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Radiation and the International Space Station: Recommendations to Reduce Risk 1 Scoping the Problem 1.1 RADIATION IN SPACE ''My God, space is radioactive!" Recoiling thus from the 1958 discovery of Earth's radiation belts, Ernie Ray, a Van Allen protégé, gave generations of space workers a motto and a challenge. For scientists the challenge is to map the radiation of space, to document its behavior, and to understand its causes. For engineers it is to design radiation-tolerant space systems and hardware. For managers it is to deploy scientific results and engineering capability to build space programs that survive despite radiation. The problem is serious. Over the past 20 years, radiation effects have caused between one and two satellites per year on average to suffer total or partial mission loss.1 Satellites at low latitudes in low Earth orbit (LEO) stay relatively safe by ducking the intense heart of the radiation belts higher up. But at higher altitudes and higher latitudes, where Earth's radiation belts reside and radiation from solar storms invades, radiation hazard cannot be ignored. James Michener, in his book Space, describes the fictional death of two astronauts on the Moon from radiation emitted by a solar storm. The scenario is accurate regarding the radiation hazard in high-altitude space, although it exaggerates doses. (Units of radiation dose are described in Section 1.3.) The scene might have been inspired by the great solar storm of August 4, 1972, which like William Tell's apple-splitting arrow, split the 8 months between the last two Apollo Moon landings evenly. It delivered a total dose of radiation over half a day that, had it missed the middle and hit the Apollo mission at either end, would have caused the crew in the lunar module to suffer acute radiation sickness and, given the uncertainty in the estimate, possibly even death.2 The International Space Station (ISS)3 will be exposed to penetrating particle radiation from three sources: terrestrial, solar, and galactic. The terrestrial source is Earth's radiation belts (a.k.a. the Van Allen belts), of which four are recognized: the inner and outer ion belts and the inner and outer electron belts. Because they reach energies that penetrate matter to significant depths, ions in the inner belt and electrons in the outer belt pose the greatest hazards to space hardware and astronauts. Figure 1.1 shows where in general the belts occur in space. Shaped in cross section like a crescent concave earthward, they peak in altitude over the equator and project down like horns to high latitudes. In this they conform to the geomagnetic field, whose lines of force Figure 1.1 depicts. The "equator" over which the belts peak is the magnetic equator, which tilts from the geographic equator by 11.4 degrees, deviating most over Peru and Sumatra. Moreover, the lines of magnetic force are not geocentric but instead are offset such that the belts come closest to Earth over the South Atlantic, giving rise to a feature known as the South Atlantic Anomaly (SAA). Consequently, whereas low-latitude satellites in low-altitude Earth orbit
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Radiation and the International Space Station: Recommendations to Reduce Risk Figure 1.1 The radiation environments of the International Space Station (adapted from Baker4). The figure shows the three regions of space around Earth where penetrating radiation occurs. The inner and outer radiation belts each have an electron and ion component. (LEO satellites) can fly under the inner belt in most places, they cannot avoid it within the SAA. (Note: except for those in equatorial orbits, which largely avoid the SAA.) Based on data from the Mir space station, cosmonauts and astronauts normally accumulate about half of their total radiation dose during the 2 to 5 percent of the time they spend in the SAA. High-inclination LEO satellites also encounter the polar caps, which are accessible to solar energetic particles. Figure 1.2 shows locations of radiation-induced upsets ("hits") suffered by computer memory in a polar-orbiting LEO satellite (UOSAT-2). The hits generate a pointillistic map of the SAA and the horns of the belts at LEO altitude. A final point to note regarding the belts, especially the outer belt, is that their intensity varies with time. The outer electron belt shows variations synchronized with distinctive and relatively common solar wind conditions known as high-speed solar wind streams. During such conditions, the intensity of energetic electrons can increase by many orders of magnitude. Space physicists call times of elevated intensities of energetic electrons highly relativistic electron events (HRE events). Penetrating particle radiation from the Sun takes the form of solar particle events (SPEs), which typically last several days to a week. Because penetrating SPEs are mainly composed of protons generated by solar storms, they share the statistical properties of these storms. They exhibit a quasi-11-year cycle loosely synchronized with the solar activity cycle. During the last solar cycle (cycle 22), 20 SPEs were officially designated as such by NOAA's Space Environment Center (SEC). (Section 2.1 includes the criterion SEC uses to declare an SPE to be in progress.) The SEC, one of NOAA's eight national centers for environmental prediction (NCEPs), is responsible for the space environment. SPEs constitute the high-altitude acute radiation hazard. The geomagnetic field shields low-latitude LEO satellites from SPEs. Shielding ceases, however, at altitudes above about 4 Earth radii (1 Earth radius, or Re = 6,370 km) or at LEO latitudes (geomagnetic) above about 60 degrees. Geomagnetic storms, which are terrestrial responses to solar storms, weaken this shielding and allow solar energetic particles to penetrate to lower altitudes and latitudes. Figure 1.1 portrays the situation during a storm, in which solar energetic particles fill the space above 50 degrees geomagnetic latitude. In rough numbers, one large SPE can impart to a high-
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Radiation and the International Space Station: Recommendations to Reduce Risk Figure 1.2 Locations of single-event memory upsets suffered by the UOSAT-2 outlining the regions of the South Atlantic Anomaly and the outer radiation belt (from CSSP/CSTR5). (Courtesy M.A. Shea.) inclination LEO satellite a dose comparable to the SAA dose accumulated by the satellite over about 100 days. If such a dose were absorbed by an astronaut in a space suit, it would be equivalent to about 1 year's accumulated SAA dose inside a space cabin. This generalization is quantified in Section 1.4. Galactic cosmic rays (GCRs) make up the third form of radiation in space. They present a low-level, continuous source of penetrating radiation. They are partially shielded by the geomagnetic field. Low-inclination LEO satellites receive about half the GCR dose received by high-inclination LEO satellites. The accumulated GCR dose to LEO satellites is, as a rule, less than or comparable to the accumulated SAA dose and is rather insensitive to the level of solar activity. Figure 1.3 shows how the penetrating power of energetic protons, the main constituent of SPEs, increases with increasing proton energy. It marks the thicknesses of the different parts of a space suit at mid-deck within a space shuttle and within the Mir space station (R-16 dosimeter and Lulin dosimeter). Protons of only 10 MeV energy can penetrate nearly three-quarters of the surface area of a space suit. It takes a 25-MeV proton to penetrate the most heavily shielded part, the visor. Above 30 MeV, protons can penetrate the mid-deck of the space shuttle. Arrows show the energies measured by two dosimeters inside Mir. The 10 MeV threshold for penetrating a space suit is also the energy that forecasters at SEC monitor to watch for the onset of an SPE. A similar energy curve for electrons shows that 0.5 MeV particles, an energy characteristic of HRE events, can penetrate space suits. HRE events typically have high flux levels, between half a million and several million electron volts, that penetrate space suits, although they rarely have dangerous fluxes of electrons at energies that can penetrate a shuttle or station hull. This report focuses on solar energetic particles with energies higher than 10 MeV and outer radiation belt electrons with energies higher than 0.5 MeV. These are the energies at which protons and electrons penetrate space suits. Because they are transient and hard to predict, these populations of penetrating particles pose the greatest challenge to ISS radiation risk managers. By comparison with highly variable SPEs and HRE events, the relatively stable SAA is well understood and predictable.6 This report does not therefore address the SAA in any detail.
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Radiation and the International Space Station: Recommendations to Reduce Risk Figure 1.3 Range-energy curve for protons showing energies capable of penetrating various items of space hardware. (Courtesy of Don Smart.) 1.2 SPACE WEATHER CONTEXT This section gives a short overview of the phenomena that make up space weather.7 The term "space weather" refers to conditions on the Sun and in the solar wind, magnetosphere, ionosphere, and thermosphere that can influence the performance and reliability of spaceborne and ground-based technological systems and that can affect human life or health.8 The Sun ultimately drives all space weather phenomena by means of the solar wind (the hot, magnetized plasma flowing nearly radially outward from the Sun at all times) and solar storms (flares, prominence eruptions, and coronal mass ejections (CMEs)). Long-term trends in space weather, which are related to the 11-year solar activity cycle, are far easier to predict than short-term variations such as individual solar storms. This is analogous to our ability to forecast Earth's atmospheric weather: we know that North America will be relatively cold next winter and warm next summer but find it difficult to predict next week's weather. Similarly, solar storms are more frequent during the years around the solar cycle maximum than during the solar minimum. Because the next solar maximum is upon us (activity is expected to peak in 2000-2001), periods of potentially dangerous space weather are likely to be more frequent in the next few years. This increased space-weather hazard coincides with the peak construction period for ISS. Until a decade ago, the primary driver of adverse space weather was thought to be solar flares. These explosive outbursts had been observed from Earth, first in optical wavelengths and then in radio and X-ray emissions, for over a century, as had been the associated ionospheric and geomagnetic disturbances. They were thus the only likely candidates until the discovery of CMEs by the first spaceborne coronagraphs in the 1970s.9-11 These huge eruptions appeared at first to be powered by flares. However the ensuing wealth of solar and interplanetary data collected by spaceborne instruments, of higher resolution and sensitivity than ever before, led to a fundamental realization: flares play a role secondary to CMEs in the initiation and propagation of geoeffective
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Radiation and the International Space Station: Recommendations to Reduce Risk disturbances.12, 13 The exact relationship between flares and CMEs is complex and the subject of much debate at present; observations indicate that some flares may be initiated by CMEs, but the reverse is rarely if ever true. As the eruption traverses the heliosphere, the magnetic topology of the CME (and the associated shock that is typically present for a "fast" CME) continue to evolve owing to variations in the surrounding plasma and magnetic field characteristics, compression and draping of the interplanetary field around the propagating disturbance, and interactions with preceding (slower) or following (faster) solar storms and solar-wind structures. Consequently, the configuration of the magnetic disruption that reaches Earth can be much different from its initial configuration near the Sun. Thereafter, the essential factor in determining the severity of a geomagnetic storm is the degree of magnetic misalignment at the point of impact between the solar eruption and Earth's self-generated magnetic field, called the magnetosphere (see below). Of prime relevance to the core issue of this report is the discovery of the role played by shocks that are caused by fast CMEs. The most energetic SPE particles can reach Earth within 10 to 100 minutes of the solar manifestations of the storm seen in visible or X-ray wavelengths. The time delay depends on whether the source of the particles is a flare or a shock and on the heliographic location of the source as seen from Earth, so particle arrival times vary widely. Moreover, their flux and energy spectra evolve in transit owing to continued acceleration and transport effects. Earth's magnetosphere is confined and shaped by the magnetized solar wind. As the solar wind passes Earth, it severely compresses the magnetospheric field on the dayside and draws it out into a long, comet like tail (the magnetotail) on the nightside. Many of the field lines that thread the magnetotail are "open": that is, they connect Earth's polar-cap ionosphere to the interplanetary medium and thus ultimately to the Sun or to interstellar space. In contrast, closed magnetospheric field lines have both "feet" on Earth, one in the southern hemisphere and one in the northern, and do not extend into the interplanetary medium. SPE particles can reach low Earth altitudes directly by spiraling along open field lines. Solar-particle access is not limited entirely to the open field lines of the polar cap, however, for very energetic solar protons can leak onto closed field lines near the polar-cap boundary between open and closed magnetic flux. The more energetic the proton, the farther it can penetrate onto closed field lines. During geomagnetic storms, the solar wind or CME compresses the magnetosphere more severely, more field lines open as a result of magnetic reconnection, and the polar cap grows. Solar particles gain access to larger regions above Earth, particularly at the highest magnetic latitudes, which will be traversed by the ISS. Thus, the dynamic response of the magnetosphere to solar disturbances will increase the solar particle fluxes encountered by ISS. Earth's radiation belts are composed of trapped energetic particles, which pose an additional radiation hazard for ISS. One component of the belts, the outer-belt MeV electrons, has long been known to be highly variable. Fluxes frequently vary by several orders of magnitude, with an interval of high flux observed typically once a month. Though MeV electrons rarely penetrate into the interior of a spacecraft, they can be hazardous to astronauts performing EVAs. Most of the other components of the belts, including highly penetrating energetic protons, are generally much more stable and predictable. However, we now know that that stability is not absolute. Radiation-belt experts were taken by surprise when on March 24, 1991, the Combined Release and Radiation Effects (CRRES) spacecraft observed a substantial energization and reorganization of the belt structure, including the energetic protons, when an exceptionally strong interplanetary shock hit Earth. A few qualitatively similar but weaker events have subsequently been found by retrospective analysis of old data. Thus, both major components of the radiation hazard faced by ISS—solar and radiation-belt particles—are strongly affected by the overall dynamics of the magnetosphere and particularly by the way in which the magnetosphere reacts to extreme events in the heliosphere. 1.3 METRICS OF RADIATION RISK How one measures radiation depends on the application. For biological applications, the quantity of interest is the radiation dose absorbed by living tissue, for which the standard units are the gray (1 Gy = 1 joule of radiation energy absorbed per kilogram of tissue) and the centigray (0.01 Gy), also called a rad. For a given dose in these units, the biological effects vary with the type of radiation. A dose of energetic particles normally causes more
