What can influence the amount of radiation exposure an astronaut receives during a mission?

There are many factors which can influence astronaut radiation exposure. This includes, but is not limited to:

• Altitude above the Earth. As altitude increases, the Earth’s magnetic field grows weaker. Consequently, there is less protection from ionizing particles and radiation exposures increase.  
• The solar cycle. The sun has periodic solar variations, which repeat approximately every 11 years. During periods of low solar activity (solar minimum), the sun’s magnetic field (which deflects ionizing particles) weakens, therefore, galactic cosmic ray (GCR) fluxes and incurred dose increase. The lowest GCR fluxes and dose, conversely, occur during times of high solar activity (solar maximum). Unlike GCRs, solar particle events (SPEs) are more likely to occur during solar maximum and correspond to large coronal mass ejections (CMEs). Exposure to a large SPE can be lethal if adequate shielding is not provided.
• Individual susceptibility. Age, gender, and genetic predisposition can affect an astronaut’s radiosensitivity.  
• Amount and type of radiation shielding provided. The design and radiation shielding materials used in the construction of spacesuits, surface vehicles, surface habitats, and spacecrafts can dramatically affect radiation exposure values. Radiation analysis tools can be used to reduce radiation exposure, through shielding design and material optimization. For example, National Aeronautics and Space Administration (NASA) has developed the On-Line Tool for the Assessment of Radiation In Space (OLTARIS), https://oltaris.larc.nasa.gov/, to assess effects of space radiation on humans and electronics. The OLTARIS website enables scientists, engineers, and designers to meet NASA’s radiation protection requirements throughout all stages of vehicle design.
• Mission duration. Radiation exposure is cumulative. Therefore, as mission duration lengthens, an astronaut’s potential radiation exposure increases.

What radiation doses would astronauts of an Apollo mission receive during a solar particle event (SPE)?

Estimating SPE exposure on the surface of the moon (or anywhere in space) is difficult, since SPEs vary in intensity and spectral shape. There is no “design basis” SPE and it is difficult to estimate the probability of SPE occurrence. SPEs are sporadic events, consisting primarily of  protons. Large SPEs have occurred only rarely; one or two per 11-year solar cycle in the past 60 years. However, exposure to a large SPE could be lethal if enough shielding is not provided. If one were to choose a historically large SPE, the resulting radiation exposure analyses and recommendations will be specific to the spectral shape of that event. Therefore, total radiation exposures assessments of “worst-case” and “more probable” SPE scenarios are equally valuable.

One of the largest recorded SPEs occurred in August 1972, between the Apollo 16 and Apollo 17 missions. The August 1972 event is one of the largest recorded SPEs in flux density and contained more high-energy (10-200 MeV) protons than most other historic events. For this event, astronauts who were thinly shielded on the Lunar surface (for example, astronauts conducting extra vehicular activities [EVAs] such as a spacewalk) could have received fatal radiation doses. Large SPEs, such as the August 1972 event, are infrequent; NASA estimates that more than 90 percent of SPEs will result in only small organ doses (<100 mGy-equivalent) for lightly shielded surface mission scenarios (Wu et. al. 2009).

Reference
Wu H, Huff JL, Casey R, Kim M-Y, Cucinotta FA. Risk of acute radiation syndromes due to solar particle events. Chapter 5 in: Human health and performance risks of space exploration missions. NASA SP-2009-3405; 2009. Available at: http://spaceradiation.usra.edu/references/Ch5SPE.pdf. Accessed 22 October 2012.

How do the radiation levels for astronauts on the moon and Mars compare?

Astronaut radiation exposures on the surface of the moon or Mars will change due to many factors, such as solar activity, altitude above the planetary surface, and the amount and type of radiation shielding provided, as mentioned previously. Numerous studies have been conducted to estimate how these factors influence or change astronauts’ radiation exposures due to omni-present galactic cosmic rays (GCRs) and sporadic solar particle events (SPEs). Recent radiation exposure assessments for Lunar and Martian surface missions include:

• Adamczyk A, Clowdsley M, Qualls G, Blattnig S, Lee K, Fry D, Stoffle N, Simonsen L, Slaba T, Walker S, Zapp E. Full mission astronaut radiation assessments for long duration surface missions. In: Proceedings of the 2011 IEEE Aerospace Conference. Big Sky, Montana; 5–12 March 2011. Available at: http://dx.doi.org/10.1109/AERO.2011.5747250. Accessed 22 October 2012.
• Adamczyk A, Werneth C, Townsend L. Comparisons of Carrington-class solar particle event radiation exposure estimates on Mars utilizing the CAM, CAF, MAX, and FAX human body models. In: Proceedings of the 13^th International Congress of the International Radiation Protection Association (IRPA). Glasgow, Scotland; May 2012.
• Adamczyk A, Werneth C, Townsend L. Estimates of significant post-Carrington solar particle event radiation exposures on Mars. In: Proceedings of the 42^nd International Conference on Environmental Systems (ICES). San Diego, California; 15–19 July 2012. Available at: arc.aiaa.org/doi/pdf/10.2514/6.2012-3634. Accessed 1 November 2012.
• Clowdsley M, Nealy J, Wilson J, Anderson B, Anderson M, Krizan S. Radiation protection for Lunar mission scenarios. In: Proceedings of AIAA Space 2005 Conference. Long Beach, California; 30 August–1 September 2005. Available at: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20050215115_2005217501.pdf. Accessed 22 October 2012.
• Drake B, ed. Human exploration of Mars design reference architecture 5.0. Houston TX: NASA report NASA/SP-2009-566; July 2009. Available at: www.nasa.gov/pdf/373665main_NASA-SP-2009-566.pdf. Accessed 1 November 2012.
• Jia Y, Lin Z. The radiation environment on the moon from galactic cosmic rays in a Lunar habitat. Radiation Research 173:238–244; 2010.
• McKenna-Lawlor S, Gonçalves P, Keating A, Reitz G, Matthiä D. Overview of energetic particle hazards during prospective manned missions to Mars. Planetary and Space Science 63–64:123–132; 2012. Available at: http://www.sciencedirect.com/science/article/pii/S0032063311002030. Accessed 1 November 2012.
• McKenna-Lawlor S, Gonçalves P, Keating A, Morgado B, Heynderickx D, Nieminen P, Santin G, Truscott P, Lei F, Foing B, Balaz J. Characterization of the particle radiation environment at three potential landing sites on Mars using ESA’s MEREM models. Icarus 218:723–734; 2012. Available at: http://www.sciencedirect.com/science/article/pii/S0019103511001254. Accessed 1 November 2012.
• National Aeronautics and Space Administration. NASA’s exploration systems architecture study. Houston TX: NASA report NASA-TM-2005-214062; November 2005. Available at: http://www.nasa.gov/exploration/news/ESAS_report.html. Accessed 22 October 2012.
• Straume T, Blattnig S, Zeitlin C. Radiation hazards and the colonization of Mars: Brain, body, pregnancy, in-utero development, cardio, cancer, degeneration. Journal of Cosmology 12:3992–4033; 2010. Available at: http://journalofcosmology.com/Mars124.html. Accessed 22 October 2012.
• Townsend L, PourArsalan M, Hall M, Anderson J, Bhatt S, DeLauder N, Adamczyk A. Estimates of Carrington-class solar particle event radiation exposures on Mars. Acta Astronautica 69:397–405; 2011. (doi:10.1016/j.actaastro.2011.05.020). Available at: http://www.sciencedirect.com/science/article/pii/S0094576511001585. Accessed 1 November 2012.
• Townsend L, Anderson J, Adamczyk A, Werneth C. Estimates of Carrington-class solar particle event radiation exposures as a function of altitude in the atmosphere of Mars. In: Proceedings of the 62^nd International Astronautical Congress (IAC). Cape Town, South Africa; 3–7 October 2011.
• Townsend L, Adamczyk A, Werneth C, PourArsalan M, Anderson A, Tsai P. Estimates of Carrington-class solar particle event radiation exposures on Mars behind polyethylene shielding. In: Proceedings of the 41^st International Conference on Environmental Systems (ICES). Portland, Oregon; 17–21 July 2011. Available at: arc.aiaa.org/doi/pdf/10.2514/6.2011-5252. Accessed 1 November 2012.
• Townsend L, PourArsalan M, Hall M. Estimates of GCR radiation exposures on Mars for female crews in hemispherical habitats. In: Proceedings of the 2010 IEEE Aerospace Conference. Big Sky, Montana; March 2010. Available at: http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=05447006. Accessed 1 November 2012.

