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Page 1. To "Glow" Where No One Has Gone Before: The Risk for Radiation to Space
Exploration Eleanor A. O'Rangers, PharmD James S. Logan, MD, MS ...
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================================================================================================ To "Glow" Where No One Has Gone Before: The Risk for Radiation to Space Exploration Eleanor A. O'Rangers, PharmD James S. Logan, MD, MS (Published in MedScape, February 26, 2008) "Before filing his report, [Sam Cottage, of the Sun Study Center in Boulder, CO] looked back to verify the astonishing size of [Region 419, a large sunspot], and as he did so he saw the area expand significantly... He reached backward for his telephone, but his attention became riveted on that distant battleground where primordial forces had reached a point of tension that could no longer be sustained. With one mighty surge, Region 419 exploded in titanic fury. It was no longer simply a threatening active region; it was one of the most violent explosions of the past two hundred years. 'Oh, Jesus!' Cottage gasped, and while he fumbled for the phone, figures and delimitations galloped through his head: Sun to Moon, less than 93,000,000 miles. What we see now happened 8.33 minutes ago. But radiation travels at [the] speed of light, so it's already hit the moon. Oh Jesus, those poor men! REMs -- 5,000, maybe 6,000 total dose? And in the brief seconds it took for him to pick up the phone, two thoughts flashed across his mind: What else might have happened during the eight minutes it took that flash to reach here? And God, God, please protect those men [on the lunar surface]." He spread the alarm, but by the time his superiors could alert NASA, two other observatories and three amateurs in the Houston area had already reported that a gigantic solar proton event was under way.[1] This fictional doomsday scenario from James A. Michener's Space depicts the discovery of a solar flare that ultimately killed 2 moonwalkers from radiation emitted during the event. Although the doses may be somewhat exaggerated, this hazard of deep space is not. Of interest, this scene may have been inspired by the "great solar storm" of August 4, 1972, which occurred between the last 2 Apollo lunar landing missions. If this event had fallen during one of those landings, it would have delivered a total radiation dose over 12 hours that would have certainly produced acute radiation illness in the lunar module crew, and possibly, given the uncertainties in total dosage estimates, resulted in the same fate as Michener's astronauts. Although concerns over radiation exposure to space travelers has existed since the dawn of the Space Age, interest has been renewed not only in the wake of NASA's plans for a return to the Moon and, ultimately, manned exploration of Mars, but also in light of the burgeoning commercial space tourism movement. What type of radiation exposures should we be concerned about? What are the actual hazards? Can they be minimized or prevented entirely, or is this all just hype to keep us terrestrially bound? In January 2004, Robert Zubrin of the Mars Society authored a provocative op-ed piece in Space News entitled "The Great Radiation Hoax.[2]" In this editorial, which followed on his demand that The New York Times retract purported inaccuracies in an article on space radiation risk,[3] Zubrin suggested that the radiation danger was overstated, and that with proper spacecraft and surface habitat shielding, radiation is "not a show-stopper for a human Mars mission.[4]" This incited a number of responses, most notably from Richard Setlow, senior biophysicist at Brookhaven National Laboratory,[5] who indicated that "we have a long way to go in terms of basic research on the ground and aboard the International Space Station (ISS) before astronauts can go to Mars." Although Zubrin's unwavering passion for manned exploration of Mars is evident through his commentary, it is also apparent that we must ensure due diligence to minimizing the known (and likely) health risks that radiation poses to future space explorers -- whether they venture into low Earth orbit (LEO) or on the Moon or Mars. What Types of Radiation Are There in Space? When considering radiation risk associated with space travel, it is useful to consider 3 major sources of ionizing radiation: the Earth's radiation belts (trapped radiation), the sun's (solar) radiation, and galactic cosmic radiation (GCR). Ionizing radiation (eg, x-rays, gamma rays, electrons, protons, alpha particles, neutrons, and heavy ions) is of particular concern because it can ionize atoms or molecules in living cells through which it passes, resulting in either direct damage to molecular structures or indirect damage (via free radical production due to the interaction with water molecules). These effects may possibly result in cell death, damage to cellular genetic material (including induction of cancer), and accelerated aging. The Earth's Radiation Belts The Earth's radiation belts (the Van Allen belts, discovered in 1958) consist of the inner and outer ion