproduced by permission of American Geophysical Union. To monitor astronaut exposure during the mission to Mars, real-time and passive tissue-equivalent dosimeters will likely be provided to each of the astronauts and distributed throughout the interior of the spacecraft crew compartments, the landing vehicle, surface base elements, and any surface transportation vehicles. This complement of dosimeters and particle detectors can be used as part of a real-time SEP event alert and warning capability. However, since this limited approach is similar to what is done today, significant advances in the understanding of SEP events will be necessary to improve on the currently available false-alarm rate, miss rate, and poor flux estimation capability.
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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop BOX 3.2 MARS MISSION PHASES Astronauts will be exposed to radiation hazards during all phases of a Mars mission: Low-Earth-orbit assembly and checkout. Partial radiation protection will be provided by Earth’s magnetic field. Earth to Mars and Mars to Earth transit. Galactic cosmic radiation (GCR) and solar energetic particle (SEP) events pose substantial risk to the health of the crew during transit between Earth and Mars. Mission timing relative to the solar cycle may have a significant impact on mission planning. GCR flux is approximately twice as large near solar minimum as it is near solar maximum. However, there is a reduced risk of a large SEP event near solar minimum. Deep space extravehicular activity (EVA). An astronaut on EVA in deep space would be exposed to potentially dangerous doses of radiation if a large SEP event occurred. If an EVA is necessary, the environment should be closely monitored, and provisions should be made for a quick return to a sheltered area (for example, to the air lock providing ingress/egress to the spacecraft). Mars orbit. A low-altitude orbit (about 500 km) will not provide significant protection analogous to the shielding in low Earth orbit, as Mars does not have an appreciable magnetic field. However, the total particle flux in orbit will be reduced by up to 40 percent through shielding by the planet’s mass. Since solar and galactic cosmic radiation are essentially isotropic, this reduction will be effective throughout the orbit, not just during a period when Mars may block the electromagnetic solar radiation. While Mars eclipses the Sun, it also blocks observations of precursors of SEP events, which may complicate the generation of alerts and warnings. Mars surface exploration. Despite Mars’s lack of a significant magnetic field, the thin atmosphere (2 percent of the thickness of Earth’s atmosphere), and the planet’s mass, which reduces particle fluxes by one-half, will provide additional shielding to protect astronauts on the martian surface from GCR and SEP event radiation. Since the incident space radiation is isotropic, it arrives at a point on the surface from all directions above the horizon with equal probablility. Hence, only a small fraction of the incident particles pass through the thinnest portion of the atmosphere, which is directly overhead. Most particles will arrive at the surface point having traversed much longer path lengths, especially particles arriving from directions near the horizon. Thus, the attenuation in the atmosphere is greatly enhanced over that which would occur if all of the incident particles arrived only from the zenith. However, there are possible extreme events or very hard events that may substantially increase the neutron flux on the surface of Mars. As with the return to the Moon, it is reasonable to assume that the Earth-based solar monitoring network will support a Mars mission with SEP event forecasts and alerts. However, there are two substantial differences: Earth will not always be in a position to directly monitor critical regions of the Sun, and separation distances of several AU may significantly limit the timeliness of communication between Earth and the astronauts. One method to overcome the limitation of Earth-viewing geometry is to provide solar and heliospheric monitoring instrumentation on the Mars spacecraft. The NASA Solar Terrestrial Relations Observatory
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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop (STEREO) mission will provide experience with a similar suite of instruments and provide confidence that they can be deployed within a reasonable mass, power, and volume limit. An astronaut could be trained or an expert system could be developed to interpret the information and produce forecasts and alerts. As the astronauts get farther from the Sun, the probability increases that a CME from an active region beyond the solar limb (as seen by the astronauts) might be the source of an SEP event. Instrumentation on additional platforms may be needed to monitor these “blind spots.” A frequently discussed concept proposes a constellation of two or three solar monitors distributed along Earth’s orbit but positioned to provide effective coverage of the Sun. Supplemental data could be obtained relatively inexpensively by placing energetic particle detectors and plasma monitors on interplanetary targets of opportunity during the years preceding the Mars mission. A distribution of in situ data points could provide valuable information on the state of the interplanetary magnetic field, solar wind conditions, and propagation conditions for shocks and SEP events. Other, more focused options are available to collect particle and plasma data to support a human mission to Mars. One approach would place a limited suite of instruments near the Sun-Mars L1 point (about a million kilometers toward the Sun from Mars), to play a role analogous to the proposed warning satellites near the Sun-Earth L1 point (about 1.5 million kilometers toward the Sun from Earth). Maintaining timely and effective communications between the various elements of a solar radiation protection network will be a serious challenge. The distances are vast, and the geometry is frequently not favorable. The Earth-based Deep Space Network or its successor will be responsible for maintaining near-continuous communication with the astronauts on the spacecraft. KNOWLEDGE-TO-OPERATIONS TRANSITION Knowledge transfer, or how to transition research understanding, models, and observational capabilities from the solar and space physics scientists to support the Vision for Space Exploration, repeatedly emerged as an issue of importance during the workshop. In some ways, this topic is at the core of the purpose of the workshop, which was to identify ways in which SSP can make significant contributions to how NASA will deal with radiation—and other space environment conditions—that affect humans and systems on missions to the Moon, Mars, and beyond. Prior to the VSE, and especially recently, knowledge-transfer issues have received much attention in the SSP community (see, for example, Chapter 5 in The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics [NRC, 2002]). Specifically, new understanding of solar and space physics processes on the Sun, in the interplanetary medium, and in Earth’s magnetosphere, thermosphere, and ionosphere, combined with new advanced computational capabilities, has led to the development of models of the space environment that can benefit those affected by space weather. NASA’s Living With a Star program is one example of a research and observation program that is structured to result in new space environment and space weather knowledge that will benefit society. There are several other SSP programs that are focused on developing understanding and models that can, with additional effort, transition from the research-and-development community to operations. The National Science Foundation (NSF) funds a multiyear Science and Technology Center that is developing coupled Sun-to-Earth space weather models. The Department of Defense has supported several Multidisciplinary University Research Initiatives in solar physics and ionospheric physics to develop models that are ripe for transition to operations. The NSF has led three highly leveraged community solar and space physics groups—SHINE, GEM, and CEDAR (Solar, Heliospheric, and Interplanetary Environment program; Geospace Environment Modeling; and Coupling, Energetics, and Dynamics of Atmospheric Regions)—that are all actively involved in bringing together scientists to develop the next generation of research under-
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Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop standing and models that can be used by scientists as well as made ready for transition to the operational community. An interagency-sponsored Community Coordinated Modeling Center has evolved as a facility for validating models that can be used by scientists as well as bringing them a step closer to transition readiness. Finally, NOAA, with cosponsors from other agencies, has held an annual and growing Space Weather Week that brings together research, operations, and users in a forum that promotes communication among these disparate groups and advances the objective of transitioning research to operations. In spite of these valuable endeavors, there is a considerable gap between what scientists have developed and what actually makes it into operations. Discussions at the workshop considered ideas for narrowing the gap so that knowledge from the solar and space physics scientists could be transferred to support the VSE. It was recognized that communication at workshops such as these, among an interdisciplinary group of participants, can identify where scientists can contribute to the high-priority needs of the users—whether it be those versed in radiation climatology talking to mission planners and hardware engineers, or space environment modelers talking to mission operations personnel. It was also clear that the practical benefit of scientific knowledge and models often requires much work beyond what is needed to make useful scientific tools. Activities such as model validation, robust code development, display design, and training all need to be supported beyond the level of scientific readiness. By communicating and working closely together with those involved with all phases of the human and robotic missions, solar and space physics scientists have much to contribute to the VSE. REFERENCES Nealy, J.E., J.W. Wilson, and L.W. Townsend. 1989. Preliminary Analyses of Space Radiation Protection for Lunar Base Surface Systems. SAE Technical Paper Series 891487. SAE International, Warrendale, Pa. NRC (National Research Council). 2000. Radiation and the International Space Station: Recommendations to Reduce Risk. National Academy Press, Washington, D.C. NRC. 2002. The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics. The National Academies Press, Washington, D.C. Simonsen, L.C., J.E. Nealy, L.W. Townsend, and J.W. Wilson. 1990. Radiation Exposure for Manned Mars Surface Missions. NASA Technical Paper 2979. March. NASA Center for AeroSpace Information, Hanover, Md. Turner, R. 2001. What we must know about solar particle events to reduce the risk to astronauts. Pp. 39-44 in Space Weather (P. Song, H.J. Singer, and G.L. Siscoe, eds.), AGU Monograph 125, AGU Code GM1259841. American Geophysical Union, Washington, D.C.
Representative terms from entire chapter: