View source for Effects of ionizing radiation in spaceflight

{{Short description|Cancer causing exposure to ionizing radiation in spaceflight}}
{{cleanup|reason = This article reads as if it is a research paper and not an encyclopedia article (in part because it is mostly cut and paste from the public domain study by NASA). It needs to be moved to a more encyclopedic topic heading, perhaps something like [[Cancer and spaceflight]], and re-written in an encyclopedia article style.|date=July 2012}}
{{Use dmy dates|date=September 2024}}
[[File:iss002e5952.jpg|thumb|300px|The Phantom Torso, as 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 72 [[millisievert]]s (mSv) while on six-month-duration missions to the [[International Space Station]] (ISS). Longer 3-year missions to Mars, however, have the potential to expose astronauts to radiation in excess of 1,000 mSv. Without the protection provided by Earth's magnetic field, the rate of exposure is dramatically increased. <ref name="Cucinotta and Durante 2006">{{cite journal |last=Cucinotta |first=FA |author2=Durante, M |title=Cancer risk from exposure to galactic cosmic rays: implications for space exploration by human beings |journal=Lancet Oncol. |year=2006 |volume=7 |pages=431–435 |doi=10.1016/S1470-2045(06)70695-7 |issue=5 |pmid=16648048 |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080029284.pdf}}</ref><ref name="Cucinotta 2008">{{cite journal|last=Cucinotta|first=FA|author2=Kim, MH |author3=Willingham, V |author4= George, KA |title=Physical and biological organ dosimetry analysis for international space station astronauts|journal=Radiation Research|date=Jul 2008|volume=170|issue=1|pages=127–38|pmid=18582161|doi=10.1667/RR1330.1|bibcode=2008RadR..170..127C|s2cid=44808142}}</ref>{{Failed verification|date=October 2015|reason=The 2 Sv figure is a figure for an theoretical unshielded Mars mission and not any actual six-month mission}} The risk of cancer caused by [[ionizing radiation]] is well documented at radiation doses beginning at 100 mSv and above.<ref name="Cucinotta and Durante 2006" /><ref name="Durante 2008">{{cite journal|last=Durante|first=M|author2=Cucinotta, FA |title=Heavy ion carcinogenesis and human space exploration|journal=Nature Reviews. Cancer|date=June 2008|volume=8|issue=6|pages=465–72|pmid=18451812|url=http://nix.nasa.gov/search.jsp?R=20080012531&qs=N%3D4294595076%2B4294290746|archive-url=https://web.archive.org/web/20160304235504/http://nix.nasa.gov/search.jsp?R=20080012531&qs=N%3D4294595076%2B4294290746|url-status=dead|archive-date=2016-03-04|doi=10.1038/nrc2391|hdl=2060/20080012531|s2cid=8394210|hdl-access=free}}</ref><ref name="BIER 2006">{{cite book |author=Committee to assess Health Risks from Exposure to Low levels of Ionizing Radiation |title=Health risks from exposure to low levels of ionizing radiation: BIER VII - Phase 2|year=2006|publisher=The National Academies Press|location=Washington, D.C.|url=http://www.nap.edu/openbook.php?isbn=030909156X|doi=10.17226/11340|isbn=978-0-309-09156-5}}</ref>

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 [[Radiation therapy|therapeutic radiation treatments]] have received low-[[linear energy transfer]] (LET) radiation ([[x-rays]] and [[gamma rays]]) doses in the same 50-2,000 mSv range.<ref>{{cite web|last=Cucinotta|first=F.A.|title=Risk of Radiation Carcinogenesis|url=http://humanresearchroadmap.nasa.gov/evidence/reports/Carcinogenesis.pdf|work=Human Health and Performance Risks of Space Exploration Missions Evidence reviewed by the NASA Human Research Program|publisher=NASA|access-date=6 June 2012|author2=Durante, M. |page=121}}</ref>

==Composition of space radiation==
While in space, astronauts are exposed to radiation which is mostly composed of high-energy [[protons]], [[helium]] nuclei ([[alpha particle]]s), and high-atomic-number ions ([[HZE ions]]), as well as [[secondary radiation]] from nuclear reactions from spacecraft parts or tissue.<ref name=122-123 />

The [[ionization]] patterns in molecules, cells, tissues and the resulting [[Radiobiology|biological effects]] are distinct from typical terrestrial radiation ([[x-ray]]s and [[gamma ray]]s, 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.<ref name=122-123 />

Prominent HZE ions:
* [[Carbon|Carbon (C)]]
* [[Oxygen|Oxygen (O)]]
* [[Magnesium|Magnesium (Mg)]]
* [[Silicon|Silicon (Si)]]
* [[Iron|Iron (Fe)]]
GCR energy spectra peaks (with median energy peaks up to 1,000 [[Electronvolt|MeV]]/[[Unified atomic mass unit|amu]]) and nuclei (energies up to 10,000 MeV/amu) are important contributors to the [[dose equivalent]].<ref name=122-123 /><ref>{{cite web|title=Galactic Cosmic Rays|url=http://helios.gsfc.nasa.gov/gcr.html|archive-url=https://web.archive.org/web/19981202132731/http://helios.gsfc.nasa.gov/gcr.html|url-status=dead|archive-date=2 December 1998|publisher=NASA|access-date=6 June 2012}}</ref>

==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.<ref name="122-123">{{cite web|last=Cucinotta|first=F.A.|title=Risk of Radiation Carcinogenesis|url=http://humanresearchroadmap.nasa.gov/evidence/reports/Carcinogenesis.pdf|work=Human Health and Performance Risks of Space Exploration Missions Evidence reviewed by the NASA Human Research Program|publisher=NASA|access-date=6 June 2012|author2=Durante, M. |pages=122–123}}</ref>
Operational parameters that need to be optimized to help mitigate these risks include:<ref name=122-123 />
* length of space missions
* crew age
* crew sex
* shielding
* biological countermeasures

===Major uncertainties===
Source:<ref name=122-123 />

* 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===
Source:<ref name=122-123 />

* 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. However as microgravity has been shown to modulate cancer progression, more research is needed into the combined effects of microgravity and radiation on carcinogenesis.<ref>{{cite journal |last1=Cortés-Sánchez |first1=José Luis |last2=Callant |first2=Jonas |last3=Krüger |first3=Marcus |last4=Sahana |first4=Jayashree |last5=Kraus |first5=Armin |last6=Baselet |first6=Bjorn |last7=Infanger |first7=Manfred |last8=Baatout |first8=Sarah |last9=Grimm |first9=Daniela |title=Cancer Studies under Space Conditions: Finding Answers Abroad |journal=Biomedicines |date=January 2022 |volume=10 |issue=1 |pages=25 |doi=10.3390/biomedicines10010025 |doi-access=free |pmid=35052703 |pmc=8773191 |language=en |issn=2227-9059}}</ref>    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.<ref name=122-123 />

==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]]<ref name=126-126 /> and acute lymphatic lymphoma <ref name=126-126 /> as well as tumors of the [[Lung cancer|lung]], [[Breast cancer|breast]], [[Stomach cancer|stomach]], [[Colorectal cancer|colon]], [[Bladder cancer|bladder]] and [[Liver cancer|liver]].  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.<ref name="NCRP 2000">{{cite book|last=NCRP|title=NCRP Report No. 132, Radiation Protection Guidance for Activities in Low-Earth Orbit|year=2000|publisher=NCRP|location=Bethseda, Md.|url=http://www.ncrponline.org/Publications/Press_Releases/132press.html|url-status=dead|archive-url=https://web.archive.org/web/20131004213805/http://www.ncrponline.org/Publications/Press_Releases/132press.html|archive-date=2013-10-04}}</ref>  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.<ref name=122-123 />

It is unknown whether high-LET radiation could cause the same types of tumors as low-LET radiation, but differences should be expected.<ref name=126-126>{{cite web|last=Cucinotta|first=F.A.|title=Risk of Radiation Carcinogenesis|url=http://humanresearchroadmap.nasa.gov/evidence/reports/Carcinogenesis.pdf|work=Human Health and Performance Risks of Space Exploration Missions Evidence reviewed by the NASA Human Research Program|publisher=NASA|access-date=8 June 2012|author2=Durante, M. |pages=126}}</ref>

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 [[neutron]]s and have RBEs that vary with the tissue type and strain.<ref name=126-126 />

==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.<ref>{{cite web|last=Rettner|first=Rachael|title=Space Radiation Doesn't Seem to Be Causing Astronauts to Die from Cancer, Study Finds|date=5 July 2019 |url=https://www.livescience.com/65878-astronauts-cancer-space-radiation-risk.html|publisher=LiveScience|access-date=7 May 2021}}</ref><ref>{{cite journal |last1=Reynolds |first1=R.J. |last2=Bukhtiyarov |first2=I.V. |last3=Tikhonova |first3=G.I. |title=Contrapositive logic suggests space radiation not having a strong impact on mortality of US astronauts and Soviet and Russian cosmonauts |journal=Scientific Reports |date=4 July 2019 |volume=9 |issue=8583 |page=8583 |doi=10.1038/s41598-019-44858-0 |pmid=31273231 |pmc=6609703 |bibcode=2019NatSR...9.8583R |url=https://doi.org/10.1038/s41598-019-44858-0 |access-date=6 May 2021}}</ref>

An earlier 1998 study came to similar conclusions, with no statistically significant increase in cancer among astronauts compared to the reference group.<ref>{{cite journal |last1=Hamm |first1=P B |last2=Billica |first2=R D |last3=Johnson |first3=G S |last4=Wear |first4=M L |last5=Pool |first5=S L |title=Risk of cancer mortality among the Longitudinal Study of Astronaut Health (LSAH) participants |journal=Aviat Space Environ Med |date=February 1998 |volume=69 |issue=2 |pages=142–4 |pmid=9491253 |url=https://pubmed.ncbi.nlm.nih.gov/9491253/ |access-date=8 May 2021}}</ref>