Note that additional references regarding exploration strategies have been provided by the Lunar and Planetary Institute at http://www.lpi.usra.edu/lunar/strategies/.
 
Radiation levels on Mars’ surface are generally lower than on the moon’s surface since Mars has a thin atmosphere, composed mainly of carbon dioxide, which acts as a radiation shield whereas the moon has no appreciable atmosphere to provide astronauts additional radiation protection. However, it should be noted that astronauts’ radiation exposures for an expedition to Mars will be much greater than for a mission to the moon. Human expeditions to Mars will be characterized by longer duration missions than those of the Apollo Program. The highest radiation exposures are likely to occur during the Martian transit spaceflight, as determined in a previous study by Townsend et al. (1992).  Radiation mitigation strategies during Martian transit must address the combined risks from the GCR and SPE environment. During the short lunar travel times, GCR exposure is only a small fraction of a long-duration lunar surface stay. A large SPE occurring during lunar transit, however, will induce non-negligible exposures.

Reference
Townsend LW, Cucinotta C, Wilson J. Interplanetary crew exposure estimates for galactic cosmic rays. Radiation Research 129:48–52; 1992. Available at: http://www.rrjournal.org/doi/abs/10.2307/3577902. Accessed 1 November 2012.

Where can I find more information regarding space radiation?

Information on space radiation can be found in the following sources.

National Council on Radiation Protection and Measurements (NCRP) Reports:

• Potential impact of individual genetic susceptibility and previous radiation exposure on radiation risk for astronauts. NCRP Report No. 167; 2012. Available at: http://www.ncrppublications.org/Reports/167. Accessed 22 October 2012.

• Information needed to make radiation protection recommendations for space missions beyond low-earth orbit. NCRP Report No. 153; 2006. Available at: http://www.ncrppublications.org/index.cfm?fm=Product.AddToCart&pid=9524417168. Accessed 22 October 2012.
• Operational radiation safety program for astronauts in low-earth orbit: a basis framework. NCRP Report No. 142; 2002. Available at: http://www.ncrppublications.org/index.cfm?fm=Product.AddToCart&pid=8877679252. Accessed 22 October 2012.
• Fluence-based and microdosimetric event-based methods for radiation protection in space. NCRP Report No. 137; 2001. Available at: http://www.ncrppublications.org/index.cfm?fm=Product.AddToCart&pid=8106603162. Accessed 22 October 2012.
• Radiation protection guidance for activities in low-earth orbit. NCRP Report No. 132; 2000. Available at: http://www.ncrppublications.org/index.cfm?fm=Product.AddToCart&pid=0613393790. Accessed 22 October 2012.
• Acceptability of risk from radiation—application to human space flight. NCRP Symposium Proceedings No. 3; 1996. Available at: http://www.ncrppublications.org/Symposia_Proceedings/3. Accessed 22 October 2012.
• Radiation guidance on radiation received in space activities. NCRP Report No. 98; 1989. Available at: http://www.ncrppublications.org/index.cfm?fm=Product.AddToCart&pid=5853854306. Accessed 22 October 2012.

National Academies of Sciences, National Research Council Reports:

• NASA space technology roadmaps and priorities: restoring NASA’s technological edge and paving the way for a new era in space. Washington, DC: The National Academies Press; 2012. Available at: http://www.nap.edu/catalog.php?record_id=13354. Accessed 22 October 2012.
• Managing space radiation risk in the new era of space exploration. Washington, DC: The National Academies Press; 2008. Available at: http://books.nap.edu/catalog.php?record_id=12045. Accessed 22 October 2012.
• Space radiation hazards and the vision for space exploration: report of a workshop. Washington, DC: The National Academies Press, 2006. Available at: http://www.nap.edu/catalog.php?record_id=11760. Accessed 22 October 2012.

