page_title: Health threat from cosmic rays
section_title: The deep-space radiation environment
hierarchical_section_title: Health threat from cosmic rays / The deep-space radiation environment
caption_attribution_description: English: PIA16938: Sources of Ionizing Radiation in Interplanetary Space http://photojournal.jpl.nasa.gov/catalog/PIA16938 The Radiation Assessment Detector (RAD) on NASA's Curiosity Mars rover monitors high-energy atomic and subatomic particles coming from the sun, distant supernovae and other sources. RAD measured the flux of this energetic-particle radiation while shielded inside the Mars Science Laboratory spacecraft on the flight delivering Curiosity from Earth to Mars, and continues to monitor the flux on the surface of Mars. This illustration depicts the two main types of radiation that RAD monitors, and how the magnetic field around Earth affects the radiation in space near Earth. Galactic cosmic rays are a variable shower of charged particles coming from supernova explosions and other events extremely far from our solar system. The sun is the other main source of energetic particles this investigation detects and characterizes. The sun spews electrons, protons and heavier ions in "solar particle events" fed by solar flares and ejections of matter from the sun's corona. The spacecraft carrying RAD departed the influence of Earth's magnetic field early during the flight from Earth to Mars. Earth's magnetic field and atmosphere provide effective shielding against the possible deadly effects of galactic cosmic rays and solar particle events. Mars lacks a global magnetic field and has only about 1 percent as much atmosphere as Earth does. Data from RAD during the trip to Mars and on the surface of Mars provide important aid to planning for astronaut safety in design of possible human missions to Mars. Southwest Research Institute, in San Antonio, Texas, and Boulder, Colo., supplied and operates the RAD instrument in collaboration with Germany's national aerospace research center, Deutsches Zentrum für Luft- und Raumfahrt. NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Mars Science Laboratory Project and the mission's Curiosity rover for NASA's Science Mission Directorate in Washington. NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology, Pasadena, manages the Mars Science Laboratory Project for NASA's Science Mission Directorate, Washington. JPL designed and built the rover. For more information about Curiosity and its mission, visit: http://www.nasa.gov/msl and http://mars.jpl.nasa.gov/msl.
context_page_description: 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.
  In October 2015, the NASA Office of Inspector General issued a health hazards report related to space exploration, including a human mission to Mars.
context_section_description: 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%), 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. 
  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.
  But of course 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 
caption: Sources of ionizing radiation in interplanetary space.
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