The health threat from cosmic rays is the danger posed by galactic cosmic rays and solar energetic particles to astronauts on interplanetary missions. Galactic cosmic rays (GCRs) consist of high energy protons (85%), helium (14%) and other high energy nuclei (HZE ions). Solar energetic particles consist primarily of protons accelerated by the Sun to high energies via proximity to solar flares and coronal mass ejections. They are one of the most important barriers standing in the way of plans for interplanetary travel by crewed spacecraft.
The deep-space radiation environment
The radiation environment of deep space is different from that on the Earth's surface or in low Earth orbit, due to the much larger flux of high-energy galactic cosmic rays (GCRs), along with radiation from solar proton events (SPEs) and the radiation belts.
Galactic cosmic rays create a continuous radiation dose throughout the Solar System that increases during solar minimum and decreases during solar maximum (solar activity). The inner and outer radiation belts are two regions of trapped particles from the solar wind that are later accelerated by dynamic interaction with the Earth's magnetic field. While always high, the radiation dose in these belts can increase dramatically during geomagnetic storms and substorms. Solar proton events are bursts of energetic protons accelerated by the Sun. They occur relatively rarely and can produce extremely high radiation levels. Without thick shielding, SPEs are sufficiently strong to cause acute radiation poisoning and death.
Life on the Earth's surface is protected from galactic cosmic rays by a number of factors:
- The Earth's atmosphere is opaque to primary cosmic rays with energies below about 1 gigaelectron volt (GeV), so only secondary radiation can reach the surface. The secondary radiation is also attenuated by absorption in the atmosphere, as well as by radioactive decay in flight of some particles, such as muons. Particles entering from a direction close to the horizon are especially attenuated. The world's population receives an average of 0.4 millisieverts (mSv) of cosmic radiation annually (separate from other sources of radiation exposure like inhaled radon) due to atmospheric shielding. At 12 km altitude, above most of the atmosphere's protection, radiation as an annual rate rises to 20 mSv at the equator to 50–120 mSv at the poles, varying between solar maximum and minimum conditions.
- Except for the very highest energy galactic cosmic rays, the radius of gyration in the Earth's magnetic field is small enough to ensure that they are deflected away from Earth. Missions beyond low Earth orbit leave the protection of the geomagnetic field, and transit the Van Allen radiation belts. Thus they may need to be shielded against exposure to cosmic rays, Van Allen radiation, or solar flares. The region between two to four Earth radii lies between the two radiation belts and is sometimes referred to as the "safe zone". See the implications of the Van Allen belts for space travel for more information.
- The interplanetary magnetic field, embedded in the solar wind, also deflects cosmic rays. As a result, cosmic ray fluxes within the heliopause are inversely correlated with the solar cycle.
Of the above factors, all but the first one apply to low Earth orbit craft, such as the Space Shuttle and the International Space Station. Exposures on the ISS average 150 mSv per year, although frequent crew rotations minimize individual risk. Astronauts on Apollo and Skylab missions received on average 1.2 mSv/day and 1.4 mSv/day respectively. Since the durations of the Apollo and Skylab missions were days and months, respectively, rather than years, the doses involved were smaller than would be expected on future long-term missions such as to a near-Earth asteroid or to Mars (unless far more shielding could be provided).
On 31 May 2013, NASA scientists reported that a possible manned mission to Mars may involve a great radiation risk based on the amount of energetic particle radiation detected by the radiation assessment detector (RAD) on the Mars Science Laboratory while traveling from the Earth to Mars in 2011–2012.
Human health effects
The potential acute and chronic health effects of space radiation, as with other ionizing radiation exposures, involve both direct damage to DNA and indirect effects due to generation of reactive oxygen species. Acute (or early radiation) effects result from high radiation doses, and these are most likely to occur after solar particle events (SPEs). Likely chronic effects of space radiation exposure include both stochastic events such as radiation carcinogenesis and deterministic degenerative tissue effects. To date, however, the only pathology associated with space radiation exposure is a higher risk for radiation cataract among the astronaut corps.
The health threat depends on the flux, energy spectrum, and nuclear composition of the radiation. The flux and energy spectrum depend on a variety of factors: short-term solar weather, long-term trends (such as an apparent increase since the 1950s), and position in the Sun's magnetic field. These factors are incompletely understood. The Mars Radiation Environment Experiment (MARIE) was launched in 2001 in order to collect more data. Estimates are that humans unshielded in interplanetary space would receive annually roughly 400 to 900 mSv) (compared to 2.4 mSv on Earth) and that a Mars mission (12 months in flight and 18 months on Mars) might expose shielded astronauts to roughly 500 to 1000 mSv. These doses approach the 1 to 4 Sv career limits advised by the National Council on Radiation Protection and Measurements for low Earth orbit activities.