belts and the inner and outer electron belts. In general, the belts roughly conform to the geomagnetic field, peaking in altitude over the magnetic equator and projecting downward at high latitudes. Thus, at the magnetic poles, the belts are at their thinnest. Of interest, the belts come closest to Earth over a region known as the South Atlantic Anomaly (SAA); it is estimated that satellites and astronauts in LEO can accumulate 2% to 5% of their total radiation exposure from passage through this area. Indeed, trapped ions from the inner belt and electrons in the outer belt pose the greatest hazard to satellites and astronauts in LEO because these particles can obtain energies capable of penetrating matter to significant depths. Finally, it is important to note that the outer electron belt can vary in intensity over time as a function of the solar wind, with electron energies increasing by several orders of magnitude during an intense flux of electrons called a "highly relativistic electron (HRE) event." Highly relativistic electrons are, unfortunately, transient and difficult to predict. These are of concern to space travelers because electron energies during such events can exceed the energy threshold that is necessary to penetrate a space-suited astronaut during an extravehicular activity (EVA). Despite repeated requests, the study authors have been unable to confirm a specific number from NASA for the level of radiation protection in grams per cubic centimeter (the standard unit for radiation shielding) provided by their current space suit. (However, several experts associated with NASA have said that, for all practical purposes, the space suit provides no radiation shielding protection whatsoever.) Solar Radiation Solar particle events (SPEs) are primarily responsible for generating high-energy proton emissions from the sun during solar storms. The peak incidence of these events tends to occur in approximately 11-year cycles (similar to the sunspot cycle). However, large SPEs, which are high-altitude acute radiation hazards, have been sporadically and unexpectedly observed during supposedly quiescent periods of the cycle. Although the magnetosphere tends to protect most low orbital inclination objects in the LEO, at orbits approaching or crossing the magnetic poles (where the radiation belts are at their thinnest) the risk for SPE exposure increases significantly. Moreover, the belts can also be deformed on the sun side of the Earth during SPEs, allowing exposure to high-energy protons at lower latitudes and altitudes. For comparative purposes, if an astronaut was exposed to a large SPE in his/her space suit during EVA, it would be analogous to absorbing about 1 year's accumulated SAA dose inside a spacecraft cabin. Like HREs, SPEs are transient and difficult to predict, but are of significant concern to space travelers because proton energies in these events are capable of penetrating space suits. GCR GCR is a low-level, constant source of penetrating radiation that is relatively insensitive to solar activity. The source of this radiation is unknown, but emanates from outside the solar system. It consists of protons and alpha particles (helium nuclei) as well as some heavier nuclei (eg, carbon, oxygen, magnesium, silicon, and iron). GCR is partially shielded by the magnetosphere, the Earth's mass, and its atmosphere; lower inclination orbits have less exposure than higher inclination orbits. GCR would be a persistent cumulative radiation risk for space travelers, particularly in deep space exploration. What Are the Hazards to Space Travelers Associated With Radiation? How do the various forms of radiation confer risk to a space traveler? It is important to note that all of the above forms of radiation contribute to cumulative radiation exposure over time for a space traveler. The sources of radiation, however, vary in their level of intensity; thus, their relative risk in terms of acute and delayed effects is variable (although, as a general rule of thumb, the larger, faster, and more energetic [penetrating] an exposure, the greater the risk). Moreover, organ tissues vary in their sensitivity to radiation exposure. When considering the amount of radiation absorbed by living tissue, the standard unit known as the gray (Gy) is employed, in which 1 Gy = 1 joule of radiation energy absorbed per kilogram of tissue; the centrigray (0.01 Gy) is known as the rad. In order to factor how energetic a source of radiation is, the gray or rad is multiplied, respectively, by a "quality factor." This factor is an experimentally determined value that defines excess or deficit radiation damage as a proportion of gamma-ray damage for the same dose (with gamma rays having a quality factor of 1.0) to produce a value indicating relative biological effectiveness of a dose of radiation. This value is measured in sieverts (Sv) or centisieverts (cSV, 0.01 Sv), which are also known as rems. For example, quality factors for the SAA vary from 1.6 to 1.9, with GCRs having a quality factor ranging from 2.9 to 3.5 (depending on orbit inclination) that is based on residing within a space shuttle or ISS. Biological effects of radiation exposure can be defined in terms of acute radiation sickness, late deterministic effects, and stochastic (probable) effects. Acute Radiation Sickness Acute radiation sickness is usually associated with radiation doses greater than 1 Sv (100 rems) occurring within 24 hours. Depending on the dose, nausea and vomiting can start within a few hours to a day (the higher the dose the greater the probability of these effects). Fatigue, loss of appetite, fever, and minor hemorrhage may also occur. The other major source of concern with acute radiation sickness is bone marrow suppression. With acute doses > 3 Sv (300 rem), bone marrow suppression is nearly total and the likelihood of death increases dramatically (Table 1). Table 1. Expected Short-term Effects in Humans From Acute Whole-Body Radiation Dose (rem)* 10-50 50-100 100-200 200-350 350-550 550-750 7501000 10002000 4500 Probable Physiologic Effects No obvious effects, except minor blood changes 5% to 10% experience nausea and vomiting for about 1 day; fatigue, but no serious disability; transient reduction in lymphocytes and neutrophils; no deaths anticipated 25% to 50% experience nausea and vomiting for about 1 day, followed by other symptoms of radiation sickness; 50% reduction in lymphocytes and neutrophils; no deaths anticipated Most experience nausea and vomiting on the first day, followed by other symptoms of radiation sickness, eg, loss of appetite, diarrhea, minor hemorrhage; up to 75% reduction in all circulating blood elements; mortality rates 5% to 50% of those exposed Nearly all experience nausea and vomiting on the first day, followed by other symptoms of radiation sickness, eg, fever, hemorrhage, diarrhea, emaciation; mortality rates of 50% to 90% within 6 weeks; survivors convalesce for about 6 months All experience nausea and vomiting within 4 hours, followed by severe symptoms of radiation sickness; death up to 100% Severe nausea and vomiting may continue into the third day; survival time reduced to less than 2.5 weeks Nausea and vomiting within 1-2 hours; always fatal within 2 weeks Incapacitation within hours, always fatal within 1 week *100 rem = 1 Sv From: Nicogossian A, Huntoon C, Pool S, eds. Space Physiology and Medicine. 3rd ed. Philadelphia: Lea and Feinger; 1994. Late Deterministic Effects Late deterministic effects from radiation exposure include temporary sterility. In addition, an increased risk for cataract formation has been suggested for those astronauts receiving higher radiation doses than their counterparts.[6] Stochastic Effects Stochastic effects from radiation exposure include induced cancer. Indeed, cancer risk is regarded as an accepted occupational hazard of spaceflight. Cancer risk appears to have no threshold below which it vanishes after exposure to radiation, although lower accumulated radiation doses confer a relatively lower cancer risk. Women overall have a nearly 2-fold greater risk for cancer than men, due to sensitivity of the breasts and ovaries to radiation exposure. Risk for cancer, in fact, forms the basis for astronaut career limits on accumulated radiation exposure. The current NASA-accepted risk for cancer death as a direct result of occupational exposure -- in this case spaceflight -- is set at a 3% lifetime excess risk, relative to risk for accidental death in "less safe" occupations (Tables 2 and 3). It is also important to note that these limits apply only to LEO and not for exploratory missions to the Moon or Mars. Currently, NASA estimates that the additional risk that a 1000-day Mars mission may lie substantially above the 3% excess lifetime risk for astronauts in LEO. Table 2. National Council on Radiation Protection & Measurements Recommended Dose Limits for All Organs and Ages (in Sieverts) Limit 30-day Annual Career Bone Marrow 0.250.5 See Table 3 Eye 1.0 2.0 4.0 Skin 1.5 3.0 6.0 From: Space Studies Board, Board on Atmospheric Studies and Climate, National Research Council. Radiation and the International Space Station: Recommendations to Reduce Risk. Washington, DC: National Academy Press; 2000. Table 3. 1999 National Council on Radiation Protection & Measurements Recommended Career Dose Limits Based on 3% Lifetime Risk for Induced Cancer (in Sieverts) Age at Exposure 25354555Female 0.5 0.9 1.3 1.7 Male 0.8 1.4 2.0 3.0 From: Space Studies Board, Board on Atmospheric Studies and Climate, National Research Council. Radiation and the International Space Station: Recommendations to Reduce Risk. Washington, DC: National Academy Press; 2000. What Is Our Current State of Knowledge in Regard to Space Radiation Risks? Despite over 45 years of manned spaceflight experience, detailed knowledge of the hazards of radiation exposure is severely lacking. Even the recommendations from the National Council on Radiation Protection & Measurements (NCRP) for an acceptable level of radiation exposure, which would increase NASA astronaut risk for cancer death by no more than 3% over the baseline risk, are extrapolated from observations of Hiroshima atomic bomb survivors and from those who have undergone radiation therapy for cancer or for medical diagnosis. It is important to note