Effects of ionizing radiation in spaceflight

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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 millisieverts (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 1000 mSv. Without the protection provided by Earth's magnetic field, the rate of exposure is dramatically increased. The risk of cancer caused by ionizing radiation is well documented at radiation doses beginning at 100 mSv and above.

Related radiological effect studies have shown that survivors of the atomic bomb explosions in Hiroshima and Nagasaki, nuclear reactor workers and patients who have undergone therapeutic radiation treatments have received low-linear energy transfer (LET) radiation (x-rays and gamma rays) doses in the same 50-2,000 mSv range.

Composition of space radiation

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

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

Prominent HZE ions:

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

GCR energy spectra peaks (with median energy peaks up to 1000 MeV/Da) and nuclei (energies up to 10000 MeV/Da) are important contributors to the dose equivalent.

Uncertainties in cancer projections

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

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

Major uncertainties

Source:

• 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:

• 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. The effects of changes in oxygen levels or in immune dysfunction on cancer risks are largely unknown and are of great concern during space flight.

Types of cancer caused by radiation exposure

Studies are being conducted on populations accidentally exposed to radiation (such as Chernobyl, production sites, and Hiroshima and Nagasaki). These studies show strong evidence for cancer morbidity as well as mortality risks at more than 12 tissue sites. The largest risks for adults who have been studied include several types of leukemia, including myeloid leukemia and acute lymphatic lymphoma as well as tumors of the lung, breast, stomach, colon, bladder and liver. Inter-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 (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.

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:

Comparison of radiation doses – includes the amount detected on the trip from Earth to Mars by the RAD on the MSL (2011–2013).

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

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

Current permissible exposure limits

Career cancer risk limits

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

Principle of 'as low as reasonably achievable'

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

Evaluating career limits

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

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

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

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

Evaluating cumulative radiation risks

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

${\displaystyle \text{Risk}=\sum _{i=1}^N{E}_i{R}_0({\mathrm{a}\mathrm{g}\mathrm{e}}_i,\mathrm{s}\mathrm{e}\mathrm{x})}$

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 the NPRC in Report No. 126 Archived 2014-03-08 at the Wayback Machine. These levels were modified to account for the uncertainty in quality factors and space dosimetry.

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

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

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

Models of cancer risks and uncertainties

Life-table methodology

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

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

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

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

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

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

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

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

${\displaystyle \mathrm{R}\mathrm{E}\mathrm{I}\mathrm{D}=\sum _{a=a_E}^{\mathrm{\infty }}m(E,a_E,a)S(E,a_E,a)}$

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

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

${\displaystyle \mathrm{L}\mathrm{L}\mathrm{E}=\sum _{a=a_E}^{\mathrm{\infty }}S(0,a_E,a)-\sum _{a=a_E}^{\mathrm{\infty }}S(E,a_E,a)}$

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

${\displaystyle \mathrm{L}\mathrm{L}\mathrm{E}-\mathrm{R}\mathrm{E}\mathrm{I}\mathrm{D}=\frac{\mathrm{L}\mathrm{L}\mathrm{E}}{\mathrm{R}\mathrm{E}\mathrm{I}\mathrm{D}}}$

Uncertainties in low-LET epidemiology data

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

${\displaystyle m(E,a_x,a)=\frac{m_0(E,a_x,a)}{\mathrm{D}\mathrm{D}\mathrm{R}\mathrm{E}\mathrm{F}}\frac{x_D{x}_s{x}_T{x}_B}{x_{Dr}}}$

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

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

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

Risk in context of exploration mission operational scenarios

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

The cancer risk projection for space missions is found by

${\displaystyle m_J(E,a_E,a{)}_{lJ}(E,a_E,a)\int dL\frac{dF}{dL}L{Q}_{trial-J}(L)X_{L-J}}$

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

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

${\displaystyle m_J(E,a_E,a)=m_{lJ}(E,a_E,a)\sum _j(E)L(E)Q_{trial-J}(L(E))x_{L-J}}$.

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

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

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

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

${\displaystyle {\chi }^2=\sum _n\frac{{\left[p_1(R_n)-p_2(R_n)\right]}^2}{\sqrt{p_1^2(R_n)+p_2^2(R_n)}}}$.