International Commission on Radiological Protection (ICRP) report:

• Dietze G, Bartlett D, Cucinotta F, Pelliccioni M, Sato T, Petrov V, Reitz G, McAulay I, Xianghong J, Cool D. Assessment of radiation exposure of astronauts in space. Draft report for consultation; July 2012. Available at: www.icrp.org/page.asp?id=163. Accessed 22 October 2012.

NASA documents:

• Radiation and human space exploration (video). Available at: http://www.nasa.gov/multimedia/videogallery/index.html?media_id=14692127. Accessed 22 October 2012.
• NASA Space Radiation Program Element. Available at: http://spaceradiation.usra.edu. Accessed 22 October 2012.
• Human adaptation and countermeasures. Available at: http://www.nasa.gov/centers/johnson/slsd/about/divisions/hacd/hrp/space-radiation.html. Accessed 22 October 2012.
• Radiation Effects and Analysis Group (REAG), NASA Goddard Space Flight Center. Available at: http://radhome.gsfc.nasa.gov/. Accessed 22 October 2012.
• Space Radiation Group (SRG), NASA Langley Research Center. Available at: http://spaceradiation.larc.nasa.gov/srgHome.html. Accessed 22 October 2012.
• Space Radiation Analysis Group (SRAG), NASA Johnson Space Center. Available at: http://srag-nt.jsc.nasa.gov/. Accessed 22 October 2012.

Where could I find information about proton radiation in space and information dealing with the penetration of protons in matter with energies typical of those found in space?

Protons are the most abundant type of charged particle in each of the three sources of naturally occurring space radiation sources, which include trapped radiation, galactic cosmic rays (GCRs), and solar particle events (SPEs). Since protons are a critical contributor to space radiation during space flights, many agencies and experimental facilities are interested in how protons affect human cells and electronics. Currently space radiation health research is being conducted at a variety of facilities, which include, but are not limited to the following:

• NASA Space Radiation Laboratory at Brookhaven National Laboratory, Long Island, New York
• Relativistic Heavy Ion Collider at Brookhaven National Laboratory, Long Island, New York
• Lawrence Berkeley National Laboratory, Berkeley, California
• Loma Linda University Medical Center, Loma Linda, California
• GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany
• National Institute of Radiological Sciences (NIRS), Chiba, Japan

More information regarding proton radiation can be found on various National Aeronautics and Space Administration (NASA) and National Oceanographic and Atmospheric Administration (NOAA) websites, which include the NOAA Space Environment Center in Boulder, Colorado; NASA Goddard and Marshall Space Flight Centers in Greenbelt, Maryland, and Huntsville, Alabama, respectively; Jet Propulsion Laboratory in Pasadena, California; NASA Langley (Hampton, Virginia) and Glenn Research Centers (Cleveland, Ohio); NASA Johnson Space Center in Houston, Texas; and education links on the NASA Headquarters website. Another source of information is the Proceedings of the Biennial Committee on Space Research (COSPAR) plenary meeting published in Advances in Space Research.
Representative websites for these organizations include the following:

• The Exploration of the Earth’s Magnetosphere
• NASA Human Spaceflight
• NASA Science, http://science.nasa.gov/
• NOAA’s Space Weather Prediction Center (proton spectra and particle data from the geostationary operational environmental satellites [GOES] can be viewed and downloaded)   
• Cosmic Rays
• Solar-Terrestrial Physics Division of the National Geophysical Data Center

What radiation protection quantities does the National Aeronautics and Space Administration (NASA) use?

NASA uses (or has used) four dosimetric quantities to calculate radiation exposure in humans and electronics (Clowdsley et al. 2004):

• Dose (gray)—The most basic radiation protection quantity, dose, is most often used to estimate risk to electronic equipment. It is rarely used to evaluate risk to humans since it does not account for the varying efficiency of different types of radiation in producing biological effects.
  
  Dose is defined as the energy absorbed per unit mass of tissue or material and is expressed in units of gray (Gy). Note that 1 Gy = 1 J kg^-1 = 100 rad.
  