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Radiation and the International Space Station: Recommendations to Reduce Risk Table 1.1 NCRP Recommended Dose Limits for All Organs and Ages (in sieverts) Limit Bone Marrow Eye Skin Thirty-day 0.25 1.0 1.5 Annual 0.5 2.0 3.0 Career See Table 1.2 4.0 6.0 Table 1.2 Updated 1999 NCRP Recommended Career Dose Limits Based on 3 Percent Lifetime Risk of Induced Cancer (in sieverts) Age at Exposure Female Male 25 0.5 0.8 35 0.9 1.4 45 1.3 2.0 55 1.7 3.0 damage than the same dose of energetic photons (X-rays or gamma rays). Particles with high atomic numbers and high energy (HZE particles) cause the greatest damage for a given dose. Units designed to measure the relative biological effectiveness (RBE) of radiation are the sievert (Sv) and the centisievert (1 cSv = 0.01 Sv), also called a rem. These are obtained from the gray and rad, respectively, by multiplying by an experimentally determined quality factor. The quality factor is defined to be unity for gamma rays. Thus, it measures the excess or deficit of radiation damage as a proportion of the gamma-ray damage for the same dose. Quality factors for the SAA vary from 1.6 to 1.9, depending on the shielding, and for GCRs, they vary from 2.9 to 3.5, higher values being associated with higher inclination orbits.14 These values apply inside a shuttle or space station. The biological effects of radiation may be classified as acute radiation sickness, late deterministic effects, and stochastic effects. Acute radiation sickness follows exposure to a radiation dose generally greater than 1 Sv in a time generally less than 1 day. Depending on dose, symptoms, which include nausea and vomiting, start within a few hours to a day.15 Symptomatic reactions to acute radiation doses vary greatly between individuals, so that dose effects must be expressed probabilistically. The probability of vomiting within 2 days is about 10 percent for an abdominal dose of 1 Sv. It reaches about 90 percent for a dose of 3 Sv.16 The probability of death is about 10 percent for a whole-body 3-Sv dose and reaches about 90 percent for a whole-body 4-Sv dose.17 Whole-body dose means a dose to the blood-forming organs (mainly bone marrow). These probabilities refer to situations in which no countermeasures are taken. Countermeasures are quite effective at ameliorating symptoms of radiation sickness. Late deterministic effects from radiation include cataract formation and temporary sterility. There are radiation thresholds for damage to tissues in various critical organs such as bone marrow, lenses of the eye, and skin. These thresholds form a basis for establishing guidelines for short-term (30-day and yearly) radiation dose limits. The risk for induced cancer appears to have no threshold below which it vanishes; it simply decreases as the accumulated dose decreases. Recommended career limits on accumulated radiation dose are therefore set by considerations of an acceptable increase in the risk of cancer. In a 1989 report, the National Council on Radiation Protection and Measurements (NCRP) published recommended limits on short-term and career limits.18 The career limit was based on a 3 percent risk of induced cancer. A recent update of this report suggests reducing the recommended career limit, still based on a 3 percent risk of induced cancer, by a factor of two. The new recommended career limits are in line with international limits set for workers in terrestrial radiation environments. Other limits in the NCRP report were judged still valid. It is believed that if these limits are observed, no acute or late deterministic effects will develop. Tables 1.1 and 1.2 give the current (updated in 1999) NCRP recommended dose limits. These are low-dose-rate values, which are appropriate to SPE situations. 1.4 RADIATION AND THE INTERNATIONAL SPACE STATION As originally conceived in the early 1980s, ISS was to have a low-inclination (28.5 degrees), low-altitude (350 km) orbit. Then, the SAA and GCRs would have been the only significant sources of radiation. SRAG
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Radiation and the International Space Station: Recommendations to Reduce Risk knows how to design mission schedules to minimize astronaut exposure to the SAA during EVAs, and there is little anyone can do to minimize GCR exposure. In 1993, however, the United States agreed with the Russian Federation to incorporate Russian launch capabilities into ISS construction and maintenance. That agreement brought with it the need to place ISS in a high-inclination orbit, essentially the same as that of the Mir space station, 51.6 degrees geographic. Consequently, ISS and the astronauts who construct and use it run the risk of being exposed to solar energetic particles and penetrating electrons in the horns of the outer belt. Exposure from these sources will be sporadic since SPEs follow solar storms and HREs follow magnetic storms and impacts by strong solar wind shocks. During the declining phase of a solar cycle—perhaps late in ISS construction—HRE events are also associated with times, lasting about a week, when solar wind streams are especially fast. Whereas satellite encounters with the SAA are as predictable as the tides, usually solar energetic particles, geomagnetic storms, and high-speed solar wind streams are not reliably predictable, nor is the intensity of the associated radiation event. The high-inclination orbit of ISS therefore introduces a new radiation risk factor. ISS construction plans call for approximately 33 U.S. shuttle flights and 10 Russian flights. The construction phase will extend from 1998 to 2004, which spans the maximum of solar cycle 23, when SPEs are expected to be most frequent (see Figure 1.4). NASA estimates that during those years astronaut and cosmonaut construction crews may have to perform more than 160 EVAs totaling more than 1,100 hours. During those same years, there will be more than 400 additional hours of EVAs by astronauts and cosmonauts to service and maintain the station. The total exceeds 1,500 hours, or 1,000 ISS orbits, of EVA time. Figure 1.4 Solar cycle 23 and EVA schedule.19
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Radiation and the International Space Station: Recommendations to Reduce Risk FIGURE 1.5 Twenty-four hours of ISS ground track overlaid with the magnetic shielding boundaries for quiet and active storm conditions and the SAA. The quasi-latitudinal pair of high-latitude lines in each hemisphere indicates the low-latitude borders of areas accessible to radiation from SPE zones. The higher-latitude line in each hemisphere represents quiet conditions and the lower-latitude line, disturbed conditions.20 Figure 1.5 illustrates the geometry relevant to discussing the risk factors faced by astronauts and cosmonauts during EVAs. It traces the sixteen 90-minute orbits ISS will make each day and shows the approximate border of the SAA. The borders of the polar caps to which solar energetic particles have access are the high-latitude curves, whose troughs and crests reflect the 11.4-degree tilt of the magnetic dipole axis relative to the geographic axis. This high-latitude pair of curves delineates the approximate areas to which solar energetic particles have access during magnetically quiet times. The lower-latitude pair of curves delineates the approximate areas to which solar energetic particles have access during severe geomagnetic storms. Severe SPEs and severe geomagnetic storms have a common cause (CMEs) and often overlap in time (see Section 2.3). The lower latitude pair of borders defines the areas for assessing the SPE risk to astronauts during EVAs in ISS orbit—the areas to which SPE particles have direct access are called the SPE zones. Cosmic ray physicists refer to the equatorial border of the SPE zones as the cosmic ray cutoff. Figure 1.5 reveals three groups of orbits that intersect radiation zones. One group crosses the SAA on the descending leg of the orbits, a second group crosses it on the ascending leg of the orbits, and a third group crosses the SPE zones. The figure also shows that as the ISS moves along its 16 orbits per day, its per-orbit risk of exposure to solar energetic particles varies from zero to a maximum that depends on the size of the SPE zones. Figure 1.6 quantifies the per-orbit SPE exposure risk over 24 hours as Earth rotates the "off-center" SPE zones toward and away from the ISS orbit. The rotation causes the SPE zones to overlap ISS orbit for a portion of each day by an amount that depends on the size of the zones at the time. The orbit's position is specified by the