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

Radiation carcinogenesis mortality rates

Age-and 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. 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:

${\displaystyle m(E,a_E,a)=\left[\mathrm{E}\mathrm{R}\mathrm{R}(a_E,a)M_{\text{c}}(a)+(1-v)\mathrm{E}\mathrm{A}\mathrm{R}(a_E,a)\right]\frac{\sum _{L}Q(L)F(L)L}{\mathrm{D}\mathrm{D}\mathrm{R}\mathrm{E}\mathrm{F}}}$

Where:

• ERR = excess relative risk per sievert
• EAR = excess additive risk per sievert
• M_c(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 may involve a great radiation risk based on the amount of energetic particle radiation detected by the RAD on the Mars Science Laboratory while traveling from the Earth to Mars in 2011-2012.

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

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

Shielding is a plausible option, but due to current launch mass restrictions, it is prohibitively costly. Also, the current uncertainties in risk projection prevent the actual benefit of shielding from being determined. Strategies such as drugs and dietary supplements to reduce the effects of radiation, as well as the selection of crew members are being evaluated as viable options for reducing exposure to radiation and effects of irradiation. Shielding is an effective protective measure for solar particle events.^[33] 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.

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.

Spacecraft shielding

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 secondary radiation. 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.

Studies of space radiation shielding should include tissue- or water-equivalent shielding along with the shielding material under study. This observation is readily understood by noting that the average tissue self-shielding of sensitive organs is about 10 cm, and that secondary radiation produced in tissue such as low energy protons, helium, and heavy ions are of high 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.^[37]
• 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.
  • Asteroids could serve to provide shielding.
• Light active radiation shields based on the charged graphene against gamma rays, where the absorption parameters can be controlled by the negative charge accumulation.
• 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 GW for a simple pure electrostatic shield (too discharged by space electrons) to a moderate 10 kilowatts (kW) by using a hybrid design. However, such complex active shielding is untried, with workability and practicalities more uncertain than material shielding.

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.

None of these strategies currently provide a method of protection that would be known to be sufficient 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. 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 mSv annually (± a factor of 2 uncertainty), less than the tens of millisieverts or more in some populated high natural background radiation areas on Earth, but the sheer mass for that level of mitigation was considered practical only because it involved first building a lunar mass driver to launch material.

Several active shielding methods have been considered that might be less massive than passive shielding, but they remain speculative. 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. 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 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.

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. 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. The materials suggested for this type of protective equipment is often polyethylene or other hydrogen rich polymers. 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.

A collaborative effort between the Israeli Space Agency, StemRad and Lockheed Martin was 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. 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.

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.

Transhumanism

Main article: 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.

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

Orbital selection

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

The orbits of the Earth-Moon system Lagrange points L₂–L₅ take them out of the protection of the Earth's magnetosphere for approximately two-thirds of the time.

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

Space radiobiology

Space radiobiology is the science that investigates the effects of space radiation in biological matter. This is an interdisciplinary field of research, aiming to create synergies between astroparticle and medical physics.

Evidence sub-pages

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

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

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

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

Effect of spaceflight on the human body

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Effect_of_spaceflight_on_the_human_body

American astronaut Marsha Ivins demonstrates the effects of microgravity on her hair in space

The effects of spaceflight on the human body are complex and largely harmful over both short and long term. Significant adverse effects of long-term weightlessness include muscle atrophy and deterioration of the skeleton (spaceflight osteopenia). Other significant effects include a slowing of cardiovascular system functions, decreased production of red blood cells (space anemia), balance disorders, eyesight disorders and changes in the immune system. Additional symptoms include fluid redistribution (causing the "moon-face" appearance typical in pictures of astronauts experiencing weightlessness), loss of body mass, nasal congestion, sleep disturbance, and excess flatulence. A 2024 assessment noted that "well-known problems include bone loss, heightened cancer risk, vision impairment, weakened immune systems, and mental health issues... [y]et what's going on at a molecular level hasn't always been clear", arousing concerns especially vis a vis private and commercial spaceflight now occurring without any scientific or medical research being conducted among those populations regarding effects.

Overall, NASA refers to the various deleterious effects of spaceflight on the human body by the acronym RIDGE (i.e., "space radiation, isolation and confinement, distance from Earth, gravity fields, and hostile and closed environments").

The engineering problems associated with leaving Earth and developing space propulsion systems have been examined for more than a century, and millions of hours of research have been spent on them. In recent years, there has been an increase in research on the issue of how humans can survive and work in space for extended and possibly indefinite periods of time. This question requires input from the physical and biological sciences and has now become the greatest challenge (other than funding) facing human space exploration. A fundamental step in overcoming this challenge is trying to understand the effects of long-term space travel on the human body.

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

On 12 April 2019, NASA reported medical results from the Astronaut Twin Study, where one astronaut twin spent a year in space on the International Space Station, while the other spent the year on Earth, which demonstrated several long-lasting changes, including those related to alterations in DNA and cognition, after the twins were compared.

In November 2019, researchers reported that astronauts experienced serious blood flow and clot problems while on board the International Space Station, based on a six-month study of 11 healthy astronauts. The results may influence long-term spaceflight, including a mission to the planet Mars, according to the researchers.

Physiological effects

Many of the environmental conditions experienced by humans during spaceflight are very different from those in which humans evolved; however, technology such as that offered by a spaceship or spacesuit is able to shield people from the harshest conditions. The immediate needs for breathable air and drinkable water are addressed by a life support system, a group of devices that allow human beings to survive in outer space. The life support system supplies air, water and food. It must also maintain temperature and pressure within acceptable limits and deal with the body's waste products. Shielding against harmful external influences such as radiation and micro-meteorites is also necessary.

Some hazards are difficult to mitigate, such as weightlessness, also defined as a microgravity environment. Living in this type of environment impacts the body in three important ways: loss of proprioception, changes in fluid distribution, and deterioration of the musculoskeletal system.

On November 2, 2017, scientists reported that significant changes in the position and structure of the brain have been found in astronauts who have taken trips in space, based on MRI studies. Astronauts who took longer space trips were associated with greater brain changes.

In October 2018, NASA-funded researchers found that lengthy journeys into outer space, including travel to the planet Mars, may substantially damage the gastrointestinal tissues of astronauts. The studies support earlier work that found such journeys could significantly damage the brains of astronauts, and age them prematurely.

In March 2019, NASA reported that latent viruses in humans may be activated during space missions, adding possibly more risk to astronauts in future deep-space missions.

Research

Main article: Space medicine

Space medicine is a developing medical practice that studies the health of astronauts living in outer space. The main purpose of this academic pursuit is to discover how well and for how long people can survive the extreme conditions in space, and how fast they can re-adapt to the Earth's environment after returning from space. Space medicine also seeks to develop preventive and palliative measures to ease the suffering caused by living in an environment to which humans are not well adapted.

Ascent and re-entry

See also: High-G training

During takeoff and re-entry, space travelers can experience several times normal gravity. An untrained person can usually withstand about 3g, but can black out at 4 to 6g. G-force in the vertical direction is more difficult to tolerate than a force perpendicular to the spine because blood flows away from the brain and eyes. First the person experiences a temporary loss of vision and then at higher g-forces loses consciousness. G-force training and a G-suit which constricts the body to keep more blood in the head can mitigate the effects. Most spacecraft are designed to keep g-forces within comfortable limits.

Space environments

The environment of space is lethal without appropriate protection: the greatest threat in the vacuum of space derives from the lack of oxygen and pressure, although temperature and radiation also pose risks. The effects of space exposure can result in ebullism, hypoxia, hypocapnia, and decompression sickness. In addition to these, there is also cellular mutation and destruction from high energy photons and sub-atomic particles that are present in the surroundings. Decompression is a serious concern during the extra-vehicular activities (EVAs) of astronauts. Current Extravehicular Mobility Unit (EMU) designs take this and other issues into consideration, and have evolved over time. A key challenge has been the competing interests of increasing astronaut mobility (which is reduced by high-pressure EMUs, analogous to the difficulty of deforming an inflated balloon relative to a deflated one) and minimising decompression risk. Investigators have considered pressurizing a separate head unit to the regular 71 kPa (10.3 psi) cabin pressure as opposed to the current whole-EMU pressure of 29.6 kPa (4.3 psi). In such a design, pressurization of the torso could be achieved mechanically, avoiding mobility reduction associated with pneumatic pressurization.

Vacuum

See also: Uncontrolled decompression

This 1768 painting, An Experiment on a Bird in the Air Pump by Joseph Wright of Derby, depicts an experiment performed by Robert Boyle in 1660 to test the effect of a vacuum on a living system.

