Health threat from cosmic rays

From Wikipedia, the free encyclopedia

The health threat from cosmic rays is the danger posed by galactic cosmic rays (GCR) and solar energetic particles to astronauts on interplanetary missions or any missions that venture through the Van-Allen Belts or outside the Earth's magnetosphere.^[1][2] They are one of the greatest barriers standing in the way of plans for interplanetary travel by crewed spacecraft,^[3][4][5] but space radiation health risks also occur for missions in low Earth orbit such as the International Space Station (ISS).^[6]

In October 2015, the NASA Office of Inspector General issued a health hazards report related to space exploration, including a human mission to Mars.^[7][8]

The deep-space radiation environment

Sources of ionizing radiation in interplanetary space.

The radiation environment of deep space is different from that on the Earth's surface or in low Earth orbit, due to the much larger flux of high-energy galactic cosmic rays (GCRs), along with radiation from solar proton events (SPEs) and the radiation belts.

Galactic cosmic rays (GCRs) consist of high energy protons (85%), helium (14%) and other high energy nuclei (HZE ions).^[1] Solar energetic particles consist primarily of protons accelerated by the Sun to high energies via proximity to solar flares and coronal mass ejections. Heavy ions and low energy protons and helium particles are highly ionizing forms of radiation, which produce distinct biological damage compared to X-rays and gamma-rays. Microscopic energy deposition from highly ionizing particles consists of a core radiation track due to direct ionizations by the particle and low energy electrons produced in ionization, and a penumbra of higher energy electrons that may extend hundreds of microns from the particles path in tissue. The core track produces extremely large clusters of ionizations within a few nanometres, which is qualitatively distinct from energy deposition by X-rays and gamma rays; hence human epidemiology data which only exists for these latter forms of radiation is limited in predicting the health risks from space radiation to astronauts.

But of course the radiation belts are within Earth’s magnetosphere and do not occur in deep space, while organ dose equivalents on the International Space Station are dominated by GCR not trapped radiation. Microscopic energy deposition in cells and tissues is distinct for GCR compared to X-rays on Earth leading to both qualitative and quantitative differences in biological effects, while there is no human epidemiology data for GCR for cancer and other fatal risks.

The solar cycle is an approximately 11-year period of varying solar activity including solar maximum where the solar wind is strongest and solar minimum where the solar wind is weakest. Galactic cosmic rays create a continuous radiation dose throughout the Solar System that increases during solar minimum and decreases during solar maximum (solar activity). The inner and outer radiation belts are two regions of trapped particles from the solar wind that are later accelerated by dynamic interaction with the Earth's magnetic field. While always high, the radiation dose in these belts can increase dramatically during geomagnetic storms and substorms. Solar proton events are bursts of energetic protons accelerated by the Sun. They occur relatively rarely and can produce extremely high radiation levels. Without thick shielding, SPEs are sufficiently strong to cause acute radiation poisoning and death.^[9]

Life on the Earth's surface is protected from galactic cosmic rays by a number of factors:

1. The Earth's atmosphere is opaque to primary cosmic rays with energies below about 1 gigaelectron volt (GeV), so only secondary radiation can reach the surface. The secondary radiation is also attenuated by absorption in the atmosphere, as well as by radioactive decay in flight of some particles, such as muons. Particles entering from a direction far from the zenith are especially attenuated. The world's population receives an average of 0.4 millisieverts (mSv) of cosmic radiation annually (separate from other sources of radiation exposure like inhaled radon) due to atmospheric shielding. At 12 km altitude, above most of the atmosphere's protection, radiation as an annual rate rises to 20 mSv at the equator to 50–120 mSv at the poles, varying between solar maximum and minimum conditions.^[10][11][12]
2. Missions beyond low Earth orbit transit the Van Allen radiation belts. Thus they may need to be shielded against exposure to cosmic rays, Van Allen radiation, or solar flares. The region between two and four Earth radii lies between the two radiation belts and is sometimes referred to as the "safe zone".^[13][14] See the implications of the Van Allen belts for space travel for more information.
3. The interplanetary magnetic field, embedded in the solar wind, also deflects cosmic rays. As a result, cosmic ray fluxes within the heliopause are inversely correlated with the solar cycle.^[15]
4. Electromagnetic radiation created by lightning in clouds only a few miles high can create a safe zone in the Van Allen radiation belts that surround the earth. This zone, known as the "Van Allen Belt slot", may be a safe haven for satellites in medium Earth orbits (MEOs), protecting them from the Sun's intense radiation.^[16][17][18]

As a result, the energy input of GCRs to the atmosphere is negligible – about 10⁻⁹ of solar radiation – roughly the same as starlight.^[19]

Of the above factors, all but the first one apply to low Earth orbit craft, such as the Space Shuttle and the International Space Station. Exposures on the ISS average 150 mSv per year, although frequent crew rotations minimize individual risk.^[20] Astronauts on Apollo and Skylab missions received on average 1.2 mSv/day and 1.4 mSv/day respectively.^[20] Since the durations of the Apollo and Skylab missions were days and months, respectively, rather than years, the doses involved were smaller than would be expected on future long-term missions such as to a near-Earth asteroid or to Mars^[3] (unless far more shielding could be provided).

On 31 May 2013, NASA scientists reported that a possible manned mission to Mars^[3] may involve a great radiation risk based on the amount of energetic particle radiation detected by the radiation assessment detector (RAD) on the Mars Science Laboratory while traveling from the Earth to Mars in 2011–2012.^[21][22][23] However, the absorbed dose and dose equivalent for a Mars mission were predicted in the early 1990s by Badhwar, Cucinotta, and others (see for example Badhwar, Cucinotta et al., Radiation Research vol. 138, 201-208, 1994) and the result of the MSL experiment are to a large extent consistent with these earlier predictions.

Human health effects

Comparison of radiation doses, includes the amount detected on the trip from Earth to Mars by the RAD on the MSL (2011–2013).^[21][22][23] The y-axis scale is in logarithmic scale. For example, the exposure from 6 months aboard the ISS is roughly a factor of 10 greater than that from an abdominal CT scan.

The potential acute and chronic health effects of space radiation, as with other ionizing radiation exposures, involve both direct damage to DNA, indirect effects due to generation of reactive oxygen species, and changes to the biochemistry of cells and tissues, which can alter gene transcription and the tissue microenvironment along with producing DNA mutations. Acute (or early radiation) effects result from high radiation doses, and these are most likely to occur after solar particle events (SPEs).^[24] Likely chronic effects of space radiation exposure include both stochastic events such as radiation carcinogenesis^[25] and deterministic degenerative tissue effects. To date, however, the only pathology associated with space radiation exposure is a higher risk for radiation cataract among the astronaut corps.^[26][27]

The health threat depends on the flux, energy spectrum, and nuclear composition of the radiation. The flux and energy spectrum depend on a variety of factors: short-term solar weather, long-term trends (such as an apparent increase since the 1950s^[28]), and position in the Sun's magnetic field. These factors are incompletely understood.^[29][30] The Mars Radiation Environment Experiment (MARIE) was launched in 2001 in order to collect more data. Estimates are that humans unshielded in interplanetary space would receive annually roughly 400 to 900 mSv (compared to 2.4 mSv on Earth) and that a Mars mission (12 months in flight and 18 months on Mars) might expose shielded astronauts to roughly 500 to 1000 mSv.^[28] These doses approach the 1 to 4 Sv career limits advised by the National Council on Radiation Protection and Measurements (NCRP) for low Earth orbit activities in 1989, and the more recent NCRP recommendations of 0.5 to 2 Sv in 2000 based on updated information on dose to risk conversion factors. Dose limits depend on age at exposure and sex due to difference in susceptibility with age, the added risks of breast and ovarian cancers to women, and the variability of cancer risks such as lung cancer between men and women.

The quantitative biological effects of cosmic rays are poorly known, and are the subject of ongoing research. Several experiments, both in space and on Earth, are being carried out to evaluate the exact degree of danger. Additionally, the impact of the space microgravity environment on DNA repair has in part confounded the interpretation of some results.^[31] Experiments over the last 10 years have shown results both higher and lower than predicted by current quality factors used in radiation protection, indicating large uncertainties exist. Experiments in 2007 at Brookhaven National Laboratory's NASA Space Radiation Laboratory (NSRL) suggest that biological damage due to a given exposure is actually about half what was previously estimated: specifically, it turns out that low energy protons cause more damage than high energy ones.^[32] This is explained by the fact that slower particles have more time to interact with molecules in the body. This may be interpreted as an acceptable result for space travel as the cells affected end up with greater energy deposition and are more likely to die without proliferating into tumors. This is in contrast to the current dogma on radiation exposure to human cells which considers lower energy radiation of higher weighting factor for tumor formation. Relative biological effectiveness (RBE) depends on radiation type described by particle charge number, Z, and kinetic energy per amu, E, and varies with tumor type with limited experimental data suggesting leukemia's having the lowest RBE, liver tumors the highest RBE, and limited or no experimental data on RBE available for cancers that dominate human cancer risks including lung, stomach, breast, and bladder cancers. Studies of Harderian gland tumors in a single strain of female mice with several heavy ions have been made, however it is not clear how well the RBE for this tumor type represents the RBE for human cancers such as lung, stomach, breast and bladder cancers nor how RBE changes with sex and genetic background.

Part of the ISS year long mission is to determine the health impacts of cosmic ray exposure over the course of one year spent aboard the International Space Station.

However, sample sizes for accurately estimating health risks directly from crew observations for the risks of concern (cancer, cataracts, cognitive and memory changes, late CNS risks, circulatory diseases, etc.) are large (typically >>10 persons) and necessarily involve long post-mission observation times (>10 years). It will be difficult for a sufficient number of astronauts to occupy the ISS and for the missions to continue long enough to make an impact on risk predictions for late effects due to statistical limitations. Hence the need for ground-based research to predict cosmic ray health risks. In addition, radiation safety requirements mandate that risks should be adequately understood prior to astronauts incurring significant risks, and methods developed to mitigate the risks if necessary.

In September 2017, NASA reported radiation levels on the surface of the planet Mars were temporarily doubled, and were associated with an aurora 25-times brighter than any observed earlier, due to a massive, and unexpected, solar storm in the middle of the month.^[33]

Central nervous system

Hypothetical early and late effects on the central nervous system are of great concern to NASA and an area of active current research interest. It is postulated short- and long-term effects of CNS exposure to galactic cosmic radiation are likely to pose significant neurological health risks to human long-term space travel.^[34][35] Estimates suggest considerable exposure to high energy heavy (HZE) ions as well as protons and secondary radiation during Mars or prolonged Lunar missions with estimates of whole body effective doses ranging from 0.17 to greater than 1.0 Sv.^[36] Given the high linear energy transfer potential of such particles, a considerable proportion of those cells exposed to HZE radiation are likely to die. Based on calculations of heavy ion fluences during space flight as well as various experimental cell models, as many as 5% of an astronaut’s cells might be killed during such missions.^[37][38] With respect to cells in critical brain regions, as many as 13% of such cells may be traversed at least once by an iron ion during a three-year Mars mission.^[3][39] Several Apollo astronauts reported seeing light flashes, although the precise biological mechanisms responsible are unclear. Likely pathways include heavy ion interactions with retinal photoreceptors^[40] and Cherenkov radiation resulting from particle interactions within the vitreous humor.^[41] This phenomenon has been replicated on Earth by scientists at various institutions.^[42][43] As the duration of the longest Apollo flights was less than two weeks, the astronauts had limited cumulative exposures and a corresponding low risk for radiation carcinogenesis. In addition, there were only 24 such astronauts, making statistical analysis of any potential health effects problematic.

In the above discussion dose equivalents is units of Sievert (Sv) are noted, however the Sv is a unit for comparing cancer risks for different types of ionizing radiation. For CNS effects absorbed doses in Gy are more useful, while the RBE for CNS effects is poorly understood. Furthermore, stating "hypothetical" risk is problematic, while space radiation CNS risk estimates have largely focused on early and late detriments to memory and cognition (e.g. Cucinotta, Alp, Sulzman, and Wang, Life Sciences in Space Research, 2014).

On 31 December 2012, a NASA-supported study reported that manned spaceflight may harm the brains of astronauts and accelerate the onset of Alzheimer's disease.^[44][45][46] This research is problematic due to many factors, inclusive of the intensity of which mice were exposed to radiation which far exceeds normal mission rates.

A review of CNS space radiobiology by Cucinotta, Alp, Sulzman, and Wang (Life Sciences in Space Research, 2014) summarizes research studies in small animals of changes to cognition and memory, neuro-inflammation, neuron morphology, and impaired neurogenesis in the hippocampus. Studies using simulated space radiation in small animals suggest temporary or long-term cognitive detriments could occur during a long-term space mission. Changes to neuron morphology in mouse hippocampus and pre-frontal cortex occur for heavy ions at low doses.
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The cumulative heavy ion doses in space are low such that critical cells and cell components will receive only 0 or 1 particle traversal. The cumulative heavy ion dose for a Mars mission near solar minimum would be ~0.05 Gy and lower for missions at other times in the solar cycle. This suggests dose-rate effects will not occur for heavy ions as long as the total doses used in experimental studies in reasonably small (<~0.1 Gy). At larger doses (>~0.1 Gy) critical cells and cell components could receive more than one particle traversal, which is not reflective of the deep space environment for extended duration missions such as a mission to Mars. An alternative assumption would be if a tissues micro-environment is modified by a long-range signaling effect or change to biochemistry, whereby a particle traversal to some cells modifies the response of other cells not traversed by particles. There is limited experimental evidence, especially for central nervous system effects, available to evaluate this alternative assumption.

Mitigation

Shielding

Standard spacecraft shielding, integrated into hull design, is strong protection from most solar radiation, but defeats this purpose with high-energy cosmic rays, as it simply splits this into showers of secondary particles. This shower of secondary and fragmented particles may be reduced by the use of hydrogen or light elements for shielding.

Material shielding can be effective against galactic cosmic rays, but thin shielding may actually make the problem worse for some of the higher energy rays, because more shielding causes an increased amount of secondary radiation, although thick shielding could counter such too.^[47] The aluminium walls of the ISS, for example, are believed to produce a net reduction in radiation exposure. In interplanetary space, however, it is believed that thin aluminium shielding would give a net increase in radiation exposure but would gradually decrease as more shielding is added to capture generated secondary radiation.^[48][49]

Studies of space radiation shielding should include tissue or water equivalent shielding along with the shielding material under study. This observation is readily understood by noting that the average tissue self-shielding of sensitive organs is about 10 cm, and that secondary radiation produced in tissue such as low energy protons, helium and heavy ions are of high LET and make significant contributions (>25%) to the overall biological damage from GCR. Studies of aluminum, polyethylene, liquid hydrogen, or other shielding materials, will involve secondary radiation not reflective of secondary radiation produced in tissue, hence the need to include tissue equivalent shielding in studies of space radiation shielding effectiveness.

Several strategies are being studied for ameliorating the effects of this radiation hazard for planned human interplanetary spaceflight:

• Spacecraft can be constructed out of hydrogen-rich plastics, rather than aluminium.^[50]
• Material shielding has been considered:
  • Liquid hydrogen, which would be brought along as fuel in any case, tends to give relatively good shielding, while producing relatively low levels of secondary radiation. Therefore, the fuel could be placed so as to act as a form of shielding around the crew. However, as fuel is consumed by the craft, the crew's shielding decreases.
  • Water, which is necessary to sustain life, could also contribute to shielding. But it too is consumed during the journey unless waste products are utilized.^[51]
  • Asteroids could serve to provide shielding.^[52][53]
• Magnetic deflection of charged radiation particles and/or electrostatic repulsion is a hypothetical alternative to pure conventional mass shielding under investigation. In theory, power requirements for the case of a 5-meter torus drop from an excessive 10 GW for a simple pure electrostatic shield (too discharged by space electrons) to a moderate 10 kilowatts (kW) by using a hybrid design.^[48] However, such complex active shielding is untried, with workability and practicalities more uncertain than material shielding.^[48]

Special provisions would also be necessary to protect against a solar proton event, which could increase fluxes to levels that would kill a crew in hours or days rather than months or years. Potential mitigation strategies include providing a small habitable space behind a spacecraft's water supply or with particularly thick walls or providing an option to abort to the protective environment provided by the Earth's magnetosphere. The Apollo mission used a combination of both strategies. Upon receiving confirmation of an SPE, astronauts would move to the Command Module, which had thicker aluminium walls than the Lunar Module, then return to Earth. It was later determined from measurements taken by instruments flown on Apollo that the Command Module would have provided sufficient shielding to prevent significant crew harm.^{[citation needed]}

None of these strategies currently provide a method of protection that would be known to be sufficient^[54] while conforming to likely limitations on the mass of the payload at present (around $10,000/kg) launch prices. Scientists such as University of Chicago professor emeritus Eugene Parker are not optimistic it can be solved anytime soon.^[54] For passive mass shielding, the required amount could be too heavy to be affordably lifted into space without changes in economics (like hypothetical non-rocket spacelaunch or usage of extraterrestrial resources) — many hundreds of metric tons for a reasonably-sized crew compartment. For instance, a NASA design study for an ambitious large spacestation envisioned 4 metric tons per square meter of shielding to drop radiation exposure to 2.5 mSv annually (± a factor of 2 uncertainty), less than the tens of millisieverts or more in some populated high natural background radiation areas on Earth, but the sheer mass for that level of mitigation was considered practical only because it involved first building a lunar mass driver to launch material.^[47]

Several active shielding methods have been considered that might be less massive than passive shielding, but they remain speculative.^[48][55] Since the type of radiation penetrating farthest through thick material shielding, deep in interplanetary space, is GeV positively charged nuclei, a repulsive electrostatic field has been proposed, but this has problems including plasma instabilities and the power needed for an accelerator constantly keeping the charge from being neutralized by deep-space electrons.^[56] A more common proposal is magnetic shielding generated by superconductors (or plasma currents). Among the difficulties with this proposal is that, for a compact system, magnetic fields up to 10–20 teslas could be required around a manned spacecraft, higher than the several teslas in MRI machines. Such high fields can produce headaches and migraines in MRI patients, and long-duration exposure to such fields has not been studied. Opposing-electromagnet designs might cancel the field in the crew sections of the spacecraft, but would require more mass. It is also possible to use a combination of a magnetic field with an electrostatic field, with the spacecraft having zero total charge. The hybrid design would theoretically ameliorate the problems, but would be complex and possibly infeasible.^[48]

Part of the uncertainty is that the effect of human exposure to galactic cosmic rays is poorly known in quantitative terms. The NASA Space Radiation Laboratory is currently studying the effects of radiation in living organisms as well as protective shielding.

Drugs

Another line of research is the development of drugs that enhance the body's natural capacity to repair damage caused by radiation. Some of the drugs that are being considered are retinoids, which are vitamins with antioxidant properties, and molecules that retard cell division, giving the body time to fix damage before harmful mutations can be duplicated.^{[citation needed]}

Timing of missions

Due to the potential negative effects of astronaut exposure to cosmic rays, solar activity may play a role in future space travel. Because galactic cosmic ray fluxes within the Solar System are lower during periods of strong solar activity, interplanetary travel during solar maximum should minimize the average dose to astronauts.

Although the Forbush decrease effect during coronal mass ejections can temporarily lower the flux of galactic cosmic rays, the short duration of the effect (1–3 days) and the approximately 1% chance that a CME generates a dangerous solar proton event limits the utility of timing missions to coincide with CMEs.

Orbital selection

Radiation dosage from the Earth's radiation belts is typically mitigated by selecting orbits that avoid the belts or pass through them relatively quickly. For example, a low Earth orbit, with low inclination, will generally be below the inner belt.

The orbits of the Earth-Moon system Lagrange points L₂ - L₅ take them out of the protection of the Earth's magnetosphere for approximately two-thirds of the time.^{[citation needed]}

The orbits of Earth-Sun system Lagrange Points L₁ and L₃ - L₅ are always outside the protection of the Earth's magnetosphere.

Spaceflight radiation carcinogenesis

From Wikipedia, the free encyclopedia

The Phantom Torso, seen here in the Destiny laboratory on the International Space Station (ISS), is designed to measure the effects of radiation on organs inside the body by using a torso that is similar to those used to train radiologists on Earth. The torso is equivalent in height and weight to an average adult male. It contains radiation detectors that will measure, in real-time, how much radiation the brain, thyroid, stomach, colon, and heart and lung area receive on a daily basis. The data will be used to determine how the body reacts to and shields its internal organs from radiation, which will be important for longer duration space flights.

Astronauts are exposed to approximately 50-2,000 millisieverts (mSv) while on six-month-duration missions to the International Space Station (ISS), the moon and beyond. The risk of cancer caused by ionizing radiation is well documented at radiation doses beginning at 50 mSv and above (DJS -- in the reference for this claim, Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2, the Public Summary states,  "At doses less than 40 times the average yearly background exposure (100 mSv), statistical limitation make it difficult to evaluate cancer risk in humans").

Related radiological effect studies have shown that survivors of the atomic bomb explosions in Hiroshima and Nagasaki, nuclear reactor workers and patients who have undergone therapeutic radiation treatments have received low-linear energy transfer (LET) radiation (x-rays and gamma rays) doses in the same 50-2000 mSv range.

Composition of space radiation

While in space, astronauts are exposed to radiation which is mostly composed of high-energy protons, helium nuclei (alpha particles), and high-atomic-number ions (HZE ions), as well as secondary radiation from nuclear reactions from spacecraft parts or tissue.

The ionization patterns in molecules, cells, tissues and the resulting biological effects are distinct from typical terrestrial radiation (x-rays and gamma rays, which are low-LET radiation). Galactic cosmic rays (GCRs) from outside the Milky Way galaxy consist mostly of highly energetic protons with a small component of HZE ions.

Prominent HZE ions:

• Carbon (C)
• Oxygen (O)
• Magnesium (Mg)
• Silicon (Si)
• Iron (Fe)

GCR energy spectra peaks (with median energy peaks up to 1,000 MeV/amu) and nuclei (energies up to 10,000 MeV/amu) are important contributors to the dose equivalent.

Uncertainties in cancer projections

One of the main roadblocks to interplanetary travel is the risk of cancer caused by radiation exposure. The largest contributors to this roadblock are: (1) The large uncertainties associated with cancer risk estimates, (2) The unavailability of simple and effective countermeasures and (3) The inability to determine the effectiveness of countermeasures. Operational parameters that need to be optimized to help mitigate these risks include:

• length of space missions
• crew age
• crew gender
• shielding
• biological countermeasures

Major uncertainties

• effects on biological damage related to differences between space radiation and x-rays
• dependence of risk on dose-rates in space related to the biology of DNA repair, cell regulation and tissue responses
• predicting solar particle events (SPEs)
• extrapolation from experimental data to humans and between human populations
• individual radiation sensitivity factors (genetic, epigenetic, dietary or "healthy worker" effects)

Minor uncertainties

• data on galactic cosmic ray environments
• physics of shielding assessments related to transmission properties of radiation through materials and tissue
• microgravity effects on biological responses to radiation
• errors in human data (statistical, dosimetry or recording inaccuracies)

Quantitative methods have been developed to propagate uncertainties that contribute to cancer risk estimates. The contribution of microgravity effects on space radiation has not yet been estimated, but it is expected to be small. The effects of changes in oxygen levels or in immune dysfunction on cancer risks are largely unknown and are of great concern during space flight.

Types of cancer caused by radiation exposure

Studies are being conducted on populations accidentally exposed to radiation (such as Chernobyl, production sites, and Hiroshima and Nagasaki). These studies show strong evidence for cancer morbidity as well as mortality risks at more than 12 tissue sites. The largest risks for adults who have been studied include several types of leukemia, including myeloid leukemia and acute lymphatic lymphoma  as well as tumors of the lung, breast, stomach, colon, bladder and liver. Inter-gender variations are very likely due to the differences in the natural incidence of cancer in males and females. Another variable is the additional risk for cancer of the breast, ovaries and lungs in females. There is also evidence of a declining risk of cancer caused by radiation with increasing age, but the magnitude of this reduction above the age of 30 is uncertain.

It is unknown whether high-LET radiation could cause the same types of tumors as low-LET radiation, but differences should be expected.

The ratio of a dose of high-LET radiation to a dose of x-rays or gamma rays that produce the same biological effect are called relative biological effectiveness (RBE) factors. The types of tumors in humans who are exposed to space radiation will be different from those who are exposed to low-LET radiation. This is evidenced by a study that observed mice with neutrons and have RBEs that vary with the tissue type and strain.

Approaches for setting acceptable risk levels

The various approaches to setting acceptable levels of radiation risk are summarized below:

Comparison of Radiation Doses - includes the amount detected on the trip from Earth to Mars by the RAD on the MSL (2011 - 2013).

• Unlimited Radiation Risk - NASA management, the families of loved ones of astronauts, and taxpayers would find this approach unacceptable.
• Comparison to Occupational Fatalities in Less-safe Industries - The life-loss from attributable radiation cancer death is less than that from most other occupational deaths. At this time, this comparison would also be very restrictive on ISS operations because of continued improvements in ground-based occupational safety over the last 20 years.
• Comparison to Cancer Rates in General Population - The number of years of life-loss from radiation-induced cancer deaths can be significantly larger than from cancer deaths in the general population, which often occur late in life (> age 70 years) and with significantly less numbers of years of life-loss.
• Doubling Dose for 20 Years Following Exposure - Provides a roughly equivalent comparison based on life-loss from other occupational risks or background cancer fatalities during a worker's career, however, this approach negates the role of mortality effects later in life.
• Use of Ground-based Worker Limits - Provides a reference point equivalent to the standard that is set on Earth, and recognizes that astronauts face other risks. However, ground workers remain well below dose limits, and are largely exposed to low-LET radiation where the uncertainties of biological effects are much smaller than for space radiation.

NCRP Report No. 153 provides a more recent review of cancer and other radiation risks. This report also identifies and describes the information needed to make radiation protection recommendations beyond LEO, contains a comprehensive summary of the current body of evidence for radiation-induced health risks and also makes recommendations on areas requiring future experimentation.

Current permissible exposure limits

Career cancer risk limits

Astronauts' radiation exposure limit is not to exceed 3% of the risk of exposure-induced death (REID) from fatal cancer over their career. It is NASA's policy to ensure a 95% confidence level (CL) that this limit is not exceeded. These limits are applicable to all missions in low Earth orbit (LEO) as well as lunar missions that are less than 180 days in duration. In the United States, the legal occupational exposure limits for adult workers is set at an effective dose of 50 mSv.

Cancer risk to dose relationship

The relationship between radiation exposure and risk is both age- and gender-specific due to latency effects and differences in tissue types, sensitivities, and life spans between genders. These relationships are estimated using the methods that are recommended by the NCRP and more recent radiation epidemiology information. The principle of As Low As Reasonably Achievable

The as low as reasonably achievable (ALARA) principle is a legal requirement intended to ensure astronaut safety. An important function of ALARA is to ensure that astronauts do not approach radiation limits and that such limits are not considered as "tolerance values." ALARA is especially important for space missions in view of the large uncertainties in cancer and other risk projection models. Mission programs and terrestrial occupational procedures resulting in radiation exposures to astronauts are required to find cost-effective approaches to implement ALARA.

Evaluating career limits

Organ (T)	Tissue weighting factor (wT)
Gonads	0.20
Bone Marrow (red)	0.12
Colon	0.12
Lung	0.12
Stomach	0.12
Bladder	0.05
Breast	0.05
Liver	0.05
Esophagus	0.05
Thyroid	0.05
Skin	0.01
Bone Surface	0.01
Remainder*	0.05
*Adrenals, brain, upper intestine, small intestine, kidney, muscle, pancreas, spleen, thymus and uterus.

The risk of cancer is calculated by using radiation dosimetry and physics methods.

For the purpose of determining radiation exposure limits at NASA, the probability of fatal cancer is calculated as shown below:

1. The body is divided into a set of sensitive tissues, and each tissue, T, is assigned a weight, w_T, according to its estimated contribution to cancer risk.
2. The absorbed dose, D_γ, that is delivered to each tissue is determined from measured dosimetry. For the purpose of estimating radiation risk to an organ, the quantity characterizing the ionization density is the LET (keV/μm).
3. For a given interval of LET, between L and ΔL, the dose-equivalent risk (in units of sievert) to a tissue, T, H_γ(L) is calculated as
   ${\displaystyle H_{\gamma }(L)=Q(L)D_{\gamma }(L)}$
   where the quality factor, Q(L), is obtained according to the International Commission on Radiological Protection (ICRP).
4. The average risk to a tissue, T, due to all types of radiation contributing to the dose is given by ${\displaystyle H_{\gamma }=\int D_{\gamma }(L)Q(L)dL}$
   or, since ${\displaystyle D_{\gamma }(L)=LF_{\gamma }(L)}$, where F_γ(L) is the fluence of particles with LET=L, traversing the organ,
   ${\displaystyle H_{\gamma }=\int dLQ(L)F_{\gamma }(L)L}$
5. The effective dose is used as a summation over radiation type and tissue using the tissue weighting factors, w_γ
   ${\displaystyle E=\sum _{\gamma }w_{\gamma }H_{\gamma }}$
6. For a mission of duration t, the effective dose will be a function of time, E(t), and the effective dose for mission i will be
   ${\displaystyle E_{i}=\int E(t)dt}$
7. The effective dose is used to scale the mortality rate for radiation-induced death from the Japanese survivor data, applying the average of the multiplicative and additive transfer models for solid cancers and the additive transfer model for leukemia by applying life-table methodologies that are based on U.S. population data for background cancer and all causes of death mortality rates. A dose-dose rate effectiveness factor (DDREF) of 2 is assumed.

Evaluating cumulative radiation risks

The cumulative cancer fatality risk (%REID) to an astronaut for occupational radiation exposures, N, is found by applying life-table methodologies that can be approximated at small values of %REID by summing over the tissue-weighted effective dose, E_i, as

${\displaystyle Risk=\sum _{i=1}^{N}E_{i}R_{0}(age_{i},gender)}$

where R₀ are the age- and gender- specific radiation mortality rates per unit dose.

For organ dose calculations, NASA uses the model of Billings et al. to represent the self-shielding of the human body in a water-equivalent mass approximation. Consideration of the orientation of the human body relative to vehicle shielding should be made if it is known, especially for SPEs.

Confidence levels for career cancer risks are evaluated using methods that are specified by the NPRC in Report No. 126. These levels were modified to account for the uncertainty in quality factors and space dosimetry.

The uncertainties that were considered in evaluating the 95% confidence levels are the uncertainties in:

• Human epidemiology data, including uncertainties in
  • statistics limitations of epidemiology data
  • dosimetry of exposed cohorts
  • bias, including misclassification of cancer deaths, and
  • the transfer of risk across populations.
• The DDREF factor that is used to scale acute radiation exposure data to low-dose and dose-rate radiation exposures.
• The radiation quality factor (Q) as a function of LET.
• Space dosimetry

The so-called "unknown uncertainties" from the NCRP report No. 126 are ignored by NASA.

Models of cancer risks and uncertainties

Life-table methodology

The double-detriment life-table approach is what is recommended by the NPRC to measure radiation cancer mortality risks. The age-specific mortality of a population is followed over its entire life span with competing risks from radiation and all other causes of death described.

For a homogenous population receiving an effective dose E at age a_E, the probability of dying in the age-interval from a to a+1 is described by the background mortality-rate for all causes of death, M(a), and the radiation cancer mortality rate, m(E,a_E,a), as:

${\displaystyle q(E,a_{E},a)={\frac {M(a)+m(E,a_{E},a)}{1+{\frac {1}{2}}\left[M(a)+m(E,a_{E},a)\right]}}}$

The survival probability to age, a, following an exposure, E at age a_E, is:

${\displaystyle S(E,a_{E},a)=\prod _{u=a_{E}}^{a-1}\left[1-q(E,a_{E},u)\right]}$

The excessive lifetime risk (ELR - the increased probability that an exposed individual will die from cancer) is defined by the difference in the conditional survival probabilities for the exposed and the unexposed groups as:

${\displaystyle ELR=\sum _{a=a_{E}}^{\infty }\left[M(a)+m(E,a_{E},a)\right]S(E,a_{E},a)-\sum _{a=a_{E}}^{\infty }M(a)S(0,a_{E},a)}$

A minimum latency-time of 10 years is often used for low-LET radiation. Alternative assumptions should be considered for high-LET radiation. The REID (the lifetime risk that an individual in the population will die from cancer caused by radiation exposure) is defined by:

${\displaystyle REID=\sum _{a=a_{E}}^{\infty }m(E,a_{E},a)S(E,a_{E},a)}$

Generally, the value of the REID exceeds the value of the ELR by 10-20%.

The average loss of life-expectancy, LLE, in the population is defined by:

${\displaystyle LLE=\sum _{a=a_{E}}^{\infty }S(0,a_{E},a)-\sum _{a=a_{E}}^{\infty }S(E,a_{E},a)}$

The loss of life-expectancy among exposure-induced-deaths (LLE-REID) is defined by:

${\displaystyle LLE-REID={\frac {LLE}{REID}}}$

Uncertainties in low-LET epidemiology data

The low-LET mortality rate per sievert, m_i is written

${\displaystyle m(E,a_{x},a)={\frac {m_{0}(E,a_{x},a)}{DDREF}}{\frac {x_{D}x_{s}x_{T}x_{B}}{x_{Dr}}}}$

where m₀ is the baseline mortality rate per sievert and x_α are quantiles (random variables) whose values are sampled from associated probability distribution functions (PDFs), P(X_a).

NCRP, in Report No. 126, defines the following subjective PDFs, P(X_a), for each factor that contributes to the acute low-LET risk projection:

1. P_dosimetry is the random and systematic errors in the estimation of the doses received by atomic-bomb blast survivors.
2. P_statistical is the distribution in uncertainty in the point estimate of the risk coefficient, r₀.
3. P_bias is any bias resulting for over- or under-reporting cancer deaths.
4. P_transfer is the uncertainty in the transfer of cancer risk following radiation exposure from the Japanese population to the U.S. population.
5. P_Dr is the uncertainty in the knowledge of the extrapolation of risks to low dose and dose-rates, which are embodied in the DDREF.

Risk in context of exploration mission operational scenarios

The accuracy of galactic cosmic ray environmental models, transport codes and nuclear interaction cross sections allow NASA to predict space environments and organ exposure that may be encountered on long-duration space missions. The lack of knowledge of the biological effects of radiation exposure raise major questions about risk prediction.

The cancer risk projection for space missions is found by

${\displaystyle m_{J}(E,a_{E},a)_{lJ}(E,a_{E},a)\int dL{\frac {dF}{dL}}LQ_{trial-J}(L)X_{L-J}}$

where ${\displaystyle {\frac {dF}{dL}}}$ represents the folding of predictions of tissue-weighted LET spectra behind spacecraft shielding with the radiation mortality rate to form a rate for trial J.

Alternatively, particle-specific energy spectra, F_j(E), for each ion, j, can be used.

${\displaystyle m_{J}(E,a_{E},a)=m_{lJ}(E,a_{E},a)\sum _{j}(E)L(E)Q_{trial-J}(L(E))x_{L-J}}$

The result of either of these equations is inserted into the expression for the REID.

Related probability distribution functions (PDFs) are grouped together into a combined probability distribution function, P_cmb(x). These PDFs are related to the risk coefficient of the normal form (dosimetry, bias and statistical uncertainties). After a sufficient number of trials have been completed (approximately 10⁵), the results for the REID estimated are binned and the median values and confidence intervals are found.

The chi-squared (χ²) test is used for determining whether two separate PDFs are significantly different (denoted p₁(R_i) and p₂(R_i), respectively). Each p(R_i) follows a Poisson distribution with variance ${\displaystyle {\sqrt {p(R_{i})}}}$.

The χ² test for n-degrees of freedom characterizing the dispersion between the two distributions is

${\displaystyle \chi ^{2}=\sum _{n}{\frac {\left[p_{1}(R_{n})-p_{2}(R_{n})\right]^{2}}{\sqrt {p_{1}^{2}(R_{n})+p_{2}^{2}(R_{n})}}}}$.

The probability, P(ņχ²), that the two distributions are the same is calculated once χ² is determined.

Radiation carcinogenesis mortality rates

Age-and gender-dependent mortality rare per unit dose, multiplied by the radiation quality factor and reduced by the DDREF is used for projecting lifetime cancer fatality risks. Acute gamma ray exposures are estimated. The additivity of effects of each component in a radiation field is also assumed.

Rates are approximated using data gathered from Japanese atomic bomb survivors. There are two different models that are considered when transferring risk from Japanese to U.S. populations.

• Multiplicative transfer model - assumes that radiation risks are proportional to spontaneous or background cancer risks.
• Additive transfer model - assumes that radiation risk acts independently of other cancer risks.

The NCRP recommends a mixture model to be used that contains fractional contributions from both methods.^[9]

The radiation mortality rate is defined as:

${\displaystyle m(E,a_{E},a)=\left[ERR(a_{E},a)M_{c}(a)+(1-v)EAR(a_{E},a)\right]{\frac {\sum _{L}Q(L)F(L)L}{DDREF}}}$

Where:

• ERR = excess relative risk per sievert
• EAR = excess additive risk per sievert
• M_c(a) = the gender- and age-specific cancer mortality rate in the U.S. population
• F = the tissue-weighted fluence
• L = the LET
• v = the fractional division between the assumption of the multiplicative and additive risk transfer models. For solid cancer, it is assumed that v=1/2 and for leukemia, it is assumed that v=0.

Biological and physical countermeasures

Identifying effective countermeasures that reduce the risk of biological damage is still a long-term goal for space researchers. These countermeasures are probably not needed for extended duration lunar missions, but will be needed for other long-duration missions to Mars and beyond. On 31 May 2013, NASA scientists reported that a possible manned mission to Mars may involve a great radiation risk based on the amount of energetic particle radiation detected by the RAD on the Mars Science Laboratory while traveling from the Earth to Mars in 2011-2012.

There are three fundamental ways to reduce exposure to ionizing radiation:

• increasing the distance from the radiation source
• reducing the exposure time
• shielding (i.e.: a physical barrier)

Shielding is a plausible option, but due to current launch mass restrictions, it is prohibitively costly. Also, the current uncertainties in risk projection prevent the actual benefit of shielding from being determined. Strategies such as drugs and dietary supplements to reduce the effects of radiation, as well as the selection of crew members are being evaluated as viable options for reducing exposure to radiation and effects of irradiation. Shielding is an effective protective measure for terrestrial radiation workers. In space, high-energy radiation is very penetrating and the effectiveness of radiation shielding depends on the atomic make-up of the material used.

Antioxidants are effectively used to prevent the damage caused by radiation injury and oxygen poisoning (the formation of reactive oxygen species), but since antioxidants work by rescuing cells from a particular form of cell death (apoptosis), they may not protect against damaged cells that can initiate tumor growth.

Evidence sub-pages

The evidence and updates to projection models for cancer risk from low-LET radiation are reviewed periodically by several bodies, which include the following organizations:

• The NAS Committee on the Biological Effects of Ionizing Radiation
• The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR)
• The ICRP
• The NCRP

These committees release new reports about every 10 years on cancer risks that are applicable to low-LET radiation exposures. Overall, the estimates of cancer risks among the different reports of these panels will agree within a factor of two or less. There is continued controversy for doses that are below 5 mSv, however, and for low dose-rate radiation because of debate over the linear no-threshold hypothesis that is often used in statistical analysis of these data. The BEIR VII report, which is the most recent of the major reports is used in the following sub-pages. Evidence for low-LET cancer effects must be augmented by information on protons, neutrons, and HZE nuclei that is only available in experimental models. Such data have been reviewed by NASA several times in the past and by the NCRP.

• Epidemiology data for low linear energy transfer radiation
• Radiobiology evidence for protons and HZE nuclei
• Radiation carcinogenesis in past space missions