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Radiation and the International Space Station: Recommendations to Reduce Risk Figure 1.6 SPE and SAA zones as a function of the longitude of the ascending node of the ISS and Mir orbits. Shading in the top panel shows the locations of the SPE and SAA zones. Circles show the fraction of the orbit inside SPE zones for four assumed radii of the zone (30 to 45 degrees in 5-degree steps) based on simplified geometry. The bottom panel shows radiation data from the Mir space station during a non-SPE orbit (line 3) and two orbits during the SPE of September 29, 1989.21
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Radiation and the International Space Station: Recommendations to Reduce Risk longitude of its ascending node, that is, the point at which it crosses the equatorial plane from south to north. The SPE exposure risk is given as the fraction of the orbit that overlaps the SPE zones. The fraction of overlap is calculated using a simple model of SPE-zone geometry in which a circle centered on the axis of Earth's magnetic dipole axis represents the zones. The axis is taken to pass through Earth's center and to penetrate Earth's surface in the northern hemisphere at 78.5 degrees latitude and 69 degrees west longitude. The model ignores the offset of the dipole axis from Earth's center (which gives rise to the SAA but does not much affect the position of the area accessible to SPE particles), and it ignores the antisolar shift of the zones, which varies with geomagnetic activity and can be several degrees.22 The antisolar shift of the zones causes the fraction of overlap to exceed the calculated value for a given orbit in one hemisphere and to lag it in the other, so that the effect tends to cancel out on a per-orbit basis. Figure 1.6 presents results for 30-, 35-, 40-, and 45-degree SPE zones, which cover conditions from typical to extreme. As an example, consider the worst case. Of the 16 ISS orbits per day, the orbit whose longitude of ascending node is nearest 210 degrees overlaps the SPE zones the most (refer to Figure 1.5). If while ISS is traversing one of these worst-case orbits the SPE zones happen to be 30 degrees in radius, corresponding to quiet conditions, ISS will be in the zones about 15 percent of the time. If, on the other hand, conditions happen to be highly disturbed, so that the zones are 45 degrees in radius, the percentage increases to 42. To estimate the worst-case fraction for an entire EVA, however, one must average the fraction per orbit over four orbits—corresponding to the wide shaded area in Figure 1.6—since EVAs nominally last 6 hours but usually extend to 7 hours (plan 6, do 7). Then, in the worst case, the average fraction per orbit spent in the SPE zones varies from about 8 percent for the 30-degree zone to about 40 percent for the 45-degree zone. (The estimate ignores the quantization introduced by discrete orbits of 90 minutes duration.) The two narrow shaded regions in Figure 1.6 show where the ISS orbit intersects the SAA twice per day on its ascending and descending swings. The bottom panel in Figure 1.6 gives an example from the Mir space station of per-orbit dose data that illustrate the information given in the top panel. The recording instrument, which was inside the station behind an average of 10 to 15 g/cm2 of shielding (compared with about 0.5 g/cm2 for a space suit), was sensitive to protons with energies above 100 MeV. Line 3 shows a non-SPE situation in which the two SAA peaks stand out as order-of-magnitude increases in the dose rate. Lines 1 and 2 show per-orbit doses during, respectively, the peak of an SPE on September 29, 1989, and three days later. The shape of line 1 suggests that the effective radius of the SPE zones at this time was between 30 and 35 degrees, since the dose per orbit drops substantially around 150 and 270 degrees longitude of ascending node. To predict the severity of the SPE radiation risk, one also needs to know the dose rates that would represent a worst-case scenario. The great solar particle event of August 4, 1972, is a commonly used benchmark for worst-case estimates. During that event, the solar proton detectors on the spacecraft monitoring SPE fluxes became saturated, so the value of the peak flux is uncertain. This uncertainty has resulted in estimates of peak dose rate that vary by a factor of two. Two studies in this area are those of Letaw et al.23 and Wilson et al.24 Letaw et al. put the peak dose rate to the blood-forming organs (BFO) of an astronaut in a space suit at about 0.3 Sv/h, whereas Wilson et al. put it at approximately 0.15 Sv/h. Letaw et al. also give a worst-case composite dose rate of about 0.5 Sv/h. These rates are assumed to apply to the SPE zones. The peak dose rate lasted about 8 hours, which gives a total dose of between 1.2 Sv and 2.4 Sv, depending on which dose-rate estimate is used. For comparison, a major SPE on October 19, 1989, is estimated to have produced a total BFO dose behind EVA-level shielding of 1.29 Sv.25 This is similar to the Wilson et al. estimate for the 1972 event, although the time histories of the two events are not similar. The difference between the two dose-rate estimates can be traced to different dose-rate protocols and to the sensitivity that these protocols have to aspects of the particle spectrum and composition that were not adequately measured during the August 1972 event. The Wilson et al. number,26 being the latest and so built on prior estimates, gives the current best estimate. But it should be noted that the procedure for calculating the dose rate is still somewhat fluid. That is, there is more than one protocol for calculating dose rates, and the results using different protocols do not always agree. A separate point of disagreement concerns the value of constructing a composite worst-case estimate. The Letaw et al.27 composite worst case is similar to an envelope over the spectra from all worst cases. It combines the worst case for low-energy fluxes (the 1972 event) with the worst case for high-energy fluxes (a 1956 event). Some radiation scientists consider this artificial "SPE from Hell" unduly