that these historical populations received acute radiation exposures that may be different from the predominance of chronic, low-level radiation exposure (interspersed with potential acute exposures) that would occur in space. Furthermore, other than the manned lunar landings, from which radiation exposure beyond LEO was also relatively short-term given the flight plans, we have little definitive data to base long-term human radiation exposure projections beyond LEO. Moreover, the specific tissue effects of individual types of ionizing radiation (eg, protons vs electrons vs GCR nuclei) and their biological impact (eg, cancer, cataracts, impaired fertility, and central nervous system changes) are also largely uncharacterized. Estimates of 10-year dose equivalents resulting in a 3% excess fatal cancer risk continue to trend downward. Between NCRP Report No. 98 (1989) and Report No. 132 (2000), the amount of radiation expected to produce the same 3% increased risk in cancer death was reduced by more than half for all age groups in males and females.[7] This change does not represent a more conservative standard (the excess cancer fatality risk was the same) but rather the realization that radiation is inherently more deleterious than previously believed. Finally, the interaction of weightlessness and radiation has yet to be examined with any degree of depth. In all likelihood, as our knowledge of these variables increases, the threshold of exposure limits to humans will proportionately decrease. Epidemiologic studies have been carried out on astronauts and pilots, who also spend prolonged times at high altitudes. Two studies on astronaut cancer risk have been inconclusive.[8,9] Another study reported an increased risk for acute leukemia and melanoma in pilots with more than 5000 hours of flight time, but the sample size is limited.[10] It is also known that individuals living at high altitudes have an increased risk for radiation exposure; although epidemiologic studies have not suggested that this increases cancer risk, definitive conclusions are difficult to make because the database for measuring such effects may not be sufficient.[11] Radiation Risks When Traveling Into LEO Given the likely introduction of routine space tourism flights into LEO (suborbital initially, followed by orbital flights) over the next decade, it is important to understand that even though radiation exposure in LEO is higher than terrestrial exposures, space travelers still receive protection from the Earth's magnetosphere, although they are still exposed to trapped ionizing radiation from the Van Allen belts, which they do not experience on the ground. Additionally, the Earth's mass blocks half of the GCR. Nevertheless, just as airlines routinely work around the impact of SPEs on polar flight trajectories and communications, experts are beginning to recommend that companies engaging in LEO flights should adopt their own contingencies around such events. This may require: � An inexpensive and reliable way to receive information concerning space weather (radiation) conditions, including "go/no-go" safety instructions just prior to liftoff of space passengers; � A full characterization of the nominal space environment for their normative flight profile, including nominal radiation dosages that passengers can expect, given the particular vehicle properties, as well as any predictable variances from those norms; � The ability to track actual radiation dosages obtained over a number of flights to verify the theoretical calculation of radiation exposure; and � Individual monitoring devices that, to the extent possible, can provide accurate postflight data as to each passenger's exposure during the flight.[12] Disclosure of potential radiation exposure is also likely to be incorporated into the informed consent form for such flights in language that is understandable by the nontechnical flying public. Radiation Risks During Interplanetary Trips Travel to Mars. During interplanetary missions, space travelers would receive no protection from the Earth's magnetosphere, its mass, or its atmosphere. They would also not have prolonged exposure to trapped radiation from the Van Allen belts. However, the radiation exposure on Mars may be a little better than that experienced during interplanetary flight. In early 2003, preliminary data from NASA's Mars Odyssey Spacecraft's Martian Radiation Environment Experiment (MARIE) were released. This experiment, which measures GCR and SPEs, suggested that over a 3-year period (eg, the approximate round-trip time for a mission to Mars), astronauts could face radiation levels approaching their lifetime exposure limits. Moreover, SPEs could last as long as a week. It is important to note that these estimates were based on data collected in martian orbit. The radiation environment on the planet's surface, while still unknown, is probably similar to the orbit environment, although with very limited shielding provided by the thin martian atmosphere and nearly absent martian magnetosphere. These data underscore the need for ensuring adequate radiation protection for future martian explorers. The Mars Science Laboratory, scheduled for a 2009 launch, will carry an instrument that will characterize radiation on the martian surface over 1 Mars year (approximately 2 Earth years.)[13] Travel to the Moon. The Moon poses even more daunting radiation challenges. In addition to exposure to GCR and SPEs, due to the lack of an atmosphere, space travelers on the lunar surface are bombarded by a secondary spray of neutrons emanating from irradiated regolith (lunar soil). A more accurate measurement of the lunar radiation environment will be part of the mission objectives for the planned Lunar Reconnaissance Orbiter, set for launch in 2008. How Can Radiation Risks During Spaceflight Be Minimized? Since 1997, the National Research Council has repeatedly called for NASA to pursue more robust research into the biological effects of deep space radiation as a necessary precursor to planning for exploratory missions to Mars. Without this research, validated spacecraft shielding will be next to impossible to develop. Specifically, the research would need to answer the following key questions[14]: � What are the cancer risks for exposure to protons and high-energy nuclei of GCR? � How do the thickness and composition of shielding affect the rate of cell death and genetic mutations induced by this radiation? Moreover, what is the risk for secondary radiation exposure due to the interaction of shielding with various forms of radiation? � Can improved estimates of human genetic risk on the basis of studies of radiation- induced genetic mutations in rodents (mice) be determined? � Is there a risk to the central nervous system and brain from exposure to heavy ions at the level that would occur during long missions into deep space? In other words, to quote Derek I. Lowenstein of Brookhaven National Laboratory, "If every neuron in your brain gets hit, do you come back being a blithering idiot, or not?[3]" � How do the selection and design of space vehicles affect the radiation environment in which a crew has to exist? In addition to the recommendations above, at a 2005 National Research Council workshop, Space Radiation Hazards and the Vision for Space Exploration, NASA was advised to create an "early-warning system" of space-based radiation events in order to better predict events that would necessitate limitation of LEO activities (such as EVAs), crew evacuation to "safe-haven" portions of spacecraft, or surface habitats in which more substantial shielding would exist. In response to this imperative, NASA launched the twin STEREO (Solar TErrestrial RElations Observatory) spacecraft in October 2006. STEREO will trace the flow of solar energy and matter from the sun to Earth. It will also determine the 3-dimensional structure of coronal mass ejections, a primary source of SPEs. In addition, NASA has initiated other space radiation research projects over the last decade. In 2001, "Fred" the Phantom Torso -- "part-dummy, part dosimeter-imbedded torso [that] is a mock- up of a human's upper body, minus a set of arms" -- was flown to the ISS and set up in Node 2 (the attachment point for the US Laboratory). Its purpose: to yield a more accurate portrait of human radiation exposure in the station.[15] In 2002, ISS crew members undergoing EVAs participated in a year-long test of Canadian radiation monitors that were tucked into suit pockets. The dosimeters were measuring radiation exposure of astronauts working outside the station. The "good" news: Radiation levels, although higher than average earthbound annual exposures, were lower outside the station than anticipated.[16] Nevertheless, a grim reminder that SPEs still pose a looming hazard to ISS crews came in January 2005, when the 2-man crew was instructed to follow an "alert regime" following an SPE event. This involved the cosmonaut and astronaut staying onboard in the most radiation- protected location of the station, particularly when it was facing the sun.[17] At Marshall Space Flight Center, work is ongoing to develop improved spacecraft shielding composed of sheets of polyethylene heavily impregnated with hydrogen. In addition to radiation shielding, the composite may also be used as a spacecraft hull as well as a micrometeoroid barrier.[18] NASA continues to sponsor innovative radiation biology research aimed at mitigating exposure risks posed to lunar or Mars-bound astronauts. In a collaboration with the National Cancer Institute, nanoscale monitors are under development that can fit into white blood cells to measure cellular damage from radiation exposure.[19] NASA also awarded Colorado State University a 5year, $9.7 million grant to determine predictors of radiation side effects, such as cancer. Ideally, this would afford physicians who monitor astronauts the ability to initiate preventive treatments many years in advance of the appearance of disease. The research could also lead to better risk prediction for various durations of radiation exposure, which could improve NASA's ability in determining radiation "career limits" for astronauts.