Effects of ionizing radiation in spaceflight

While in space, astronauts are exposed to radiation which is mostly composed of high-energy protons, helium nuclei (alpha particles), and high-atomic-number ions (HZE ions), as well as secondary radiation from nuclear reactions from spacecraft parts or tissue.[6]

The ionization patterns in molecules, cells, tissues and the resulting biological effects are distinct from typical terrestrial radiation (x-rays and gamma rays, which are low-LET radiation). Galactic cosmic rays (GCRs) from outside the Milky Way galaxy consist mostly of highly energetic protons with a small component of HZE ions.[6]

Prominent HZE ions:

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

GCR energy spectra peaks (with median energy peaks up to 1,000 MeV/amu) and nuclei (energies up to 10,000 MeV/amu) are important contributors to the dose equivalent.[6][7]

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.[6] Operational parameters that need to be optimized to help mitigate these risks include:[6]

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

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[8] and acute lymphatic lymphoma [8] as well as tumors of the lung, breast, stomach, colon, bladder and 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.[9] 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.[6]

It is unknown whether high-LET radiation could cause the same types of tumors as low-LET radiation, but differences should be expected.[8]

The ratio of a dose of high-LET radiation to a dose of x-rays or gamma rays that produce the same biological effect are called relative biological effectiveness (RBE) factors. The types of tumors in humans who are exposed to space radiation will be different from those who are exposed to low-LET radiation. This is evidenced by a study that observed mice with neutrons and have RBEs that vary with the tissue type and strain.[8]

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.[10][11]

An earlier 1998 study came to similar conclusions, with no statistically significant increase in cancer among astronauts compared to the reference group.[12]

The various approaches to setting acceptable levels of radiation risk are summarized below:[13]

Comparison of radiation doses - includes the amount detected on the trip from Earth to Mars by the RAD on the MSL (2011 - 2013).[14][15][16][17]

• 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.[18] 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.[13]

The risk of cancer is calculated by using radiation dosimetry and physics methods.[19]

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

1. The body is divided into a set of sensitive tissues, and each tissue, T, is assigned a weight, w_T, according to its estimated contribution to cancer risk.[19]
2. The absorbed dose, D_γ, that is delivered to each tissue is determined from measured dosimetry. For the purpose of estimating radiation risk to an organ, the quantity characterizing the ionization density is the LET (keV/μm).[19]
3. For a given interval of LET, between L and ΔL, the dose-equivalent risk (in units of sievert) to a tissue, T, H_γ(L) is calculated as
   ${\displaystyle H_{\gamma }(L)=Q(L)D_{\gamma }(L)}$
   where the quality factor, Q(L), is obtained according to the International Commission on Radiological Protection (ICRP).[19]
4. The average risk to a tissue, T, due to all types of radiation contributing to the dose is given by [19]
   ${\displaystyle H_{\gamma }=\int D_{\gamma }(L)Q(L)dL}$
   or, since ${\displaystyle D_{\gamma }(L)=LF_{\gamma }(L)}$, where F_γ(L) is the fluence of particles with LET=L, traversing the organ,
   ${\displaystyle H_{\gamma }=\int dLQ(L)F_{\gamma }(L)L}$
5. The effective dose is used as a summation over radiation type and tissue using the tissue weighting factors, w_γ [19]
   ${\displaystyle E=\sum _{\gamma }w_{\gamma }H_{\gamma }}$
6. For a mission of duration t, the effective dose will be a function of time, E(t), and the effective dose for mission i will be [19]
   ${\displaystyle E_{i}=\int E(t)dt}$
7. The effective dose is used to scale the mortality rate for radiation-induced death from the Japanese survivor data, applying the average of the multiplicative and additive transfer models for solid cancers and the additive transfer model for leukemia by applying life-table methodologies that are based on U.S. population data for background cancer and all causes of death mortality rates. A dose-dose rate effectiveness factor (DDREF) of 2 is assumed.[19]

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.[31]

The cancer risk projection for space missions is found by [31]

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

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

Alternatively, particle-specific energy spectra, F_j(E), for each ion, j, can be used [31]

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

The result of either of these equations is inserted into the expression for the REID.[31]

Related probability distribution functions (PDFs) are grouped together into a combined probability distribution function, P_cmb(x). These PDFs are related to the risk coefficient of the normal form (dosimetry, bias and statistical uncertainties). After a sufficient number of trials have been completed (approximately 10⁵), the results for the REID estimated are binned and the median values and confidence intervals are found.[31]

The chi-squared (χ²) test is used for determining whether two separate PDFs are significantly different (denoted p₁(R_i) and p₂(R_i), respectively). Each p(R_i) follows a Poisson distribution with variance ${\displaystyle {\sqrt {p(R_{i})}}}$.[31]

The χ² test for n-degrees of freedom characterizing the dispersion between the two distributions is [31]

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

The probability, P(ņχ²), that the two distributions are the same is calculated once χ² is determined.[31]

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,[3] but will be needed for other long-duration missions to Mars and beyond.[31] On 31 May 2013, NASA scientists reported that a possible human mission to Mars may involve a great radiation risk based on the amount of energetic particle radiation detected by the RAD on the Mars Science Laboratory while traveling from the Earth to Mars in 2011-2012.[14][15][16][17]

There are three fundamental ways to reduce exposure to ionizing radiation:[31]

• 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.[32] 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.[31]

Antioxidants are effectively used to prevent the damage caused by radiation injury and oxygen poisoning (the formation of reactive oxygen species), but since antioxidants work by rescuing cells from a particular form of cell death (apoptosis), they may not protect against damaged cells that can initiate tumor growth.[31]

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:[19]

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

These committees release new reports about every 10 years on cancer risks that are applicable to low-LET radiation exposures. Overall, the estimates of cancer risks among the different reports of these panels will agree within a factor of two or less. There is continued controversy for doses that are below 5 mSv, however, and for low dose-rate radiation because of debate over the linear no-threshold hypothesis that is often used in statistical analysis of these data. The BEIR VII report,[4] 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.[9][19][33][34]

• Epidemiology data for low linear energy transfer radiation
• Radiobiology evidence for protons and HZE nuclei

• Asaithamby, A; Uematsu, N; Chatterjee, A; Story, MD; Burma, S; Chen, DJ (April 2008). "Repair of HZE-particle-induced DNA double-strand breaks in normal human fibroblasts". Radiation Research. 169 (4): 437–46. Bibcode:2008RadR..169..437A. doi:10.1667/RR1165.1. PMID 18363429. S2CID 731451.
• Chatterjee, A.; Borak, T.H. (2001). "Physical and biological studies with protons and HZE particles in a NASA supported research center in radiation health" (PDF). Physica Medica. 17 (1): 59–66. PMID 11770539. Archived from the original (PDF) on 13 May 2006. Retrieved 5 June 2012.
• Snyder, Kendra (August 2006). "One-Two Particle Punch Poses Greater Risk for Astronauts". Brookhaven National Laboratory. Retrieved 6 June 2012.
• Bucker, H; Facius, R (Sep–Oct 1981). "The role of HZE particles in space flight: results from spaceflight and ground-based experiments". Acta Astronautica. 8 (9–10): 1099–107. Bibcode:1981AcAau...8.1099B. doi:10.1016/0094-5765(81)90084-9. PMID 11543100.
• 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 (2008). Managing space radiation risk in the new era of space exploration. Washington, D.C.: National Academies Press. ISBN 978-0-309-11383-0.{{cite book}}: CS1 maint: multiple names: authors list (link)
• The Health Risks of Extraterrestrial Environments

 This article incorporates public domain material from Human Health and Performance Risks of Space Exploration Missions (PDF). National Aeronautics and Space Administration. (NASA SP-2009-3405).