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Radiation and the International Space Station: Recommendations to Reduce Risk extreme, since no single event is likely to exhibit the worst features of all prior events. On the other hand, it is also unlikely that the maximum dose rate of all prior events will never be exceeded. CSSP/CSTR therefore used all three estimates in constructing Table 1.3. The lowest values came from Wilson et al. and should be regarded as the current best worst-case estimates. The intermediate values came from Letaw et al.28 and should be taken as representing the uncertainty in constructing a worst case from the August 1972 event. The highest values came from the Letaw et al.29 composite and should be considered not a worst case but a conservative upper limit. For the sake of making a worst-case estimate for each dose rate, CSSP/CSTR assumed that the EVA occurred during the peak of the 1972 event and that it was centered on the maximum excursion into the SPE zones (i.e., the wide gray area in Figure 1.6). Table 1.3 gives the total dose based on different SPE zone radii and different dose rate assumptions. These numbers can be compared with an upper limit of 0.15 Sv estimated for the dose to the Mir cosmonauts inside the station during the October 1989 SPE.30 This upper limit estimate should be multiplied by 3.5 to be applicable to an EVA situation, as determined by direct comparison of the dose rates inside and outside the station (Gautam Badhwar, SRAG, JSC, private communication, 1999). The lower range of worst-case estimates (first row of Table 1.3) encroaches on an astronaut's short-term radiation limits (Table 1.1). The upper range (second row) encroaches on an astronaut's career radiation limit (Table 1.2). The high end of conservative upper-limit values (third row) approaches and somewhat overlaps the range in which symptoms of acute radiation sickness begin to occur. A similar table for estimates of worst-case doses to the skin and ocular lens would show that the short-term limits are reached at doses lower by a factor of about two.31 Of course, by definition, worst-case scenarios are unlikely to happen. It is unlikely, for example, that the peak of the SPE dose rate would coincide with a 6-hour EVA. But if the construction schedule calls for consecutive EVAs, one EVA shift might receive a dose of about 50 percent or more of the total possible. As another example, it is unlikely that an SPE will occur during any given construction flight. But because ISS requires a large number of construction flights and because the construction will occur during the peak of solar cycle 23, the probability is high that some construction flights will experience SPE radiation. Section 2.2 of this report presents a calculation showing that for specified assumptions about the severity of solar cycle 23, the probability that at least two flights will experience SPE conditions is close to 100 percent. The probability drops to about 50 percent for five to seven flights experiencing SPE conditions. Section 2.3 shows that SPEs with high dose rates tend to occur when the SPE zones are big, 35 degrees or more. Chapter 2 therefore reaches an important conclusion: there is a nonnegligible likelihood that while performing an EVA during the construction phase of ISS, astronauts could receive a radiation dose that is significant in terms of increased cancer risk and of reaching allowable dose limits. The likelihood of this happening could diminish, however, if the ability of NASA to specify and forecast SPE severity at the ISS were to improve. The analysis presented here deals with the radiation dose received during a single SPE. One must add this dose to the accumulated dose that an astronaut receives from all sources during a mission and from prior missions. Table 1.3 Worst-Case Estimates of BFO Dose for a 6-Hour EVA with Maximum Exposure Within SPE Zones Having Radii and Dose Rates As Shown Dose Rate (Sv/hr) Dose (Sv) 30º 35º 40º 45º 0.15 0.06 0.20 0.29 0.36 0.30 0.13 0.40 0.58 0.72 0.50 0.21 0.66 0.96 1.20 NOTE: Space suit shielding is assumed to be equivalent to about 0.5 g/cm2 Al.
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Radiation and the International Space Station: Recommendations to Reduce Risk Insofar as it would be more likely to cross a radiation limit threshold, the incremental dose that an SPE delivers toward the end of a mission could be more serious than a dose delivered in the early part of a mission. Figure 1.6 illustrates a fairly obvious but operationally very important point: if a flight director knows that a serious SPE is in progress or will be in progress and has the flexibility to change preset EVA schedules, then scheduling EVAs that avoid deep penetration of the SPE zones, even if they straddle the SAA, would greatly reduce the particle radiation dose to astronauts. This point demonstrates the value of employing data and implementing models that reliably specify and predict SPE occurrence, intensity, and duration. Such data are discussed in Chapters 3 and 4, and such models are discussed in Appendix A. 1.5 ISSUES IN MANAGING RADIATION RISK DURING ISS CONSTRUCTION The flight director at JSC has overall responsibility for the safe execution of a mission. Radiation risk is only one issue the flight director must consider in deciding whether to delay a launch or an EVA or whether to end a flight or an EVA early. In some construction activities, the negative impact on construction logistics of delaying an EVA or ending it early because of an SPE could exceed the negative impact of the additional dose the astronauts would receive by keeping to the original schedule. NASA has accepted the radiation dose limits recommended in the 1989 NCRP report.32 It is likely to accept the new NCRP recommendation that calls for reducing the career limit by one half (Table 1.2). In general, these limits are applied retrospectively, that is, after a flight. During the flight, the primary operative flight rule is to keep the dose as low as reasonably achievable (ALARA). This rule gives the flight director much discretion. For example, it does not preclude an astronaut's reaching his or her 30-day or annual limit during one flight, although this has never happened. If the SPE dose rate should become high enough and the SPE zones become wide enough to threaten doses in the career-limit range or in the acute radiation sickness range during an EVA, then obviously radiation would become a bigger component in the risk equation that the flight director must consider. To factor this component into the risk equation, however, the flight director must have sufficiently reliable information about the radiation environment. Part of this report has to do with assessing which information is now or can become sufficiently reliable. An unofficial flight rule, but one flight directors nonetheless observe, is the real-time, on-site flight rule, which says that any decision made in response to a radiation situation must be based on radiation measurements made in real time and on site, that is, in the external ISS environment. This requirement cannot be fulfilled if current plans are followed, because from now until June of 2000 there will be no capability for obtaining such measurements, and only retrospective dose data will be available. As already stated, this report discloses a high probability that a flight director will face an SPE-radiation situation during ISS construction and that the dose increments during an EVA while an SPE is in progress can account for a significant fraction of an astronaut's short-term limits or even exceed those limits. Whatever a flight director decides in response to a radiation situation, the primary input to that decision pertaining to astronaut health and safety comes in the form of a recommendation from the flight surgeon.33 The flight surgeon, who knows the astronauts' radiation histories, can tell how much an incremental dose of radiation advances each astronaut toward his or her radiation limits. To make a real-time recommendation to the flight director to alter an EVA schedule or assignment based on a radiation situation, the flight surgeon needs a complete and accurate description of the real-time, on-site radiation situation. The unit at JSC responsible for informing the flight surgeon about radiation situations is SRAG. This unit has multiple responsibilities: provide preflight crew exposure projections; provide real-time astronaut radiation protection support; provide radiation monitoring to meet medical and legal requirements; maintain comprehensive crew exposure modeling capability; provide preflight planning and analysis support; and provide in-flight support. In-flight support entails specific responsibilities, among them the following: provide updated EVA exposure analysis to the flight surgeon; provide EVA start and stop times to the flight surgeon; provide an EVA go/no go recommendation prior to egress; monitor real-time space weather; recommend whether to continue or terminate an EVA during a radiation event; track exposure from the nominal radiation environment; monitor extravehicular charged-particle directional spectrometer data when they become available starting in June of 2000; and provide a