[20] Perhaps of the greatest significant advance, however, is the $34 million NASA Space Radiation Laboratory (NSRL), constructed at the Department of Energy's Brookhaven National Laboratory in Upton, New York. Opened in October 2003, this facility was specifically developed to study the effects of space radiation on cells, tissue samples, and lab animals, such as mice. Particle accelerators on-site can simulate GCR. Over the next 10 years or so, the program will focus on providing more accurate estimates on risk associated with space radiation exposure as well as developing potential countermeasures, such as medications, to mitigate that risk. In September 2006, one study supported by the NSRL suggested that human cells exposed to dual-particle radiation that is analogous to the space environment (eg, high-energy proton and high atomic number particles, such as iron or titanium) were 3 times more likely to induce precancerous mutations when the particle exposures were close together temporally. Although the probability of such "one-two particle punches" occurring in space during short time spans is unlikely, such research will add to the database needed to effectively predict risk during spaceflight.[21] Conclusions Increasing characterization of radiation environments beyond the Earth's magnetosphere suggests increasing risk for long-duration exposure of humans engaged in space exploration. Radiation environments may severely constrain prolonged human habitation of the surface of the Moon and Mars. The first casualty of these constraints will likely be the accepted concept of repeated surface EVAs on the Moon and Mars by the same crew. The radiation threat is real, not hype (as Zubrin[4] has alleged): We are woefully unprepared for prolonged human operations in space. Radiation hazards (along with other life science issues) may indeed be the "tall pole in the tent" for long-duration human space habitation. References 1. Michener JA. Space. New York: Ballantine Books; 1982. 2. Zubrin R. The great radiation hoax. Space News. December 26, 2004; page 14. 3. Wald M. Mars missions invisible enemy: radiation. NY Times. December 9, 2003. 4. Zubrin R. NY Times misrepresents Mars missions radiation danger. Space Daily. December 14, 2003. Available at: http://www.spacedaily.com/news/mars-general03w. html Accessed January 31, 2008. 5. Setlow RB. Mitigating hazards of space travel. Space News. March 15, 2004; page 13. 6. Cucinotta FA, Manuel FK, Jones J, et al. Space radiation and cataracts in astronauts. Radiat Res. 2001;156:460-466. Abstract 7. Townsend LW, Fry RJM. Radiation protection guidance for activities in low-Earth orbit. Adv Space Res. 2002;30:957-963. Abstract 8. Peterson LE, Pepper LJ, Hamm PB, Gilbert SL. Longitudinal study of astronaut health: mortality in the years 1959-1991. Radiat Res. 1993;133:257-264. Abstract 9. Hamm PB, et al. Risk of cancer mortality among the Longitudinal Study of Astronaut Health (LSAH) participants. Aviat, Space Environ Med. 1998;69:142-144. 10. Gundestrup M, Storm HH. Radiation-induced acute myeloid leukemia and other cancers in commercial jet cockpit crew: a population-based cohort study. Lancet. 1999;354:20292031. Abstract 11. Mason TJ, Miller RW. Cosmic radiation at high altitudes and U.S. cancer mortality, 19501969. Radiat Res. 1974;60:302-306. Abstract 12. David L. Space travel industry needs more data on radiation risk. May 3, 2006. Available at: http://www.space.com/news/060503_public_travel.html Accessed January 31, 2008. 13. Mars instrument to assess astronaut radiation risk [press release]. San Antonio: Southwest Research Institute: Spaceflight Now; January 18, 2005. Available at: http://spaceflightnow.com/news/n0501/18radiation/ Accessed January 31, 2008. 14. Eisele A. NASA urged to pursue study of radiation effects. Space News. January 6, 1997; page 6. 15. David L. The phantom torso: testing the effects of radiation on space travelers. December 4, 2000. Available at: http://www.space.com/missionlaunches/missions/phantom_torso_001201.html Accessed January 31, 2008. 16. Malik T. CSA study gauges astronaut radiation exposure. July 16, 2002. Available at: http://www.space.com/scienceastronomy/radiation_evarm_020716.html Accessed January 31, 2008. 17. Astronauts warned of radiation risk. ABC News Online. January 19, 2005. Available at: http://www.abc.net.au/news/newsitems/200501/s1284868.htm Accessed January 31, 2008. 18. NASA works on radiation protection shield. Associated Press. December 1, 2003. Available at: http://www.space.com/scienceastronomy/nasa_radiation_031201.html Accessed January 31, 2008. 19. Sparks H. How miniature radiation detectors will keep astronauts safe in deep space. July 17, 2002. Available at: http://www.space.com/businesstechnology/technology/radiation_nanobots_020717.html Accessed January 31, 2008. 20. Banke J. NASA study to monitor astronauts' susceptibility to cancer. November 18, 2003. Available at: http://www.space.com/spacenews/archive03/cancerarch_111803.html Accessed 1/24/2008). 21. One-two particle punch poses greater risk for astronauts. Brookhaven National Laboratory. ScienceDaily. September 9, 2006. Available at: http://www.sciencedaily.com/releases/2006/08/060824222016.htm Accessed January 31, 2008. Return to the current Home-Page