Human physiology is adapted to living within the atmosphere of Earth, and a certain amount of oxygen is required in the air we breathe. If the body does not get enough oxygen, then the astronaut is at risk of becoming unconscious and dying from hypoxia. In the vacuum of space, gas exchange in the lungs continues but results in the removal of all gases, including oxygen, from the bloodstream. After 9 to 12 seconds, the deoxygenated blood reaches the brain, and it results in the loss of consciousness. Exposure to vacuum for up to 30 seconds is unlikely to cause permanent physical damage. Animal experiments show that rapid and complete recovery is normal for exposures shorter than 90 seconds, while longer full-body exposures are fatal and resuscitation has never been successful. There is only a limited amount of data available from human accidents, but it is consistent with animal data. Limbs may be exposed for much longer if breathing is not impaired.

In December 1966, aerospace engineer and test subject Jim LeBlanc of NASA was participating in a test to see how well a pressurized space suit prototype would perform in vacuum conditions. To simulate the effects of space, NASA constructed a massive vacuum chamber from which all air could be pumped. At some point during the test, LeBlanc's pressurization hose became detached from the space suit. Even though this caused his suit pressure to drop from 3.8 psi (26.2 kPa) to 0.1 psi (0.7 kPa) in less than 10 seconds, LeBlanc remained conscious for about 14 seconds before losing consciousness due to hypoxia; the much lower pressure outside the body causes rapid de-oxygenation of the blood. "As I stumbled backwards, I could feel the saliva on my tongue starting to bubble just before I went unconscious and that's the last thing I remember", recalls LeBlanc. A colleague entered the chamber within 25 seconds and gave LeBlanc oxygen. The chamber was repressurized in 1 minute instead of the normal 30 minutes. LeBlanc recovered almost immediately with just an earache and no permanent damage.

Another effect from a vacuum is a condition called ebullism which results from the formation of bubbles in body fluids due to reduced ambient pressure. The steam may bloat the body up to twice its normal size and slow down circulation, but tissues are elastic and porous enough to prevent rupture. Technically, ebullism is considered to begin at an elevation of around 19 kilometres (12 mi; 62,000 ft) or pressures less than 6.3 kPa (47 mm Hg), known as the Armstrong limit. Experiments with other animals have revealed an array of symptoms that could also apply to humans. The least severe of these is the freezing of bodily secretions due to evaporative cooling. Severe symptoms, such as loss of oxygen in tissue, followed by circulatory failure and flaccid paralysis would occur in about 30 seconds. The lungs also collapse in this process, but will continue to release water vapour leading to cooling and ice formation in the respiratory tract. A rough estimate is that a human will have about 90 seconds to be recompressed, after which death may be unavoidable. Swelling from ebullism can be reduced by containment in a flight suit which is necessary to prevent ebullism above 19 km. During the Space Shuttle program astronauts wore a fitted elastic garment called a Crew Altitude Protection Suit (CAPS) which prevented ebullism at pressures as low as 2 kPa (15 mm Hg).

The only humans known to have died of exposure to vacuum in space are the three crew-members of the Soyuz 11 spacecraft; Vladislav Volkov, Georgi Dobrovolski, and Viktor Patsayev. During preparations for re-entry from orbit on June 30, 1971, a pressure-equalisation valve in the spacecraft's descent module unexpectedly opened at an altitude of 168 kilometres (551,000 ft), causing rapid depressurisation and the subsequent death of the entire crew.

Temperature

In a vacuum, there is no medium for removing heat from the body by conduction or convection. Loss of heat is by radiation from the 310 K temperature of a person to the 3 K of outer space. This is a slow process, especially in a clothed person, so there is no danger of immediately freezing. Rapid evaporative cooling of skin moisture in a vacuum may create frost, particularly in the mouth, but this is not a significant hazard.

Exposure to the intense radiation of direct, unfiltered sunlight would lead to local heating, though that would likely be well distributed by the body's conductivity and blood circulation. Other solar radiation, particularly ultraviolet rays, however, may cause severe sunburn.

Radiation

Main article: Health threat from cosmic rays

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

Without the protection of Earth's atmosphere and magnetosphere astronauts are exposed to high levels of radiation. High levels of radiation damage lymphocytes, cells heavily involved in maintaining the immune system; this damage contributes to the lowered immunity experienced by astronauts. Radiation has also recently been linked to a higher incidence of cataracts in astronauts. Outside the protection of low Earth orbit, galactic cosmic rays present further challenges to human spaceflight, as the health threat from cosmic rays significantly increases the chances of cancer over a decade or more of exposure. A NASA-supported study reported that radiation may harm the brain of astronauts and accelerate the onset of Alzheimer's disease. Solar flare events (though rare) can give a fatal radiation dose in minutes. It is thought that protective shielding and protective drugs may ultimately lower the risks to an acceptable level.

Crew living on the International Space Station (ISS) are partially protected from the space environment by Earth's magnetic field, as the magnetosphere deflects solar wind around the Earth and the ISS. Nevertheless, solar flares are powerful enough to warp and penetrate the magnetic defences, and so are still a hazard to the crew. The crew of Expedition 10 took shelter as a precaution in 2005 in a more heavily shielded part of the station designed for this purpose. However, beyond the limited protection of Earth's magnetosphere, interplanetary human missions are much more vulnerable. Lawrence Townsend of the University of Tennessee and others have studied the most powerful solar flare ever recorded. Radiation doses astronauts would receive from a flare of this magnitude could cause acute radiation sickness and possibly even death.

There is scientific concern that extended spaceflight might slow down the body's ability to protect itself against diseases. Radiation can penetrate living tissue and cause both short and long-term damage to the bone marrow stem cells which create the blood and immune systems. In particular, it causes 'chromosomal aberrations' in lymphocytes. As these cells are central to the immune system, any damage weakens the immune system, which means that in addition to increased vulnerability to new exposures, viruses already present in the body—which would normally be suppressed—become active. In space, T-cells (a form of lymphocyte) are less able to reproduce properly, and the T-cells that do reproduce are less able to fight off infection. Over time immunodeficiency results in the rapid spread of infection among crew members, especially in the confined areas of space flight systems.

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.

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

Weightlessness

Astronauts on the ISS in weightless conditions. Michael Foale can be seen exercising in the foreground.

Following the advent of space stations that can be inhabited for long periods of time, exposure to weightlessness has been demonstrated to have some deleterious effects on human health. Humans are well-adapted to the physical conditions at the surface of the Earth, and so in response to weightlessness, various physiological systems begin to change, and in some cases, atrophy. Though these changes are usually temporary, some do have a long-term impact on human health.

Short-term exposure to microgravity causes space adaptation syndrome, self-limiting nausea caused by derangement of the vestibular system. Long-term exposure causes multiple health problems, one of the most significant being loss of bone and muscle mass. Over time these deconditioning effects can impair astronauts' performance, increase their risk of injury, reduce their aerobic capacity, and slow down their cardiovascular system. As the human body consists mostly of fluids, gravity tends to force them into the lower half of the body, and our bodies have many systems to balance this situation. When released from the pull of gravity, these systems continue to work, causing a general redistribution of fluids into the upper half of the body. This is the cause of the round-faced 'puffiness' seen in astronauts, and may contribute to observations of altered speech motor control in astronauts. Redistributing fluids around the body itself causes balance disorders, distorted vision, and a loss of taste and smell.

A 2006 Space Shuttle experiment found that Salmonella typhimurium, a bacterium that can cause food poisoning, became more virulent when cultivated in space. On April 29, 2013, scientists in Rensselaer Polytechnic Institute, funded by NASA, reported that, during spaceflight on the International Space Station, microbes seem to adapt to the space environment in ways "not observed on Earth" and in ways that "can lead to increases in growth and virulence". In 2017, bacteria were found to be more resistant to antibiotics and to thrive in the near-weightlessness of space. Microorganisms have been observed to survive the vacuum of outer space.

Motion sickness

Main article: Space adaptation syndrome

Bruce McCandless II floating free in orbit with a space suit and Manned Maneuvering Unit.

The most common problem experienced by humans in the initial hours of weightlessness is known as space adaptation syndrome or SAS, commonly referred to as space sickness. It is related to motion sickness, and arises as the vestibular system adapts to weightlessness. Symptoms of SAS include nausea and vomiting, vertigo, headaches, lethargy, and overall malaise. The first case of SAS was reported by cosmonaut Gherman Titov in 1961. Since then, roughly 45% of all people who have flown in space have suffered from this condition.

Bone and muscle deterioration

Main article: Spaceflight osteopenia

Aboard the International Space Station, astronaut Frank De Winne is attached to the COLBERT with bungee cords

A major effect of long-term weightlessness involves the loss of bone and muscle mass. In a weightless environment, astronauts put almost no weight on the back muscles or leg muscles used for standing up. Those muscles then start to weaken and eventually get smaller. Consequently, some muscles atrophy rapidly, and without regular exercise astronauts can lose up to 20% of their muscle mass in just 5 to 11 days. The types of muscle fibre prominent in muscles also change. Slow-twitch endurance fibres used to maintain posture are replaced by fast-twitch rapidly contracting fibres that are insufficient for any heavy labour. Advances in research on exercise, hormone supplements, and medication may help maintain muscle and body mass.

Bone metabolism also changes. Normally, bone is laid down in the direction of mechanical stress. However, in a microgravity environment, there is very little mechanical stress. This results in a loss of bone tissue approximately 1.5% per month especially from the lower vertebrae, hip, and femur. Due to microgravity and the decreased load on the bones, there is a rapid increase in bone loss, from 3% cortical bone loss per decade to about 1% every month the body is exposed to microgravity, for an otherwise healthy adult. The rapid change in bone density is dramatic, making bones frail and resulting in symptoms that resemble those of osteoporosis. On Earth, the bones are constantly being shed and regenerated through a well-balanced system which involves signaling of osteoblasts and osteoclasts. These systems are coupled, so that whenever bone is broken down, newly formed layers take its place—neither should happen without the other, in a healthy adult. In space, however, there is an increase in osteoclast activity due to microgravity. This is a problem because osteoclasts break down the bones into minerals that are reabsorbed by the body. Osteoblasts are not consecutively active with the osteoclasts, causing the bone to be constantly diminished with no recovery. This increase in osteoclasts activity has been seen particularly in the pelvic region because this is the region that carries the biggest load with gravity present. A study demonstrated that in healthy mice, osteoclasts appearance increased by 197%, accompanied by a down-regulation of osteoblasts and growth factors that are known to help with the formation of new bone, after only sixteen days of exposure to microgravity. Elevated blood calcium levels from the lost bone result in dangerous calcification of soft tissues and potential kidney stone formation. It is still unknown whether bone recovers completely. Unlike people with osteoporosis, astronauts eventually regain their bone density. After a 3–4 month trip into space, it takes about 2–3 years to regain lost bone density. New techniques are being developed to help astronauts recover faster. Research on diet, exercise, and medication may hold the potential to aid the process of growing new bone.

To prevent some of these adverse physiological effects, the ISS is equipped with two treadmills (including the COLBERT), and the aRED (advanced Resistive Exercise Device), which enable various weight-lifting exercises which add muscle but do nothing for bone density, and a stationary bicycle; each astronaut spends at least two hours per day exercising on the equipment. Astronauts use bungee cords to strap themselves to the treadmill. Astronauts subject to long periods of weightlessness wear pants with elastic bands attached between waistband and cuffs to compress the leg bones and reduce osteopenia.

Currently, NASA is using advanced computational tools to understand how to best counteract the bone and muscle atrophy experienced by astronauts in microgravity environments for prolonged periods of time. The Human Research Program's Human Health Countermeasures Element chartered the Digital Astronaut Project to investigate targeted questions about exercise countermeasure regimes.NASA is focusing on integrating a model of the advanced Resistive Exercise Device (ARED) currently on board the International Space Station with OpenSim musculoskeletal models of humans exercising with the device. The goal of this work is to use inverse dynamics to estimate joint torques and muscle forces resulting from using the ARED, and thus more accurately prescribe exercise regimens for the astronauts. These joint torques and muscle forces could be used in conjunction with more fundamental computational simulations of bone remodeling and muscle adaptation in order to more completely model the end effects of such countermeasures, and determine whether a proposed exercise regime would be sufficient to sustain astronaut musculoskeletal health.

Fluid redistribution

The effects of microgravity on fluid distribution around the body (greatly exaggerated).

The Beckman Physiological and Cardiovascular Monitoring System in the Gemini and Apollo suits would inflate and deflate cuffs to stimulate blood flow to lower limbs

Astronaut Clayton Anderson observes as a water bubble floats in front of him on the Space Shuttle Discovery. Water cohesion plays a bigger role in microgravity than on Earth

In space, astronauts lose fluid volume—including up to 22% of their blood volume. When the astronauts return to Earth, low blood volume can cause orthostatic intolerance or dizziness when standing. Under the influence of the earth's gravity, when a person is standing, blood and other body fluids are pulled towards the lower body, increasing pressure at the feet. In a microgravity environment, hydrostatic pressures throughout the body are removed and the resulting change in blood distribution is analogous to an individual changing from standing up to lying down. The persistent change in the redistribution of blood volume may result in facial edema and other unwelcome side effects. Upon return to Earth, the reduced blood volume creates orthostatic hypotension. Orthostatic tolerance after spaceflight has been greatly improved by fluid loading countermeasures taken by astronauts before touchdown.