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Radiation and the International Space Station: Recommendations to Reduce Risk post-EVA final estimate of additional crew exposure to the flight surgeon and the flight director. SRAG consists of a small number of health physicists, physicists, and programmers. It has one civil servant (or possibly none) and four or five contractors. Considering the number of scheduled EVAs during ISS construction, it may be overtasked. SRAG has available a number of resources with which to execute these functions: statistical radiation-belt models, models for specifying and predicting SPE conditions, real-time data from SEC on the space weather and the radiation environment, and codes to evaluate the radiation situation at ISS from the statistical models and the real-time data. Equipment for monitoring the ISS radiation environment comprises crew passive dosimeters (CPDs) and a tissue-equivalent proportional counter (TEPC). CPDs provide postflight data. The TEPC gives dose-rate and cumulative-dose data every several seconds to the crew, but the data are not telemetered to SRAG. TEPC data refer to the radiation environment inside the shuttle or station, not to the EVA environment. An external radiation monitor is scheduled to be mounted on the station during the eighth shuttle flight, in June of 2000, after 46 ISS EVAs will already have been performed. Although SRAG uses state-of-the-art models, the (unofficial) flight rule excluding nonlocal, non-real-time data suggests that such data specify the radiation environment at a confidence level too low to allow convincing a flight director to change a flight schedule or an EVA schedule in the absence of on-site data. This implies that SRAG would be more effective if its models could achieve a confidence level high enough that a flight director would consider the model's specifications as adequate substitutes for on-site data. Astronauts are the stakeholders most directly affected by how radiation risk is managed. A heavy radiation dose while performing an EVA during an SPE puts an astronaut in double jeopardy: no astronaut wants to reach the short-term radiation limits, much less the career limit. Reaching a radiation limit is the first jeopardy. The second is the increased risk of stochastic radiation effects, that is, induced cancer. This jeopardy does not go away after the flight or even after the career. There is also another party involved in radiation risk management that is not directly involved in ISS construction. This is the community that studies space weather and provides space weather services. The space weather community is made up of physicists at universities, government laboratories, and industry who study the environment of space and researchers and forecasters at space weather operation centers. This community currently possesses resources and capabilities that could be marshaled to develop tools to specify and forecast the radiation environment at ISS orbit more completely and accurately than is now done. 1.6 THE APOLLO EXPERIENCE During the Apollo program a network of solar observatories was used to give a real-time warning of SPEs and to estimate their possible impact on the lunar missions. The Solar Particle Alert Network (SPAN) was implemented by the NASA Manned Spacecraft Center (MSC)—now the Johnson Space Center—in Houston.34 It consisted of seven observatories located around the world to ensure 24-hour observations of solar activity. Hydro-gen-alpha (0.5 Å bandwidth) telescopes were installed at all seven observatories to observe optical solar flares that produce solar particle events. The observatories were located at Houston; Boulder, Colorado; Honolulu, Hawaii; Carnarvon, Australia; Culgoora, Australia; Teheran, Iran; and the Grand Canary Islands, Spain. Each location had real-time communications with Mission Control Center (MCC) in Houston. In addition, there were radio-frequency telescopes operating at 2695 MHz at three locations: Houston, Carnarvon, and the Canary Islands. A blue-ribbon committee of space scientists, chaired by Wilmot Hess, oversaw the implementation of the network. It was recognized early in the Apollo program that high-energy particles from solar flares could pose a radiation hazard to the astronauts. They were especially vulnerable while they were in the thinly shielded lunar excursion module (LEM) or on the lunar surface. However, the command and service module provided enough protection to reduce exposures from solar particle events to acceptable levels. The Apollo missions were scheduled to take place during solar maximum years, when large solar particle events are more apt to occur. Research had established that virtually all particle events during solar cycle 19 were preceded by type IV solar radio bursts. However, not all type IV bursts were followed by particle events. (The same is true for solar flares observed in the hydrogen-alpha line, but there are many more flares than type IV radio bursts.) Studies carried out at MSC established a correlation between large type IV solar radio bursts and SPE size (time-integrated proton flux
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Radiation and the International Space Station: Recommendations to Reduce Risk > 30 MeV). The radio flux was integrated over time to obtain a measure of the energy of the burst. The hypothesis was that the radio burst was produced by synchrotron radiation from electrons that are accelerated at the same time as the protons. Data from radio observatories at Ottawa, Canada (which operated at 2800 MHz) and Nagoya University, in Japan (which operated at 3000 MHz) were used for the study. Particle event data were taken from the Solar Proton Manual and a Boeing Company report.35 While some solar flares produce relativistic-energy protons that can arrive in the Earth-Moon region within 30 minutes, the arrival times for most events are 4 to 6 hours after the flare and radio burst. Peak particle intensities do not occur until another 4 to 6 hours after the arrival of particles. The strategy was to use this time to move the Apollo astronauts off the lunar surface and have them return to the more heavily shielded command and service module. Information on the occurrence of a solar flare (observed by the hydrogen-alpha telescopes) and data from a large radio-frequency (10 cm wavelength) burst were transmitted back to MCC in Houston. Radiation specialists working on the radiation console, located in one of the MCC ''back rooms," analyzed these data. If an event of a certain (estimated) size was believed to produce a substantial radiation dose to the astronauts, the flight director would be advised so that action could be taken to minimize their exposure. Particle spectrometers and dosimeters onboard the Apollo spacecraft then detected the increase in the radiation environment, to verify that the particle event had arrived in cislunar space. This reduced the impact of a false alarm when a flare and type