The quantitative biological effects of cosmic rays are poorly known, and are the subject of ongoing research. Several experiments, both in space and on Earth, are being carried out to evaluate the exact degree of danger. Experiments in 2007 at Brookhaven National Laboratory's NASA Space Radiation Laboratory (NSRL) suggest that biological damage due to a given exposure is actually about half what was previously estimated: specifically, it turns out that low energy protons cause more damage than high energy ones. This is explained by the fact that slower particles have more time to interact with molecules in the body. This may be interpreted as an acceptable result for space travel as the cells affected end up with greater energy deposition and are more likely to die without proliferating into tumors. This is in contrast to the current dogma on radiation exposure to human cells which considers lower energy radiation of higher weighting factor for tumor formation.
Central nervous system
Hypothetical early and late effects on the central nervous system are of great concern to NASA and an area of active current research interest. It is postulated short and long term effects of CNS exposure to galactic cosmic radiation are likely to pose significant neurological health risks to human long-term space travel. Estimates suggest considerable exposure to high energy heavy (HZE) ions as well as protons and secondary radiation during Mars or prolonged Lunar missions with estimates of whole body effective doses ranging from 0.17 to greater than 1.0 Sv. Given the high linear energy transfer potential of such particles, a considerable proportion of those cells exposed to HZE radiation are likely to die. Based on calculations of heavy ion fluences during space flight as well as various experimental cell models, as many as 5% of an astronaut’s cells might be killed during such missions. With respect to cells in critical brain regions, as many as 13% of such cells may be traversed at least once by an iron ion during a three-year Mars mission. Several Apollo astronauts reported seeing light flashes, although the precise biological mechanisms responsible are unclear. Likely pathways include heavy ion interactions with retinal photoreceptors and Cerenkov radiation resulting from particle interactions within the vitreous humor. This phenomenon has been replicated on Earth by scientists at various institutions. As the duration of the longest Apollo flights was less than two weeks, the astronauts had limited cumulative exposures and a corresponding low risk for radiation carcinogenesis. In addition, there were only 24 such astronauts, making statistical analysis of any potential health effects problematic.
On 31 December 2012, a NASA-supported study reported that manned spaceflight may harm the brains of astronauts and accelerate the onset of Alzheimer's disease. This research is problematic due to many factors, inclusive of the intensity of which mice were exposed to radiation which far exceeds normal mission rates.
Material shielding can be effective against galactic cosmic rays, but thin shielding may actually make the problem worse for some of the higher energy rays, because more shielding causes an increased amount of 
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.
Material shielding has been considered:
- Liquid hydrogen, which would be brought along as fuel in any case, tends to give relatively good shielding, while producing relatively low levels of secondary radiation. Therefore, the fuel could be placed so as to act as a form of shielding around the crew. However, as fuel is consumed by the craft, the crew's shielding decreases.
- Water, which is necessary to sustain life, could also contribute to shielding. But it too is consumed during the journey unless waste products are utilized.
- Asteroids could serve to provide shielding.
- Magnetic deflection of charged radiation particles and/or electrostatic repulsion is a hypothetical alternative to pure conventional mass shielding under investigation. In theory, power requirements for the case of a 5 meter torus drop from an excessive 10 
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 any time 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 spacestation envisioned 4 metric tons per square meter of shielding to drop radiation exposure to 2.5 mSv annually (± a factor of 2 uncertainty), less than the tens of millisieverts or more in some populated high natural background radiation areas on Earth, but the sheer mass for that level of mitigation was considered practical only because it involved first building a lunar mass driver to launch material.
Several active shielding methods have been considered for lesser mass than passive mass shielding, but they remain in the realm of uncertain speculation at the present time. Since the segment of space radiation penetrating farthest through thick material shielding, deep in interplanetary space, is gigaelectron-volt-level positively charged nuclei, a repulsive positively charged electrostatic shield has been hypothesized, but issues include plasma instabilities and power needs for an accelerator constantly keeping the charge from being neutralized by deep-space electrons. A more common proposal is magnetic shielding using superconductors (or plasma currents), although, among other complications, if designing a relatively compact system, magnetic fields up to 10–20 teslas could be required around a manned spacecraft, higher than the several teslas in 
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.
Another line of research is the development of drugs that mimic or 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.
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.
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.
- Electromagnetic radiation and health
- Background radiation
- Effect of spaceflight on the human body
- Lagrange point colonization
- List of microorganisms tested in outer space
- NASA Space Radiation Laboratory
- Proton: Exposure
- Solar flare: Hazards
- Solar proton event
- Solar wind
- Space medicine
- Van Allen belt
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