Disruption of senses

Vision

In 2013 NASA published a study that found changes to the eyes and eyesight of monkeys with spaceflights longer than 6 months. Noted changes included a flattening of the eyeball and changes to the retina. Space travelers' eyesight can become blurry after too much time in space. Another effect is known as cosmic ray visual phenomena.

[a] NASA survey of 300 male and female astronauts, about 23 percent of short-flight and 49 percent of long-flight astronauts said they had experienced problems with both near and distance vision during their missions. Again, for some people vision problems persisted for years afterward.

— NASA

Since dust can not settle in zero gravity, small pieces of dead skin or metal can get in the eye, causing irritation and increasing the risk of infection.

Long spaceflights can also alter a space traveler's eye movements (particularly the vestibulo-ocular reflex).

Intracranial pressure

Main article: Visual impairment due to intracranial pressure

Because weightlessness increases the amount of fluid in the upper part of the body, it has been hypothesized that astronauts experience pathologically elevated intracranial pressure. This would increase pressure on the backs of the eyeballs, affecting their shape and slightly crushing the optic nerve. This was noticed in 2012 in a study using MRI scans of astronauts who had returned to Earth following at least one month in space. However, direct evidence of pathologically elevated intracranial pressures in microgravity has yet to be obtained. Invasive measures of intracranial pressure on parabolic flights showed that pressures were actually reduced relative to supine levels and slightly higher than seated levels, meaning pressures were within normal physiological variation. Without elevated intracranial pressures, a force that flattens the posterior of the eye is still created by the removal of hydrostatic gradients in the intracranial and intraocular spaces.

Such eyesight problems could be a major concern for future deep space flight missions, including a crewed mission to the planet Mars. If indeed elevated intracranial pressure is the cause, artificial gravity might present one solution, as it would for many human health risks in space. However, such artificial gravitational systems have yet to be proven. More, even with sophisticated artificial gravity, a state of relative microgravity may remain, the risks of which remain unknown.

Taste

One effect of weightlessness on humans is that some astronauts report a change in their sense of taste when in space. Some astronauts find that their food is bland, others find that their favorite foods no longer taste as good (one who enjoyed coffee disliked the taste so much on a mission that he stopped drinking it after returning to Earth); some astronauts enjoy eating certain foods that they would not normally eat, and some experience no change whatsoever. Multiple tests have not identified the cause, and several theories have been suggested, including food degradation, and psychological changes such as boredom. Astronauts often choose strong-tasting food to combat the loss of taste.

Additional physiological effects

Within one month the human skeleton fully extends in weightlessness, causing height to increase by 2,5 cm (1 inch). After two months, calluses on the bottoms of feet molt and fall off from lack of use, leaving soft new skin. Tops of feet become, by contrast, raw and painfully sensitive, as they rub against the handrails feet are hooked into for stability. Tears cannot be shed while crying, as they stick together into a ball. In microgravity odors quickly permeate the environment, and NASA found in a test that the smell of cream sherry triggered the gag reflex. Various other physical discomforts such as back and abdominal pain are common because of the readjustment to gravity, where in space there was no gravity and these muscles could freely stretch. These may be part of the asthenization syndrome reported by cosmonauts living in space over an extended period of time, but regarded as anecdotal by astronauts. Fatigue, listlessness, and psychosomatic worries are also part of the syndrome. The data is inconclusive; however, the syndrome does appear to exist as a manifestation of the internal and external stress crews in space must face.

Psychological effects

See also: Psychological and sociological effects of spaceflight

Studies of Russian cosmonauts, such as those on Mir, provide data on the long-term effects of space on the human body.

Research

The psychological effects of living in space have not been clearly analyzed but analogies on Earth do exist, such as Arctic research stations and submarines. The enormous stress on the crew, coupled with the body adapting to other environmental changes, can result in anxiety, insomnia and depression.

Stress

There has been considerable evidence that psycho social stressors are among the most important impediments to optimal crew morale and performance. Cosmonaut Valery Ryumin, twice Hero of the Soviet Union, quotes this passage from "The Handbook of Hymen" by O. Henry in his autobiographical book about the Salyut 6 mission: "If you want to instigate the art of manslaughter just shut two men up in an eighteen by twenty-foot cabin for a month. Human nature won't stand it."

NASA's interest in psychological stress caused by space travel, initially studied when their crewed missions began, was rekindled when astronauts joined cosmonauts on the Russian space station Mir. Common sources of stress in early American missions included maintaining high performance while under public scrutiny, as well as isolation from peers and family. On the ISS, the latter is still often a cause of stress, such as when NASA Astronaut Daniel Tani's mother died in a car accident, and when Michael Fincke was forced to miss the birth of his second child.

Sleep

Main article: Sleep in space

The amount and quality of sleep experienced in space is poor due to highly variable light and dark cycles on flight decks and poor illumination during daytime hours in the spacecraft. Even the habit of looking out of the window before retiring can send the wrong messages to the brain, resulting in poor sleep patterns. These disturbances in circadian rhythm have profound effects on the neurobehavioural responses of the crew and aggravate the psychological stresses they already experience. Sleep is disturbed on the ISS regularly due to mission demands, such as the scheduling of incoming or departing space vehicles. Sound levels in the station are unavoidably high because the atmosphere is unable to thermosiphon; fans are required at all times to allow processing of the atmosphere, which would stagnate in the freefall (zero-g) environment. Fifty percent of Space Shuttle astronauts took sleeping pills and still got 2 hours less sleep each night in space than they did on the ground. NASA is researching two areas which may provide the keys to a better night's sleep, as improved sleep decreases fatigue and increases daytime productivity. A variety of methods for combating this phenomenon are constantly under discussion.

Duration of space travel

A study of the longest spaceflight concluded that the first three weeks represent a critical period where attention is adversely affected because of the demand to adjust to the extreme change of environment. While Skylab's three crews remained in space 1, 2, and 3 months respectively, long-term crews on Salyut 6, Salyut 7, and the ISS remain about 5–6 months, while MIR expeditions often lasted longer. The ISS working environment includes further stress caused by living and working in cramped conditions with people from very different cultures who speak different languages. First-generation space stations had crews who spoke a single language, while 2nd and 3rd generation stations have crews from many cultures who speak many languages. The ISS is unique because visitors are not classed automatically into 'host' or 'guest' categories as with previous stations and spacecraft, and may not suffer from feelings of isolation in the same way.

Children

So far no children have been to space, nor are any planned to go.

Raising children in space has been confronted with ethical questions about the impact of a non-terrestrial environment and rights to it.

Reproduction

For other uses, see Space sexology and Sex in space.

Reproductive health and fertility

Research has been conducted in frozen sperm integrity of mice after a stay on a space station.

Development

Further information: Pantropy

Research into carrying an unborn child in space has been conducted with rats unsuccessfully in 1983 and with fish successfully in 1994.

The company SpaceBorn United aims to grow a baby in space, enabling humans through its research to reproduce extraterrestrially, stating its reasons as for making humanity sustainable without Earth, seeing the need for becoming multiplanetary, as a backup plan. The company is as of 2023 planning preliminary research in space with rats, making them live and reproduce in space.

Future use

Space colonization efforts must take into account the effects of space on the human body.

The sum of human experience has resulted in the accumulation of 58 solar years in space and a much better understanding of how the human body adapts. In the future, industrialisation of space and exploration of inner and outer planets will require humans to endure longer and longer periods in space. The majority of current data comes from missions of short duration and so some of the long-term physiological effects of living in space are still unknown. A round trip to Mars with current technology is estimated to involve at least 18 months in transit alone. Knowing how the human body reacts to such time periods in space is a vital part of the preparation for such journeys. On-board medical facilities need to be adequate for coping with any type of trauma or emergency as well as contain a huge variety of diagnostic and medical instruments in order to keep a crew healthy over a long period of time, as these will be the only facilities available on board a spacecraft for coping not only with trauma but also with the adaptive responses of the human body in space.

At the moment only rigorously tested humans have experienced the conditions of space. If off-world colonization someday begins, many types of people will be exposed to these dangers, and the effects on the very young are completely unknown. On October 29, 1998, John Glenn, one of the original Mercury 7, returned to space at the age of 77. His space flight, which lasted 9 days, provided NASA with important information about the effects of space flight on older people. Factors such as nutritional requirements and physical environments which have so far not been examined will become important. Overall, there is little data on the manifold effects of living in space, and this makes attempts toward mitigating the risks during a lengthy space habitation difficult. Testbeds such as the ISS are currently being utilized to research some of these risks.

The environment of space is still largely unknown, and there will likely be as-yet-unknown hazards. Meanwhile, future technologies such as artificial gravity and more complex bioregenerative life support systems may someday be capable of mitigating some risks.