IV burst did not produce particles that propagated to cislunar space. Flight rules precluded launching into an SPE or landing on the Moon during such an event and terminated a lunar excursion if exposures were estimated to be above an acceptable level. Fortunately, no large SPEs occurred during any Apollo mission. The large solar particle event of August 1972 occurred between the Apollo 16 and Apollo 17 missions and did not, therefore, affect them. A solar flare and radio burst occurred during the Apollo 12 mission, which had exercised the operational procedures. NASA operated SPAN until late 1970, after Congress gave NOAA responsibility for space weather. NOAA tasked the Space Environment Forecast Center in Boulder with carrying out this responsibility, and NASA transferred SPAN to NOAA, which operated it throughout the remaining Apollo flights and during the Skylab missions. Although NOAA continues to participate in the space radiation protection program for shuttle missions, it has replaced SPAN with other observational methods, including spaceborne X-ray and particle instruments. It is of interest that the Soviet space program used similar SPE warning criteria throughout the 1970s and 1980s (Vladislav M. Petrov, Institute for Biomedical Problems, Moscow, personal communication). Russians performing EVAs at ISS will be directed out of the mission control center in Moscow. Further, it is likely that U.S. and international crew members on ISS will also participate in EVAs directed out of MCC-Moscow, whose flight rules pertaining to radiation may differ from those of MCC-Houston. While this report is focused on U.S. policy regarding radiation risk and ISS, CSSP/CSTR believes some of the recommendations in this report might also be implementable by MCC-Moscow. 1.7 SUMMARY AND RECOMMENDATION ISS construction and concurrent station maintenance will entail more than 1,500 hours of EVAs over a 4-year period, 1998 to 2002, that straddles the peak in the current solar cycle. The station's high-inclination orbit (51.6 degrees) cuts through radiation environments more severe than those of the originally planned low-inclination orbit (28 degrees). The high-latitude radiation environments (energetic particles from solar storms and relativistic electrons in Earth's outer radiation belt) are highly variable. At the height of their variability, they are intense enough to pose a hazard to astronauts engaged in EVAs, although even doses estimated for worst-case scenarios fall short of life-threatening. (This is in contrast to the situation astronauts could face in flights beyond the protective shield of Earth's magnetic field—for example, on a flight to Mars.36) Although the fraction of time during which the high-latitude radiation environments reach threatening levels is small, the amount of time committed to EVAs is large, so the probability that an SPE will occur when an EVA is scheduled is not small. Estimates based on calculations referred to in this report put at near certainty the likelihood that at least two ISS construction flights will be in progress when energetic particles from a solar storm
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Radiation and the International Space Station: Recommendations to Reduce Risk invade a volume of space above the polar atmosphere that at times of solar storms often significantly overlaps the ISS orbit. Flight directors will almost certainly be faced with flights impacted by high-latitude radiation. This finding directs attention to the issue of radiation risk management. The responsibilities at JSC in this area are well defined. The CSSP/CSTR review of current JSC flight rules shows, however, that radiation risk management would benefit by exploiting resources and capabilities that currently exist elsewhere. This report discusses the relevant resources and capabilities, addresses issues related to their application to radiation risk management, and makes recommendations aimed at reducing radiation risk. One main recommendation emerges from Chapter 1. Recommendation 1: Because it denies access to valid information and thus unnecessarily restrains flight-director options, flight directors should not adhere rigidly to the (unofficial) real-time, on-site data rule. Simply put, reducing the dose to astronauts from solar energetic particles during EVAs entails avoiding EVAs during orbits that penetrate the SPE zones when SPE particles are present. To implement an operational procedure for SPE-zone avoidance, the flight director must act on information on the size and shape of the zones and on the occurrence, intensity, and duration of the SPE. Information on SPE zone geometry and SPE start, strength, and length must be reliable enough to gain the flight director's confidence and timely enough to allow the flight director to act on it. The status of the resources (existing or in development) that are needed to acquire such information is reviewed in Chapters 3 and 4 and Appendix A. One might draw an analogy to the influence of terrestrial weather in the execution of space missions. A flight director will routinely delay a launch or a landing because of a thunderstorm forecast based on nonlocal data. Forecasts of space weather based on nonlocal data could, likewise, help the flight director reduce the probability of radiation exposure to astronauts during EVAs. The principal difference in these two cases is that a terrestrial weather forecast has consequences for hardware, whereas a space-weather (radiation) forecast affects the rotation of astronauts and their health and future flight opportunities. For Recommendation 1 to be successful, flight directors should acquire ownership of any flight rule that replaces the one whose removal is recommended. They should therefore work with SRAG to approve sources of nonlocal data and models that use these data to specify and forecast radiation levels at ISS. Radiation conditions at ISS should be inferred from nonlocal data until resources become available to augment the nonlocal data and models with real-time, on-site data and modeling. One instance indicates that flight directors might be able to participate more in reducing radiation risk. SRAG proposed putting a radiation monitor in the shuttle bay to provide real-time, on-site radiation data on flights before Flight 8A, which will install a radiation monitor on the station. However, the proposal failed to receive flight director approval in time to implement it before Flight 8A. This delay guaranteed that, under current operating procedures, no change to flight or EVA schedules in response to a radiation situation is possible prior to Flight 8A. Timely action by the flight directors could have made it possible to respond to a radiation situation under current flight rules. 1.8 NOTES AND REFERENCES 1. The 1999 report on space weather of the National Security Space Architect finds that during the preceding 16 years at least 13 satellites suffered total mission failure attributable to space weather. There were more failures in which space weather was implicated, but the evidence was not definitive. 2. J.R. Letaw, R. Silberberg, and C.H. Tsao, "Galactic cosmic radiation doses to astronauts outside the magnetosphere," in Terrestrial Space Radiation and its Biological Effects, P.D. McCormack, C.E. Swenberg, and H. Bucker, eds., Plenum Press, New York, 1988; J.W. Wilson, F.A. Cucinotta, J.L. Shinn, L.C. Simonsen, R.R. Dubey, W.R. Jordan, T.D. Jones, C.K. Chang, and M.Y. Kim, "Shielding from solar particle event exposures in deep space," in Proceedings of Workshop on Impact of Solar Energetic Particle Events for Design of Human Missions, September 9-11, 1997, Center for Advanced Space Studies, Houston.