==Approaches for setting acceptable risk levels==
The various approaches to setting acceptable levels of radiation risk are summarized below:<ref name=137-138>{{cite web|last=Cucinotta|first=F.A.|title=Risk of Radiation Carcinogenesis|url=http://humanresearchroadmap.nasa.gov/evidence/reports/Carcinogenesis.pdf|work=Human Health and Performance Risks of Space Exploration Missions Evidence reviewed by the NASA Human Research Program|publisher=NASA|access-date=8 June 2012|author2=Durante, M. |pages=137–138}}</ref>
[[File:PIA17601-Comparisons-RadiationExposure-MarsTrip-20131209.png|thumb|250px|right|Comparison of radiation doses - includes the amount detected on the trip from Earth to Mars by the [[Radiation assessment detector|RAD]] on the [[Mars Science Laboratory|MSL]] (2011 - 2013).<ref name="SCI-20130531a">{{cite journal |last=Kerr |first=Richard |title=Radiation Will Make Astronauts' Trip to Mars Even Riskier |date=31 May 2013 |journal=[[Science (journal)|Science]] |volume=340 |page=1031 |doi=10.1126/science.340.6136.1031 |issue=6136 |pmid=23723213 |bibcode=2013Sci...340.1031K}}</ref><ref name="SCI-20130531b">{{cite journal |title=Measurements of Energetic Particle Radiation in Transit to Mars on the Mars Science Laboratory |journal=[[Science (journal)|Science]] |date=31 May 2013 |volume=340 |pages=1080–1084 |doi=10.1126/science.1235989 |pmid=23723233 |issue=6136 |display-authors=8 |last1=Zeitlin |first1=C. |last2=Hassler |first2=D. M. |last3=Cucinotta |first3=F. A. |last4=Ehresmann |first4=B. |last5=Wimmer-Schweingruber |first5=R. F. |last6=Brinza |first6=D. E. |last7=Kang |first7=S. |last8=Weigle |first8=G. |last9=Bottcher |first9=S. |last10=Bohm |first10=E. |last11=Burmeister |first11=S. |last12=Guo |first12=J. |last13=Kohler |first13=J. |last14=Martin |first14=C. |last15=Posner |first15=A. |last16=Rafkin |first16=S. |last17=Reitz |first17=G. |bibcode=2013Sci...340.1080Z |s2cid=604569 |url=https://semanticscholar.org/paper/d4f68022dd4b96755933bccdc586bbeb2e031eb3 }}</ref><ref name="NYT-20130530">{{cite news |last=Chang |first=Kenneth |title=Data Point to Radiation Risk for Travelers to Mars |url=https://www.nytimes.com/2013/05/31/science/space/data-show-higher-cancer-risk-for-mars-astronauts.html |date=30 May 2013 |work=[[New York Times]] |access-date=31 May 2013 }}</ref><ref name="SN-20130629">{{cite journal |last=Gelling |first=Cristy|title=Mars trip would deliver big radiation dose; Curiosity instrument confirms expectation of major exposures|url=http://www.sciencenews.org/view/generic/id/350728/description/Mars_trip_would_deliver_big_radiation_dose|volume=183 |issue=13 |page=8 |journal=[[Science News]] |date=June 29, 2013 |access-date=July 8, 2013 |doi=10.1002/scin.5591831304}}</ref> ]]
* 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.

[[National Council on Radiation Protection and Measurements|NCRP]] Report No. 153 provides a more recent review of cancer and other radiation risks.<ref>{{cite book|last1=NCRP|title=Information needed to make radiation protection recommendations for space missions beyond low-earth orbit|date=2006|publisher=National Council on Radiation Protection and Measurements|location=Bethesda, MD|isbn=978-0-929600-90-1|url=https://www.ncrppublications.org/Reports/153|access-date=2017-11-04|archive-date=2017-11-07|archive-url=https://web.archive.org/web/20171107025808/https://www.ncrppublications.org/Reports/153|url-status=dead}}</ref> 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.<ref name=137-138 />

==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|low Earth orbit (LEO)]] as well as lunar missions that are less than 180 days in duration.<ref name="127-131">{{cite web|last=Cucinotta|first=F.A.|title=Risk of Radiation Carcinogenesis|url=http://humanresearchroadmap.nasa.gov/evidence/reports/Carcinogenesis.pdf|work=Human Health and Performance Risks of Space Exploration Missions Evidence reviewed by the NASA Human Research Program|publisher=NASA|access-date=12 June 2012|author2=Durante, M. |pages=127–131}}</ref> In the United States, the legal occupational exposure limits for adult workers is set at an effective dose of 50 mSv annually.<ref>{{cite web|title=NRC: 10 CFR 20.1201 Occupational dose limits for adults.|url=https://www.nrc.gov/reading-rm/doc-collections/cfr/part020/part020-1201.html|website=Nuclear Regulatory Commission|access-date=4 November 2017|language=en}}</ref>

===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 <ref name="NCRP 2000" /> and more recent radiation epidemiology information <ref name="Cucinotta and Durante 2006" /><ref name=127-131 /><ref name="Preston 2003">{{cite journal|last=Preston|first=DL|author2=Shimizu, Y|author3=Pierce, DA|author4=Suyama, A|author5=Mabuchi, K|title=Studies of mortality of atomic bomb survivors. Report 13: Solid cancer and noncancer disease mortality: 1950-1997|journal=Radiation Research|date=Oct 2003|volume=160|issue=4|pages=381–407|pmid=12968934|url=http://cerrie.org/committee_papers/Paper_12-04b.pdf|doi=10.1667/RR3049|url-status=dead|archive-url=https://web.archive.org/web/20111028021256/http://www.cerrie.org/committee_papers/Paper_12-04b.pdf |archive-date=2011-10-28|bibcode=2003RadR..160..381P|s2cid=41215245}}</ref>

===The principle of As Low As Reasonably Achievable===
The [[ALARP|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.<ref name=127-131 />

==Evaluating career limits==
<div style="text-align: center;">
{| class="wikitable"  style="float:right;"
!Organ (''T'')
!Tissue weighting factor (''w<sub>T</sub>'')
|-
|Gonads
| align="center" | 0.20
|-
|Bone Marrow (red)
| align="center" | 0.12
|-
|Colon
| align="center" | 0.12
|-
|Lung
| align="center" | 0.12
|-
|Stomach
| align="center" | 0.12
|-
|Bladder
| align="center" | 0.05
|-
|Breast
| align="center" | 0.05
|-
|Liver
| align="center" | 0.05
|-
|Esophagus
| align="center" | 0.05
|-
|Thyroid
| align="center" | 0.05
|-
|Skin
| align="center" | 0.01
|-
|Bone Surface
| align="center" | 0.01
|-
|Remainder*
| align="center" | 0.05
|-
|colspan=2|<sub>*Adrenals, brain, upper intestine, small intestine,<br />kidney, muscle, pancreas, spleen, thymus and uterus.</sub>
|}</div>