  
• Dose equivalent (sievert)—The radiation protection quantity of dose equivalent accounts for the varying efficiency of different types of incident radiation in producing stochastic effects. This dosimetric quantity is given in units of sievert (Sv), where 1 Sv = 1 J kg^-1 = 100 rem. The dose equivalent at a given point in a tissue or organ is given by
  
  Dose equivalent (Sv) = ∫Quality factor × Dose (Gy),
  
  where the quality factor is a function of the linear energy transfer and is obtained from the International Commission for Radiological Protection (ICRP) Report No. 60 (ICRP 1991).
  
  Note that dose equivalent differs from equivalent dose, which has been defined in the National Council on Radiation Protection and Measurements (NCRP) Report No. 116 (NCRP 2001) and ICRP Report No. 60 (ICRP 1991). Instead of quality factors, equivalent dose calculations use radiation weighting factors, which model stochastic effects.
  
  
• Gray-equivalent—For the estimation of deterministic acute effects to astronauts from a solar particle event (SPE) during a mission, the dosimetric quantity of gray-equivalent is used by NASA, as recommended by NCRP in Report No. 132 (NCRP 2010). A resulting dose in units of gray-equivalent (Gy-Eq) is obtained from
  
  Dose (Gy-Eq) = dose (Gy) x RBE,
  
  where dose (gray) is the mean absorbed dose in an organ or tissue. The multiplicative term RBE stands for the relative biological effectiveness factor and is radiation-field dependent. For SPE protons, an RBE value of 1.5 is assumed as recommended by the NCRP. Therefore, a "dose" in units of gray-equivalent indicates that the absorbed dose (gray) has been multiplied by the RBE. To ensure that space operations during a mission meet NASA guidelines, calculated gray-equivalent values are compared to NASA 30-day, one-year, and career-permissible exposure limits (PELs) for the skin, lens, blood-forming organs, central nervous system, and heart. If exposures are found to be below these limits and are as low as reasonably achievable (ALARA), a mission-design concept can be finalized. However, the conventional amount of shielding materials used in the construction of a specific mission element often does not provide enough radiation protection to keep doses below the NASA limits. Radiation mitigation strategies then need to be employed to reduce radiation exposures to more acceptable levels. Note that the PELs vary depending on the specific space agency. For example, NASA's limits are different from the European Space Agency’s limits.
  
  
• Effective dose (sievert)—The effective dose is a weighted average of the dose equivalent to numerous organs or tissues and is important for evaluating risk from stochastic effects. Effective dose is given in units of sievert (Sv) and is calculated using
  
  Effective dose (Sv) = ∑ Tissue weighting factor × Organ dose equivalent (Sv),
  
  where the sum is taken over all measurements of organs or tissues. NASA uses tissue weighting factors recommended in NCRP Report No. 132 (2000) for a tissue or an organ of interest. Note that NCRP Report No. 132 suggests that the dosimetric quantity of effective dose replace dose equivalent; therefore NASA has adopted effective dose for evaluating risk from stochastic effects. NASA has established career-effective dose limits, which are deemed necessary for preventing radiation carcinogenesis. The limits were defined in NASA Standard 3001 (2009) and established so that astronauts would not exceed a 3 percent risk of exposure-induced death from carcinogenesis. This risk limit must not be exceeded by male and female NASA astronauts at a 95 percent confidence level.

References
Clowdsley M, Wilson J, Kim M, Anderson B, Nealy J. Radiation protection quantities for near earth environments. In: Proceedings of the AIAA Space 2004 Conference and Exposition. San Diego, CA: American Institute of Aeronautics and Astronautics; 2004.

International Commission for Radiological Protection. 1990 Recommendations of the International Commission for Radiological Protection. New York: Elsevier Science; ICRP Report No. 60; Annals of the ICRP 21 (1–3); 1991.

National Aeronautics and Space Administration. NASA space flight human system standard; Volume 1: Crew health. Washington, DC: National Aeronautics and Space Administration; NASA-STD-3001; 2009.

National Council on Radiation Protection and Measurements. Limitation of exposure to ionizing radiation. Bethesda, MD: National Council on Radiation Protection and Measurements; NCRP Report No. 116; 2001.

National Council on Radiation Protection and Measurements. Radiation protection guidance for activities in low earth orbit. Bethesda, MD: National Council on Radiation Protection and Measurements; NCRP Report No. 132; 2010.