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Radiation and the International Space Station: Recommendations to Reduce Risk 3. Construction of the International Space Station, a project of the United States (lead), Canada, Japan, the European Space Agency, and the Russian Federation, began in late 1998. The ISS is in orbit at an altitude of 250 statute miles with an inclination of 51.6 degrees. The first crew to live aboard the (partially assembled) space station is scheduled to arrive in March 2000. Assembly of the ISS is scheduled to continue until late 2004. 4. D.N. Baker, "Solar wind-magnetosphere drivers of space weather," J. Atmos. Terr. Phys., 58, pp. 1509-1526. 5. National Research Council, Committee on Solar and Space Physics and Committee on Solar-Terrestrial Research, Space Weather: A Research Perspective, available on the Internet at <www.nas.edu/ssb/cover.html> 6. See, for example, J.F. Lemaire, D. Heynderickx, and D.N. Baker, eds., Radiation Belt Models: Models and Standards, Geophysical Monograph 97, Washington, D.C.: American Geophysical Union, 1996. The SAA has been monitored in detail sufficient to follow its westward drift of roughly 1 degree every 3 years owing to the secular variation of the geomagnetic field. G.D. Badhwar, "Drift rate of the South Atlantic Anomaly," J. Geophys. Res., 102, 1997, pp.2343-2349. 7. The National Research Council has produced a document on this topic (see footnote 5), which may be consulted for a fuller treatment and more detail. 8. Definition taken from the Strategic Plan of the National Space Weather Program, 1995, obtainable from the Upper Atmospheric Section of the Division of Atmospheric Sciences, National Science Foundation. 9. R.A. Howard, M.J. Koomen, D.J. Michels, R. Tousey, C.R. Detwiler, D.E. Roberts, R.T. Seal, and J.D. Whitney (U.S. Naval Research Laboratory, Washington, D.C.) and R.T. Hansen, S.F. Hansen, C.J. Garcia, and E. Yasukawa (High Altitude Observatory, NCAR, Boulder, Colo.), "Synoptic observations of the solar corona during Carrington rotations," 11 October 1971-15 January 1973, pp. 1580-1596 (Reissue of UAG-48 with quality images, February 1976, 200 pp. Supersedes UAG-48). 10. R.M. MacQueen, J.A. Eddy, J.T. Gosling, E. Hildner, R.H. Munro, G.A. Newkirk, A.I. Poland, and C.L. Ross, "The outer corona as observed from Skylab," Astrophys. J., 187, 1974, p. L85. 11. For more information about our current understanding of CMEs and their relation to other manifestations of solar and geomagnetic activity, see N.U. Crooker, J.A. Joselyn, and J. Feynman, eds., Coronal Mass Ejections, Geophys. Monogr. Ser.,99, American Geophysical Union, Washington, D.C., 1997. 12. S.Kahler, "Solar flares and coronal mass ejections," Ann. Rev. Astron. Astrophys., 30,1992, pp. 113-141. 13. J.T. Gosling, "The solar flare myth," J. Geophys. Res., 98, 1993,p. 18937. 14. Gautam Badhwar, Presentation to CSSP/CSTR. This material, which was presented to CSSP at its meeting on January 26, 1998, is available for viewing in the National Research Council's Public Access Records Office. 15. National Council on Radiation Protection and Measurement, Guidance on Radiation Received in Space Activities, Report No. 98, 1989, p. 70; R.W. Young, "Acute radiation syndrome," in Military Radiobiology , J.J. Conklin and R.I. Walker, eds., Academic Press, New York, 1987. 16. National Council on Radiation Protection and Measurement, Guidance on Radiation Received in Space Activities, Report No. 98, 1989, p. 71. 17. National Council on Radiation Protection and Measurement, Guidance on Radiation Received in Space Activities, Report No. 98, 1989, p. 73. 18. National Council on Radiation Protection and Measurement, Guidance on Radiation Received in Space Activities, Report No. 98, 1989. 19. Figure from R. Turner and C. Kemere, "Solar particle events and International Space Station," Report submitted to the Committee on Solar and Space Physics, August 12, 1998. 20. The ground track was computed by the Analytical Graphics Incorporated (AGI) Satellite Tool Kit from the recent ISS ephemeris. The magnetic shielding boundaries are 30 MeV geomagnetic vertical cutoff calculations by Don Smart for Kp = 0 (quiet) and Kp = 9+ (active), from D.F. Smart, M.A. Shea, E.O. Flueckiger, A.J. Tylka, and P.R. Boberg, Changes in Calculated Vertical Cutoff Rigidities at the Altitude of the International Space Station As a Function of Geomagnetic Activity, 26th International Cosmic Ray Conference, Contributed Papers, Vol. 7, 1999, pp. 337-340. The South Atlantic Anomaly boundary is for 100 MeV protons with flux greater than 100 particles/cm2-sec, as calculated by C. Dyer, A. Sims, and C. Underwood using the NASA AP-8 model with a 1991 magnetic field model, in "Radiation belt observations from CREAM and CREDO," Geophysical Monograph 97, Radiation Belts: Models and Standards, J.F. Lemaire, D. Heynderickx, and D.N. Baker, eds., American Geophysical Union, Washington, D.C., 1996. The compilation was produced by Ron Turner and Stephen Thomas, ANSER, Arlington, Va. 21. V.A. Shurshakov et al., "Solar particle events observed on Mir station," in Proceedings of Workshop on Impact of Solar Energetic Particle Events for Design of Human Missions, September 9-11, 1997 , Center for Advanced Space Studies, Houston, 1998, pp. 1-18. 22. G.L. Siscoe, "What determines the size of the auroral oval?" in Auroral Physics, C.-I. Meng, M. Rycroft, and L.A. Frank, eds., Cambridge University Press, 1991, pp. 159-175. 23. See Note 2. 24. See Note 2. 25. J.T. Lett, W. Atwell, and M.J. Golightly, "Radiation hazards to humans in deep space: A summary with special reference to large solar particle events," in Solar-Terrestrial Predictions, Proceedings of a Workshop at Leura, Australia, October 16-20, 1989, Vol. 1, NOAA/ERL, 1990, pp. 140-153. 26. See Note 2. 27. See Note 2. 28. See Note 2.
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Radiation and the International Space Station: Recommendations to Reduce Risk 29. See Note 2. 30. G. Badhwar, Presentation to CSSP. This material, which was presented to CSSP at its meeting on January 26, 1998, is available for viewing in the National Research Council's Public Access Records Office. 31. J. Wilson, L. Townsend, W. Schimmerling, G. Khandelwal, F. Khan, J. Nealy, F. Cucinota, L. Simonsen, J. Shinn, and J. Norbury, Transport Methods and Interactions for Space Radiation, NASA Reference Publication 1257, 1991. 32. See pp. 7 and 8 in the reference at note 16. 33. A proposed flight rule, submitted to the ISS program office for approval, also gives a prominent role in real-time decision making during ISS missions to the radiation health officer (RHO). Under this rule, the flight surgeon would consult with the RHO regarding potential radiation effects (based on radiation dose projections from SRAG) before making recommendations to the flight director regarding actions to reduce crew radiation exposure. Such a consultation might occur, for example, as astronauts prepare to leave the relative safety of the ISS for an EVA, or while an EVA is under way. 34. J.L. Modisette, M.D. Lopez, and J.W. Snyder, "Radiation plan for the Apollo lunar mission," AIAA paper 69-19; J.L. Modisette, T.M. Vinson, and A.C. Hardy, "Model solar proton environments for manned spacecraft design," Manned Spacecraft Center, NASA TN D-2746, April 1965; M.D. Lopez, A.L. Bragg, and J.L. Modisette, "Preliminary warning criteria for Solar Particle Alert Network," NASA Program Apollo Working Paper No. 1193, NASA Manned Spacecraft Center, 1966; D.E. Robbins and J.H. Reid, "Solar physics at the NASA Manned Spacecraft Center," Solar Physics, 10, 1969, pp. 502-510. 35. H.H. Malitson and W.R. Weber, Solar Proton Manual, F.G. McDonald, ed., NASA Goddard Space Flight Center X-611-62-122, 1963; W.R. Weber, "An evaluation of the radiation hazard due to solar-particle events," Boeing Co. Report D2-90469, December 1963. 36. Space Studies Board, National Research Council, Radiation Hazards to Crews of Interplanetary Missions, National Academy Press, Washington, D.C., 1996, p. 15.
Representative terms from entire chapter: