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Astronauts are exposed to approximately 50-2,000
millisievert The sievert (symbol: SvNot be confused with the sverdrup or the svedberg, two non-SI units that sometimes use the same symbol.) is a unit in the International System of Units (SI) intended to represent the stochastic health risk of ionizing radi ...
s (mSv) while on six-month-duration missions to the
International Space Station The International Space Station (ISS) is the largest modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos (Russia), JAXA ...
(ISS), the Moon and beyond. The risk of cancer caused by
ionizing radiation Ionizing radiation (or ionising radiation), including nuclear radiation, consists of subatomic particles or electromagnetic waves that have sufficient energy to ionize atoms or molecules by detaching electrons from them. Some particles can travel ...
is well documented at radiation doses beginning at 100mSv and above. Related radiological effect studies have shown that survivors of the atomic bomb explosions in
Hiroshima is the capital of Hiroshima Prefecture in Japan. , the city had an estimated population of 1,199,391. The gross domestic product (GDP) in Greater Hiroshima, Hiroshima Urban Employment Area, was US$61.3 billion as of 2010. Kazumi Matsui h ...
and
Nagasaki is the capital and the largest city of Nagasaki Prefecture on the island of Kyushu in Japan. It became the sole port used for trade with the Portuguese and Dutch during the 16th through 19th centuries. The Hidden Christian Sites in the ...
, nuclear reactor workers and patients who have undergone therapeutic radiation treatments have received low-
linear energy transfer In dosimetry, linear energy transfer (LET) is the amount of energy that an ionizing particle transfers to the material traversed per unit distance. It describes the action of radiation into matter. It is identical to the retarding force acting on ...
(LET) radiation (
x-rays An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 Picometre, picometers to 10 Nanometre, nanometers, corresponding to frequency, ...
and
gamma rays A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...
) doses in the same 50-2,000 mSv range.


Composition of space radiation

While in space, astronauts are exposed to radiation which is mostly composed of high-energy
protons A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
,
helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
nuclei (
alpha particle Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be produce ...
s), and high-atomic-number ions (
HZE ions HZE ions are the high-energy nuclei component of galactic cosmic rays (GCRs) which have an electric charge of +3  or greater – that is, they must be the nucleii of heavier elements than hydrogen or helium. The abbreviation "HZE" comes fro ...
), as well as
secondary radiation A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically sh ...
from nuclear reactions from spacecraft parts or tissue. The
ionization Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule i ...
patterns in molecules, cells, tissues and the resulting biological effects are distinct from typical terrestrial radiation (
x-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10  picometers to 10  nanometers, corresponding to frequencies in the range 30&nb ...
s and
gamma ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...
s, which are low-LET radiation).
Galactic cosmic rays Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...
(GCRs) from outside the
Milky Way galaxy The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...
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 In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy gained by a single electron accelerating from rest through an Voltage, electric potential difference of one volt i ...
/ amu) and nuclei (energies up to 10,000 MeV/amu) are important contributors to the
dose equivalent Equivalent dose is a dose quantity '' H '' representing the stochastic health effects of low levels of ionizing radiation on the human body which represents the probability of radiation-induced cancer and genetic damage. It is derived from the ph ...
.


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 sex * 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 DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA dam ...
, cell regulation and tissue responses * predicting solar particle events (SPEs) *
extrapolation In mathematics, extrapolation is a type of estimation, beyond the original observation range, of the value of a variable on the basis of its relationship with another variable. It is similar to interpolation, which produces estimates between know ...
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 The term micro-g environment (also μg, often referred to by the term microgravity) is more or less synonymous with the terms ''weightlessness'' and ''zero-g'', but emphasising that g-forces are never exactly zero—just very small (on the I ...
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 An immune disorder is a dysfunction of the immune system. These disorders can be characterized in several different ways: * By the component(s) of the immune system affected * By whether the immune system is overactive or underactive * By whether t ...
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 Chernobyl ( , ; russian: Чернобыль, ) or Chornobyl ( uk, Чорнобиль, ) is a partially abandoned city in the Chernobyl Exclusion Zone, situated in the Vyshhorod Raion of northern Kyiv Oblast, Ukraine. Chernobyl is about no ...
, production sites, and
Hiroshima is the capital of Hiroshima Prefecture in Japan. , the city had an estimated population of 1,199,391. The gross domestic product (GDP) in Greater Hiroshima, Hiroshima Urban Employment Area, was US$61.3 billion as of 2010. Kazumi Matsui h ...
and
Nagasaki is the capital and the largest city of Nagasaki Prefecture on the island of Kyushu in Japan. It became the sole port used for trade with the Portuguese and Dutch during the 16th through 19th centuries. The Hidden Christian Sites in the ...
). 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 Leukemia ( also spelled leukaemia and pronounced ) is a group of blood cancers that usually begin in the bone marrow and result in high numbers of abnormal blood cells. These blood cells are not fully developed and are called ''blasts'' or ' ...
, including
myeloid leukemia Myeloid leukemia is a type of leukemia affecting myeloid tissue. Types include: * Acute myeloid leukemia * Chronic myelogenous leukemia * Acute megakaryoblastic leukemia * Blastic plasmacytoid dendritic cell neoplasm See also * Hematological m ...
and acute lymphatic lymphoma as well as tumors of the
lung The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...
,
breast The breast is one of two prominences located on the upper ventral region of a primate's torso. Both females and males develop breasts from the same embryological tissues. In females, it serves as the mammary gland, which produces and secret ...
,
stomach The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the digestive system. The stomach i ...
, colon,
bladder The urinary bladder, or simply bladder, is a hollow organ in humans and other vertebrates that stores urine from the kidneys before disposal by urination. In humans the bladder is a distensible organ that sits on the pelvic floor. Urine enters ...
and
liver The liver is a major Organ (anatomy), organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of proteins and biochemicals necessary for ...
. Inter-sex 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 In radiobiology, the relative biological effectiveness (often abbreviated as RBE) is the ratio of biological effectiveness of one type of ionizing radiation relative to another, given the same absorbed dose, amount of absorbed energy. The RBE i ...
(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
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...
s and have RBEs that vary with the tissue type and strain.


Measured rate of cancer among astronauts

The measured change rate of cancer is restricted by limited statistics. A study published in Scientific Reports looked over 301 U.S. astronauts and 117 Soviet and Russian cosmonauts, and found no measurable increase in cancer mortality compared to the general population, as reported by LiveScience. An earlier 1998 study came to similar conclusions, with no statistically significant increase in cancer among astronauts compared to the reference group.


Approaches for setting acceptable risk levels

The various approaches to setting acceptable levels of radiation risk are summarized below: * 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 In frequentist statistics, a confidence interval (CI) is a range of estimates for an unknown parameter. A confidence interval is computed at a designated ''confidence level''; the 95% confidence level is most common, but other levels, such as 9 ...
(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 annually.


Cancer risk to dose relationship

The relationship between radiation exposure and risk is both age- and sex-specific due to latency effects and differences in tissue types, sensitivities, and life spans between sexes. 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

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: # The body is divided into a set of sensitive tissues, and each tissue, ''T'', is assigned a weight, ''wT'', according to its estimated contribution to cancer risk. # The absorbed dose, ''Dγ'', 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). # For a given interval of LET, between L and ΔL, the dose-equivalent risk (in units of
sievert The sievert (symbol: SvNot be confused with the sverdrup or the svedberg, two non-SI units that sometimes use the same symbol.) is a unit in the International System of Units (SI) intended to represent the stochastic health risk of ionizing radi ...
) to a tissue, ''T'', ''Hγ(L)'' is calculated as
H_\gamma (L) = Q(L)D_\gamma (L)
where the quality factor, Q(L), is obtained according to th
International Commission on Radiological Protection (ICRP)
# The average risk to a tissue, ''T'', due to all types of radiation contributing to the dose is given by
H_\gamma = \int D_\gamma (L)Q(L)dL
or, since D_\gamma (L) = LF_\gamma (L), where ''Fγ(L)'' is the fluence of particles with ''LET=L'', traversing the organ,
H_\gamma = \int dLQ(L)F_\gamma (L)L # The effective dose is used as a summation over radiation type and tissue using the tissue weighting factors, ''wγ''
E=\sum_\gamma w_\gamma H_\gamma # 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
E_i = \int E(t) dt # 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, ''Ei'', as
:Risk = \sum_^N E_i R_0 (age_i, sex) where ''R0'' are the age- and sex- 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 th
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 aE, 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,aE,a)'', as: :q(E,a_E,a)=\frac The survival probability to age, ''a'', following an exposure, ''E'' at age ''aE'', is: :S(E,a_E,a)= \prod_^ \left 1-q(E,a_E,u)\right/math> 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: :ELR=\sum_^ \infty \left (a)+m(E,a_E,a)\right(E,a_E,a)-\sum_^ \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: :REID=\sum_^\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: :LLE=\sum_^\infty S(0,a_E,a) - \sum_^\infty S(E,a_E,a) The loss of life-expectancy among exposure-induced-deaths (LLE-REID) is defined by: :LLE-REID=\frac


Uncertainties in low-LET epidemiology data

The low-LET mortality rate per sievert, ''mi'' is written
:m(E,a_x,a) = \frac \frac where ''m0'' is the baseline mortality rate per sievert and ''xα'' are
quantile In statistics and probability, quantiles are cut points dividing the range of a probability distribution into continuous intervals with equal probabilities, or dividing the observations in a sample in the same way. There is one fewer quantile tha ...
s (random variables) whose values are sampled from associated probability distribution functions (PDFs), ''P(Xa)''. NCRP, in Report No. 126, defines the following subjective PDFs, ''P(Xa)'', for each factor that contributes to the acute low-LET risk projection: # ''Pdosimetry'' is the random and systematic errors in the estimation of the doses received by atomic-bomb blast survivors. # ''Pstatistical'' is the distribution in uncertainty in the point estimate of the risk coefficient, ''r0''. # ''Pbias'' is any bias resulting for over- or under-reporting cancer deaths. # ''Ptransfer'' is the uncertainty in the transfer of cancer risk following radiation exposure from the Japanese population to the U.S. population. # ''PDr'' 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
:m_J(E,a_E,a)_(E,a_E,a) \int dL \fracLQ_(L)X_ where \frac 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, ''Fj(E)'', for each ion, ''j'', can be used
:m_J (E,a_E,a) = m_(E,a_E,a) \sum_j (E)L(E)Q_(L(E))x_. 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, ''Pcmb(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(Ri)'' and ''p2(Ri)'', respectively). Each ''p(Ri)'' follows a Poisson distribution with variance \sqrt. The χ2 test for n-degrees of freedom characterizing the dispersion between the two distributions is
:\chi^2=\sum_n \frac. The probability, ''P(ņχ2)'', that the two distributions are the same is calculated once χ2 is determined.


Radiation carcinogenesis mortality rates

Age-and sex-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. The radiation mortality rate is defined as: : m(E,a_E,a)=\left RR(a_E, a)M_c(a)+(1-v)EAR(a_E,a)\right/math> Where: * ERR = excess relative risk per sievert * EAR = excess additive risk per sievert * Mc(a) = the sex- 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
human mission to Mars The idea of sending humans to Mars has been the subject of aerospace engineering and scientific studies since the late 1940s as part of the broader exploration of Mars. Some have also considered exploring the Martian moons of Phobos and Deimos. ...
may involve a great radiation risk based on the amount of energetic particle radiation detected by the
RAD RAD or Rad may refer to: People * Robert Anthony Rad Dougall (born 1951), South African former racing driver * Rad Hourani, Canadian fashion designer and artist * Nickname of Leonardus Rad Kortenhorst (1886–1963), Dutch politician * Radley R ...
on the
Mars Science Laboratory Mars Science Laboratory (MSL) is a robotic spacecraft, robotic space probe mission to Mars launched by NASA on November 26, 2011, which successfully landed ''Curiosity (rover), Curiosity'', a Mars rover, in Gale (crater), Gale Crater on August ...
while traveling from the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
to
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
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 solar particle events. As far as shielding from GCR, high-energy radiation is very penetrating and the effectiveness of radiation shielding depends on the atomic make-up of the material used.
Antioxidant Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. This can lead to polymerization and other chain reactions. They are frequently added to industrial products, such as fuels and lubricant ...
s 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 The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) was set up by resolution of the United Nations General Assembly in 1955. 21 states are designated to provide scientists to serve as members of the committee which ...
(UNSCEAR) * The
ICRP The International Commission on Radiological Protection (ICRP) is an independent, international, non-governmental organization, with the mission to protect people, animals, and the environment from the harmful effects of ionising radiation. Its r ...
* 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


See also

* Central nervous system effects from radiation exposure during spaceflight *
Dosimetry Radiation dosimetry in the fields of health physics and radiation protection is the measurement, calculation and assessment of the ionizing radiation dose absorbed by an object, usually the human body. This applies both internally, due to ingested o ...
*
Health threat from cosmic rays Health threats from cosmic rays are the dangers posed by cosmic rays to astronauts on interplanetary missions or any missions that venture through the Van-Allen Belts or outside the Earth's magnetosphere. They are one of the greatest barriers ...
* Radiation Syndrome *
Radiation protection Radiation protection, also known as radiological protection, is defined by the International Atomic Energy Agency (IAEA) as "The protection of people from harmful effects of exposure to ionizing radiation, and the means for achieving this". Expos ...


References


External links

* * * * *
The Health Risks of Extraterrestrial Environments
{{Space medicine Space medicine