health threat from cosmic rays health threat from cosmic rays health threats from cosmic rays are the dangers posed by cosmic rays to astronauts on interplanetary missions or any missions that venture through the van-allen belts or outside the earth's magnetosphere. they are one of the greatest barriers standing in the way of plans for interplanetary travel by crewed spacecraft, but space radiation health risks also occur for missions in low earth orbit such as the international space station (iss). this page is currently being merged. after a discussion, consensus to merge this page with effects of ionizing radiation in spaceflight was found. you can help implement the merge by following the instructions at help:merging and the resolution on the discussion. process started in september 2022. in october 2015, the nasa office of inspector general issued a health hazards report related to space exploration, including a human mission to mars. the deep-space radiation environment sources of ionizing radiation in interplanetary space. 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 (gcrs) consist of high energy protons (85%), alpha particles (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. heavy ions and low energy protons and helium particles are highly ionizing forms of radiation, which produce distinct biological damage compared to x-rays and gamma-rays. microscopic energy deposition from highly ionizing particles consists of a core radiation track due to direct ionizations by the particle and low energy electrons produced in ionization, and a penumbra of higher energy electrons that may extend hundreds of microns from the particles path in tissue. the core track produces extremely large clusters of ionizations within a few nanometres, which is qualitatively distinct from energy deposition by x-rays and gamma rays; hence human epidemiology data which only exists for these latter forms of radiation is limited in predicting the health risks from space radiation to astronauts. the radiation belts are within earth's magnetosphere and do not occur in deep space, while organ dose equivalents on the international space station are dominated by gcr not trapped radiation. microscopic energy deposition in cells and tissues is distinct for gcr compared to x-rays on earth, leading to both qualitative and quantitative differences in biological effects, while there is no human epidemiology data for gcr for cancer and other fatal risks. the solar cycle is an approximately 11-year period of varying solar activity including solar maximum where the solar wind is strongest and solar minimum where the solar wind is weakest. 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 (spes) 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 far from the zenith 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. missions beyond low earth orbit 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 and 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. electromagnetic radiation created by lightning in clouds only a few miles high can create a safe zone in the van allen radiation belts that surround the earth. this zone, known as the "van allen belt slot", may be a safe haven for satellites in medium earth orbits (meos), protecting them from the sun's intense radiation. as a result, the energy input of gcrs to the atmosphere is negligible – about 10−9 of solar radiation – roughly the same as starlight. 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 human 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. however, the absorbed dose and dose equivalent for a mars mission were predicted in the early 1990s by badhwar, cucinotta, and others (see for example badhwar, cucinotta et al., radiation research vol. 138, 201–208, 1994) and the result of the msl experiment are to a large extent consistent with these earlier predictions. human health effects comparison of radiation doses, includes the amount detected on the trip from earth to mars by the rad on the msl (2011–2013). the y-axis scale is in logarithmic scale. for example, the exposure from 6 months aboard the iss is roughly a factor of 10 greater than that from an abdominal ct scan. the potential acute and chronic health effects of space radiation, as with other ionizing radiation exposures, involve both direct damage to dna, indirect effects due to generation of reactive oxygen species, and changes to the biochemistry of cells and tissues, which can alter gene transcription and the tissue microenvironment along with producing dna mutations. 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 (ncrp) for low earth orbit activities in 1989, and the more recent ncrp recommendations of 0.5 to 2 sv in 2000 based on updated information on dose to risk conversion factors. dose limits depend on age at exposure and sex due to difference in susceptibility with age, the added risks of breast and ovarian cancers to women, and the variability of cancer risks such as lung cancer between men and women. a 2017 laboratory study on mice, estimates that the risk of developing cancer due to galactic cosmic rays (gcr) radiation exposure after a mars mission could be two times greater than what scientists previously thought. 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. additionally, the impact of the space microgravity environment on dna repair has in part confounded the interpretation of some results. experiments over the last 10 years have shown results both higher and lower than predicted by current quality factors used in radiation protection, indicating large uncertainties exist. 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 suggested that low energy protons cause more damage than high energy ones. this was 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. relative biological effectiveness (rbe) depends on radiation type described by particle charge number, z, and kinetic energy per amu, e, and varies with tumor type with limited experimental data suggesting leukemia's having the lowest rbe, liver tumors the highest rbe, and limited or no experimental data on rbe available for cancers that dominate human cancer risks including lung, stomach, breast, and bladder cancers. studies of harderian gland tumors in a single strain of female mice with several heavy ions have been made, however it is not clear how well the rbe for this tumor type represents the rbe for human cancers such as lung, stomach, breast and bladder cancers nor how rbe changes with sex and genetic background. part of the iss year long mission is to determine the health impacts of cosmic ray exposure over the course of one year spent aboard the international space station. however, sample sizes for accurately estimating health risks directly from crew observations for the risks of concern (cancer, cataracts, cognitive and memory changes, late cns risks, circulatory diseases, etc.) are large (typically >>10 persons) and necessarily involve long post-mission observation times (>10 years). the small number of astronauts on the iss and the limited length of missions puts statistical limits on how accurate risk predictions can be. hence the need for ground-based research to predict cosmic ray health risks. in addition, radiation safety requirements mandate that risks should be adequately understood prior to astronauts incurring significant risks, and methods developed to mitigate the risks if necessary. noting these limitations, 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 over time. an earlier 1998 study came to similar conclusions, with no statistically significant increase in cancer among astronauts compared to the reference group. see spaceflight radiation carcinogenesis for further details on cancer risks. central nervous system