The risk of cancer is calculated by using [[Dosimetry|radiation dosimetry]] and physics methods.<ref name=127-131 />

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, ''w<sub>T</sub>'', according to its estimated contribution to cancer risk.<ref name=127-131 />
# The absorbed dose, ''D<sub>γ</sub>'', 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).<ref name=127-131 />
# For a given interval of LET, between L and ΔL, the dose-equivalent risk (in units of [[sievert]]) to a tissue, ''T'', ''H<sub>γ</sub>(L)'' is calculated as<br /><math>H_\gamma (L) = Q(L)D_\gamma (L)</math><br />where the quality factor, Q(L), is obtained according to the [http://www.icrp.org International Commission on Radiological Protection (ICRP)].<ref name=127-131 />
# The average risk to a tissue, ''T'', due to all types of radiation contributing to the dose is given by <ref name=127-131 /><br /><math>H_\gamma = \int D_\gamma (L)Q(L)dL</math><br />or, since <math>D_\gamma (L) =  LF_\gamma (L)</math>, where ''F<sub>γ</sub>(L)'' is the fluence of particles with ''LET=L'', traversing the organ,<br /><math>H_\gamma = \int dLQ(L)F_\gamma (L)L</math>
# The effective dose is used as a summation over radiation type and tissue using the tissue weighting factors, ''w<sub>γ</sub>'' <ref name=127-131 /><br /><math>E=\sum_\gamma w_\gamma H_\gamma</math>
# 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  <ref name=127-131 /><br /><math>E_i = \int E(t) dt</math>
# 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|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.<ref name=127-131 />

===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<sub>i</sub>'', as<br />
:<math>Risk = \sum_{i=1}^N E_i R_0 (age_i, sex)</math>
where ''R<sub>0</sub>'' are the age- and sex- specific radiation mortality rates per unit dose.<ref name=127-131 />

For organ dose calculations, NASA uses the model of Billings et al.<ref name=Billings>{{cite book|last=Billings|first=MP|title=Body self-shielding data analysis|year=1973|publisher=McDonnell-Douglas Astronautics Company West|edition=MDC-G4131|author2=Yucker, WR |author3=Heckman, BR }}</ref> 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 <ref name=Wilson>{{cite journal|last=Wilson|first=JW|author2=Kim, M |author3=Schimmerling, W |author4=Badavi, FF |author5=Thibeaullt, SA |author6=Cucinotta, FA |author7=Shinn, JL |author8= Kiefer, R |title=Issues in space radiation protection|journal=Health Phys|year=1993|volume=68|issue=1|pages=50–58|url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19970006977_1997005102.pdf|doi=10.1097/00004032-199501000-00006|pmid=7989194}}</ref>

Confidence levels for career cancer risks are evaluated using methods that are specified by the [http://www.ncrponline.org/Publications/Press_Releases/126press.html NPRC in Report No. 126] {{Webarchive|url=https://web.archive.org/web/20140308155704/http://www.ncrponline.org/Publications/Press_Releases/126press.html |date=2014-03-08 }}.<ref name=127-131 />  These levels were modified to account for the uncertainty in quality factors and space dosimetry.<ref name="Cucinotta and Durante 2006" /><ref name=127-131 /><ref name="Cucinotta 2001">{{cite journal|last=Cucinotta|first=FA|author2=Schimmerling, W |author3=Wilson, JW |author4=Peterson, LE |author5=Badhwar, GD |author6=Saganti, PB |author7= Dicello, JF |title=Space radiation cancer risks and uncertainties for Mars missions|journal=Radiation Research|date=Nov 2001|volume=156|issue=5 Pt 2|pages=682–8|pmid=11604093|jstor=3580473|doi=10.1667/0033-7587(2001)156[0682:SRCRAU]2.0.CO;2|bibcode=2001RadR..156..682C|s2cid=25236859 }}</ref>

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 <ref name="NCRP 1997a">{{cite book|last=NCRP|title=NCRP Report No. 126, Uncertainties in Fatal Cancer Risk Estimates Used in Radiation Protection|year=1997|publisher=NCRP|location=Bethesda, Md|url=http://www.ncrponline.org/Publications/Press_Releases/126press.html|url-status=dead|archive-url=https://web.archive.org/web/20140308155704/http://www.ncrponline.org/Publications/Press_Releases/126press.html|archive-date=2014-03-08}}</ref> 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 <ref name="NCRP 2000" /> 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.<ref name="Bunger 1981">{{cite journal|last=Bunger|first=BM|author2=Cook, JR |author3=Barrick, MK |title=Life table methodology for evaluating radiation risk: an application based on occupational exposures|journal=Health Physics|date=Apr 1981|volume=40|issue=4|pages=439–55|pmid=7228696|doi=10.1097/00004032-198104000-00002|s2cid=40538338 }}</ref><ref name="144-145">{{cite web|last=Cucinotta|first=F.A.|title=Risk of Radiation Carcinogenesis|url=http://humanresearchroadmap.nasa.gov/evidence/reports/Carcinogenesis.pdf|work=Human Health and Performance Risks of Space Exploration Missions Evidence reviewed by the NASA Human Research Program|publisher=NASA|access-date=8 June 2012|author2=Durante, M. |pages=144–145}}</ref>

For a homogenous population receiving an effective dose E at age a<sub>E</sub>, 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<sub>E</sub>,a)'', as:<ref name=144-145 />

:<math>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]}</math>

The survival probability to age, ''a'', following an exposure, ''E'' at age ''a<sub>E</sub>'', is:<ref name=144-145 />

:<math>S(E,a_E,a)= \prod_{u=a_E}^{a-1} \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:<ref name=144-145 />

:<math>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)</math>

A minimum latency-time of 10 years is often used for low-LET radiation.<ref name="NCRP 2000" />  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:<ref name=144-145 />

:<math>REID=\sum_{a=a_E}^\infty m(E,a_E,a)S(E,a_E,a)</math>

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:<ref name=144-145 />

:<math>LLE=\sum_{a=a_E}^\infty S(0,a_E,a) - \sum_{a=a_E}^\infty S(E,a_E,a)</math>

The loss of life-expectancy among exposure-induced-deaths (LLE-REID) is defined by:<ref name=144-145 /><ref name="Vaeth 1990">{{cite journal|last=Vaeth|first=M|author2=Pierce, DA |title=Calculating excess lifetime risk in relative risk mdels|journal=Environmental Health Perspectives|year=1990|volume=81|pages=83–94|jstor=3431010|doi=10.1289/ehp.908783|pmid=2269245|pmc=1567825}}</ref>

:<math>LLE-REID=\frac {LLE}{REID}</math>

===Uncertainties in low-LET epidemiology data===
The low-LET mortality rate per sievert, ''m<sub>i</sub>'' is written<br />
:<math>m(E,a_x,a) = \frac {m_0 (E,a_x,a)}{DDREF} \frac {x_D x_s x_T x_B}{x_{Dr}}</math>
where ''m<sub>0</sub>'' is the baseline mortality rate per sievert and ''x<sub>α</sub>'' are [[quantile]]s (random variables) whose values are sampled from associated probability distribution functions (PDFs), ''P(X<sub>a</sub>)''.<ref name=145-147>{{cite web|last=Cucinotta|first=F.A.|title=Risk of Radiation Carcinogenesis|url=http://humanresearchroadmap.nasa.gov/evidence/reports/Carcinogenesis.pdf|work=Human Health and Performance Risks of Space Exploration Missions Evidence reviewed by the NASA Human Research Program|publisher=NASA|access-date=8 June 2012|author2=Durante, M. |pages=145–147}}</ref>

NCRP, in Report No. 126, defines the following subjective PDFs, ''P(X<sub>a</sub>)'', for each factor that contributes to the acute low-LET risk projection:<ref name=145-147 /><ref>{{cite book|last1=NCRP|title=Uncertainties in fatal cancer risk estimates used in radiation protection|date=1997|publisher=National Council on Radiation Protection and Measurements|location=Bethesda, Md.|isbn=978-0-929600-57-4|url=https://archive.org/details/uncertaintiesinf00nati|url-access=registration}}</ref>
# ''P<sub>dosimetry</sub>'' is the random and systematic errors in the estimation of the doses received by atomic-bomb blast survivors.
# ''P<sub>statistical</sub>'' is the distribution in uncertainty in the point estimate of the risk coefficient, ''r<sub>0</sub>''.
# ''P<sub>bias</sub>'' is any bias resulting for over- or under-reporting cancer deaths.
# ''P<sub>transfer</sub>'' is the uncertainty in the transfer of cancer risk following radiation exposure from the Japanese population to the U.S. population.
# ''P<sub>Dr</sub>'' 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.<ref name="155-161">{{cite web|last=Cucinotta|first=F.A.|title=Risk of Radiation Carcinogenesis|url=http://humanresearchroadmap.nasa.gov/evidence/reports/Carcinogenesis.pdf|work=Human Health and Performance Risks of Space Exploration Missions Evidence reviewed by the NASA Human Research Program|publisher=NASA|access-date=6 June 2012|author2=Durante, M. |pages=155–161}}</ref>

The cancer risk projection for space missions is found by <ref name=155-161 /><br />
:<math>m_J(E,a_E,a)_{lJ}(E,a_E,a) \int dL \frac{dF}{dL}LQ_{trial-J}(L)X_{L-J}</math>
where <math>\frac{dF}{dL}</math> 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<sub>j</sub>(E)'', for each ion, ''j'', can be used <ref name=155-161 /><br />
:<math>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}</math>.

The result of either of these equations is inserted into the expression for the REID.<ref name=155-161 />

Related probability distribution functions (PDFs) are grouped together into a combined probability distribution function, ''P<sub>cmb</sub>(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 10<sup>5</sup>), the results for the REID estimated are binned and the median values and confidence intervals are found.<ref name=155-161 />

The [[Chi-squared test|chi-squared (χ<sup>2</sup>) test]] is used for determining whether two separate PDFs are significantly different (denoted ''p<sub>1</sub>(R<sub>i</sub>)'' and ''p<sub>2</sub>(R<sub>i</sub>)'', respectively).  Each ''p(R<sub>i</sub>)'' follows a Poisson distribution with variance <math>\sqrt{p(R_i)}</math>.<ref name=155-161 />

The χ<sup>2</sup> test for n-degrees of freedom characterizing the dispersion between the two distributions is <ref name=155-161 /><br />
:<math>\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)}}</math>.

The probability, ''P(ņχ<sup>2</sup>)'', that the two distributions are the same is calculated once χ<sup>2</sup> is determined.<ref name=155-161 />

===Radiation carcinogenesis mortality rates===
Age-and sex-dependent mortality rate 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.<ref name="NCRP 2000" />  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.<ref name="NCRP 2000" />

The radiation mortality rate is defined as:

:<math> m(E,a_E,a)=\left[ERR(a_E, a)M_c(a)+(1-v)EAR(a_E,a)\right]{\frac{\sum_LQ(L)F(L)L}{DDREF}}</math>

Where:
* ERR = excess relative risk per sievert
* EAR = excess additive risk per sievert
* M<sub>c</sub>(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,<ref name="Durante 2008" /> but will be needed for other long-duration missions to Mars and beyond.<ref name=155-161 /> On 31 May 2013, NASA scientists reported that a possible [[human mission to Mars]] may involve a great [[radiation|radiation risk]] based on the amount of [[radiation|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.<ref name="SCI-20130531a" /><ref name="SCI-20130531b" /><ref name="NYT-20130530" /><ref name="SN-20130629" />

There are three fundamental ways to reduce exposure to ionizing radiation:<ref name=155-161 />
* 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.<ref>{{cite journal |last1=Nelson |first1=Gregory |title=Space Radiation and Human Exposures, A Primer |journal=Radiation Research |date=April 2016 |volume=185 |issue=4 |pages=349–358 |doi=10.1667/rr14311.1|pmid=27018778 |bibcode=2016RadR..185..349N |doi-access=free }}</ref>  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.<ref name=155-161 />

[[Antioxidant]]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.<ref name=155-161 />

===Spacecraft shielding===
[[File:Cosmic ray shielding with secondary radiation.jpg|thumb|Standard spacecraft shielding, integrated into hull design, is strong protection from most solar radiation. This usefulness is defeated by high-energy cosmic rays, which it effectively splits into showers of secondary particles. This shower of secondary particles may be reduced by the use of hydrogen-dense materials 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 of the increased amount of [[Bremsstrahlung|secondary radiation]].<ref name = mass_shielding>NASA SP-413 [https://web.archive.org/web/20100531210412/http://settlement.arc.nasa.gov/75SummerStudy/5appendE.html Space Settlements: A Design Study. Appendix E Mass Shielding] Retrieved 3 May 2011.</ref> 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; thicker shielding would be needed to block the secondary radiation.<ref name="islandone.org">{{cite web | url=http://www.islandone.org/Settlements/MagShield.html | title=Magnetic Radiation Shielding: An Idea Whose Time Has Returned? | author=G.Landis | date=1991 | access-date=8 March 2008 | archive-date=5 April 2016 | archive-url=https://web.archive.org/web/20160405143918/http://www.islandone.org/Settlements/MagShield.html | url-status=dead }}</ref><ref>{{cite journal | journal=Popular Science | title=Juno Probe, Built to Study Jupiter's Radiation Belt, Gets A Titanium Suit of Interplanetary Armor | author=Rebecca Boyle | date=13 July 2010 | url=http://www.popsci.com/science/article/2010-07/juno-probe-built-study-jupiters-radiation-belt-gets-coat-armor}}</ref>

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&nbsp;cm, and that secondary radiation produced in tissue such as low energy protons, helium, and heavy ions are of high [[linear energy transfer]] (LET) and make significant contributions (>25%) to the overall biological damage from GCR. Studies of aluminium, polyethylene, liquid hydrogen, or other shielding materials should include their combination effects against primary and secondary radiation, plus their ability to limit the secondary radiation produced in tissue.

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.<ref name="nasa3">{{cite web|url=https://science.nasa.gov/headlines/y2005/25aug_plasticspaceships.htm|publisher=science.nasa.gov|title=NASA - Plastic Spaceships|access-date=2 April 2017|url-status=dead|archive-url=https://web.archive.org/web/20100323201842/http://science.nasa.gov/headlines/y2005/25aug_plasticspaceships.htm|archive-date=23 March 2010|df=dmy-all}}</ref>
* Mass and material shielding:
** Liquid hydrogen, often used as fuel, 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.
** Fresh or waste water can contribute to shielding.<ref name = newscientist>{{cite web|url=https://www.newscientist.com/article.ns?id=dn7753|publisher=New Scientist|title= Cosmic rays may prevent long-haul space travel|date= 1 August 2005 |access-date=2 April 2017}}</ref>
** Asteroids could serve to provide shielding.<ref>Morgan, P. (2011) [http://blogs.discovermagazine.com/discoblog/2011/02/11/to-hitch-a-ride-to-mars-just-flag-down-an-asteroid/ "To Hitch a Ride to Mars, Just Flag Down an Asteroid"] {{Webarchive|url=https://web.archive.org/web/20110413224955/http://blogs.discovermagazine.com/discoblog/2011/02/11/to-hitch-a-ride-to-mars-just-flag-down-an-asteroid/ |date=13 April 2011 }} ''Discover'' magazine blog</ref><ref>{{cite journal |author1=Matloff G.L. |author2=Wilga M. | date = 2011 | title = NEOs as stepping stones to Mars and main-belt asteroids | doi = 10.1016/j.actaastro.2010.02.026 | journal = Acta Astronautica | volume = 68 | issue = 5–6| pages = 599–602 |bibcode=2011AcAau..68..599M }}</ref>
* Light active radiation shields based on the charged graphene against gamma rays, where the absorption parameters can be controlled by the negative charge accumulation.<ref>{{cite journal |last1=Fornalski |first1=Krzysztof W. |last2=Adamowski |first2=Łukasz |last3=Bonczyk |first3=Michał |last4=Winkowska-Struzik |first4=Magdalena |title=Ionizing radiation interaction with charged graphene: An experimental evaluation attempt |journal=Radiation Physics and Chemistry |date=September 2020 |volume=174 |pages=108901 |doi=10.1016/j.radphyschem.2020.108901|bibcode=2020RaPC..17408901F |s2cid=216229192 }}</ref>
* 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 a 5-meter torus drop from an excessive 10 [[Gigawatt|GW]] for a simple pure electrostatic shield (too discharged by space electrons) to a moderate 10&nbsp;[[kilowatt]]s (kW) by using a hybrid design.<ref name="islandone.org"/> However, such complex active shielding is untried, with workability and practicalities more uncertain than material shielding.<ref name="islandone.org"/>

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|date=April 2010}}

None of these strategies currently provide a method of protection that would be known to be sufficient<ref name="parker" /> 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.<ref name="parker">{{cite journal | author=Eugene N. Parker | title=Shielding Space Travelers | journal=Scientific American | volume=294 | issue=3 | pages=40–7 | date=March 2006| bibcode=2006SciAm.294c..40P | doi=10.1038/scientificamerican0306-40 | pmid=16502610 }}</ref> 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 space station envisioned 4 metric tons per square meter of shielding to drop radiation exposure to 2.5&nbsp;mSv annually (± a factor of 2 uncertainty), less than the tens of millisieverts or more in some populated [[Background Radiation#Natural background radiation|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.<ref name = mass_shielding/>

Several active shielding methods have been considered that might be less massive than passive shielding, but they remain speculative.<ref name="islandone.org"/><ref>[http://engineering.dartmouth.edu/~simon_g_shepherd/research/Shielding/docs/ToMaSS.pdf Simulations of Magnetic Shields for Spacecraft]. Retrieved 3 May 2011.</ref><ref>{{cite journal |last1=Fornalski |first1=Krzysztof Wojciech |title=Theoretical considerations on charged graphene as active gamma radiation shields |journal=The European Physical Journal Applied Physics |date=March 2018 |volume=81 |issue=3 |pages=30401 |doi=10.1051/epjap/2018170387|bibcode=2018EPJAP..8130401F }}</ref> 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.<ref>NASA SP-413 [https://web.archive.org/web/20100531210211/http://settlement.arc.nasa.gov/75SummerStudy/4appendD.html Space Settlements: A Design Study. Appendix D The Plasma Core Shield] Retrieved 3 May 2011.</ref> 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 20 [[Tesla_(unit)|tesla]] could be required around a crewed spacecraft, higher than the several tesla 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.<ref name="islandone.org"/>

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.

=== Wearable radiation shielding ===
Apart from passive and active radiation shielding methods, which focus on protecting the spacecraft from harmful space radiation, there has been much interest in designing personalized radiation protective suits for astronauts. The reason behind choosing such methods of radiation shielding is that in passive shielding, adding a certain thickness to the spacecraft can increase the mass of the spacecraft by several thousands of kilograms.<ref>{{Cite journal|last=Singleterry|first=R. C.|date=2013-10-01|title=Radiation engineering analysis of shielding materials to assess their ability to protect astronauts in deep space from energetic particle radiation|url=http://www.sciencedirect.com/science/article/pii/S0094576513001252|journal=Acta Astronautica|language=en|volume=91|pages=49–54|doi=10.1016/j.actaastro.2013.04.013|bibcode=2013AcAau..91...49S|s2cid=120839628 |issn=0094-5765}}</ref> This mass can surpass the launch constraints and costs several millions of dollars. 

On the other hand, active radiation shielding methods is an emerging technology which is still far away in terms of testing and implementation. Even with the simultaneous use of active and passive shielding, wearable protective shielding may be useful, especially in reducing the health effects of SPEs, which generally are composed of particles that have a lower penetrating force than GCR particles.<ref>{{Cite journal|last1=Desai|first1=Mihir|last2=Giacalone|first2=Joe|date=December 2016|title=Large gradual solar energetic particle events|url= |journal=Living Reviews in Solar Physics|language=en|volume=13|issue=1|pages=3|doi=10.1007/s41116-016-0002-5|issn=2367-3648|pmc=7175685|pmid=32355890|bibcode=2016LRSP...13....3D}}</ref> The materials suggested for this  type of protective equipment is often  polyethylene or other hydrogen rich polymers.<ref name=life>{{Cite journal|date=2020-08-01|title=Investigation of shielding material properties for effective space radiation protection|journal=Life Sciences in Space Research|language=en|volume=26|pages=69–76|doi=10.1016/j.lssr.2020.05.001|issn=2214-5524|last1=Naito|first1=Masayuki|last2=Kodaira|first2=Satoshi|last3=Ogawara|first3=Ryo|last4=Tobita|first4=Kenji|last5=Someya|first5=Yoji|last6=Kusumoto|first6=Tamon|last7=Kusano|first7=Hiroki|last8=Kitamura|first8=Hisashi|last9=Koike|first9=Masamune|last10=Uchihori|first10=Yukio|last11=Yamanaka|first11=Masahiro|last12=Mikoshiba|first12=Ryo|last13=Endo|first13=Toshiaki|last14=Kiyono|first14=Naoki|last15=Hagiwara|first15=Yusuke|last16=Kodama|first16=Hiroaki|last17=Matsuo|first17=Shinobu|last18=Takami|first18=Yasuhiro|last19=Sato|first19=Toyoto|last20=Orimo|first20=Shin-Ichi|pmid=32718689|bibcode=2020LSSR...26...69N|doi-access=free}}</ref> Water has also been suggested as a shielding material. The limitation with wearable protective solutions is that they need to be ergonomically compatible with crew needs such as movement inside crew volume.  One attempt at creating wearable protection for space radiation was done by the Italian Space Agency, where a garment was proposed that could be filled with recycled water on the signal of incoming SPE.<ref>{{Cite journal|last1=Vuolo|first1=M.|last2=Baiocco|first2=G.|last3=Barbieri|first3=S.|last4=Bocchini|first4=L.|last5=Giraudo|first5=M.|last6=Gheysens|first6=T.|last7=Lobascio|first7=C.|last8=Ottolenghi|first8=A.|date=2017-11-01|title=Exploring innovative radiation shielding approaches in space: A material and design study for a wearable radiation protection spacesuit|url=http://www.sciencedirect.com/science/article/pii/S2214552417300329|journal=Life Sciences in Space Research|language=en|volume=15|pages=69–78|doi=10.1016/j.lssr.2017.08.003|pmid=29198316|bibcode=2017LSSR...15...69V|issn=2214-5524}}</ref> 

A collaborative effort between the [[Israel Space Agency|Israeli Space Agency]], [[StemRad]] and [[Lockheed Martin]] was [[Astrorad|AstroRad]], tested aboard the ISS. The product is designed as an ergonomically suitable protective vest, which can minimize the effective dose by SPE to an extent similar to onboard storm shelters.<ref>Waterman, G., Milstein, O., Knight, L., Charles, J., Coderre, K., Posey, J., Semones, E. "AstroRad Radiation Protective Equipment Evaluations On Orion AND ISS", IAC-19,A1,5,5,x52629, 70 th International Astronautical Congress (IAC)</ref> It also has potential to mildly reduce the effective dose of GCR through extensive use during the mission during such routine activities such as sleeping. This radiation protective garment uses selective shielding methods to protect most radiation-sensitive organs such as BFO, stomach, lungs, and other internal organs, thereby reducing the mass penalty and launch cost.{{fact|date=September 2024}}

===Drugs and medicine===
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|date=April 2018}}

===Transhumanism===
{{main|Transhumanism}}
It has also been suggested that only through substantial improvements and modifications could the human body endure the conditions of space travel. While not constrained by basic laws of nature in the way technical solutions are, this is far beyond current science of medicine.

===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.{{fact|date=September 2024}}

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.{{fact|date=September 2024}}

===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|Lagrange points]] {{L2}} - {{L5}} take them out of the protection of the Earth's [[magnetosphere]] for approximately two-thirds of the time.{{citation needed|date=November 2015}}

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

==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:<ref name=127-131 />
* 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 model|linear no-threshold hypothesis]] that is often used in statistical analysis of these data.  The BEIR VII report,<ref name="BIER 2006" /> 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.<ref name="NCRP 2000" /><ref name=127-131 /><ref name="NCRP 1989">{{cite book|last=NCRP|first=NCRP Report No. 98|title=Guidance on radiation received in space activities|year=1989|publisher=NCRP|location=Bethesda, Md.|url=http://www.ncrppublications.org/Reports/098}}</ref><ref name="NCRP 2006">{{cite book|last=NCRP|first=NCRP Report No. 153|title=Information needed to make radiation protection recommendations for space missions beyond low-Earth orbit|year=2006|publisher=NCRP|location=Bethesda, Md.|url=http://www.ncrponline.org/Publications/Press_Releases/153press.html|url-status=dead|archive-url=https://web.archive.org/web/20150610232701/http://www.ncrponline.org/Publications/Press_Releases/153press.html|archive-date=2015-06-10}}</ref>
* [[Epidemiology data for low-linear energy transfer radiation|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]]
* [[Health threat from cosmic rays]]
* [[Radiation syndromes#Spaceflight|Radiation Syndrome]]
* [[Radiation protection]]

==References==
{{Reflist}}

==External links==
* {{cite journal |last1=Asaithamby |first1=A |last2=Uematsu |first2=N |last3=Chatterjee |first3=A |last4=Story |first4=MD |last5=Burma |first5=S |last6=Chen |first6=DJ |title=Repair of HZE-particle-induced DNA double-strand breaks in normal human fibroblasts |journal=Radiation Research |date=April 2008 |volume=169 |issue=4 |pages=437–46 |pmid=18363429 |doi=10.1667/RR1165.1|bibcode=2008RadR..169..437A |s2cid=731451 }}
* {{cite journal |last1=Chatterjee |first1=A. |last2=Borak |first2=T.H. |title=Physical and biological studies with protons and HZE particles in a NASA supported research center in radiation health |journal=Physica Medica |year=2001 |volume=17 |issue=1 |pages=59–66 |pmid=11770539 |url=http://www.physicamedica.com/VOLXVII_S1/13-CHATTERJEE-BORAK.pdf |access-date=5 June 2012 |url-status=dead |archive-url=https://web.archive.org/web/20060513194826/http://www.physicamedica.com/VOLXVII_S1/13-CHATTERJEE-BORAK.pdf |archive-date=13 May 2006 }}
* {{cite web |last=Snyder |first=Kendra |title=One-Two Particle Punch Poses Greater Risk for Astronauts |url=http://www.bnl.gov/bnlweb/pubaf/pr/pr_display.asp?prid=06-99 |publisher=Brookhaven National Laboratory |access-date=6 June 2012 |date=August 2006}}
* {{cite journal |last1=Bucker |first1=H |last2=Facius |first2=R |title=The role of HZE particles in space flight: results from spaceflight and ground-based experiments |journal=Acta Astronautica |date=Sep–Oct 1981 |volume=8 |issue=9–10 |pages=1099–107 |pmid=11543100 |doi=10.1016/0094-5765(81)90084-9|bibcode=1981AcAau...8.1099B }}
* {{cite book |author=Committee on the Evaluation of Radiation Shielding for Space Exploration, Aeronautics and Space Engineering Board, Division on Engineering and Physical Sciences, National Research Council of the National Academies |title=Managing space radiation risk in the new era of space exploration |year=2008 |publisher=National Academies Press |location=Washington, D.C. |isbn=978-0-309-11383-0}}
* [https://web.archive.org/web/20120903041231/http://three.usra.edu/ The Health Risks of Extraterrestrial Environments]

{{NASA|article=Human Health and Performance Risks of Space Exploration Missions|url=http://humanresearchroadmap.nasa.gov/evidence/reports/EvidenceBook.pdf|comment=NASA SP-2009-3405}}

{{Space medicine}}

[[Category:Space medicine]]