Space Radiation in Earth Orbit

15 11 2011

The Light Flash phenomenon is only one effect that is caused by cosmic rays in Earth Orbit or in the Solar System. Our planet with its atmosphere and its magnetic field shields us from cosmic rays and their dangerous effects.

Earth magnetic field offers a partial protection from cosmic radiation to astronauts in Low Earth Orbit (LEO) during Shuttle or ISS missions, while exiting the magnetosphere (missions to the Moon or to Mars) would expose astronauts to higher radiation doses. Anyway during all space missions, even LEO ones, ionizing radiation levels are higher than on Earth surface.

The main radiation sources in LEO are galactic cosmic rays, solar particles and particles trapped in the Van Allen belts.

Galactic cosmic rays (GCR) are charged particles, mainly nuclei:

imageimage

Nuclei are mainly composed by protons (87%) and helium (12%), while heavy nuclei are mostly carbon, nitrogen and oxygen nuclei. Heavier nuclei up to uranium are present in smaller quantities

Cosmic rays kinetic energies range from some MeVs up to 1012 MeV (who does not like powers of ten would like to know that 1012 is equal to ten with twelve zeroes, that is 1.000.000.000.000). For energies higher than 2 GeV for nucleon (that is 2000 MeV for nucleon) these particles travel at speed near to speed of light: they come from our galaxy after being accelerated by supernovae explosions; particles of higher kinetic energy are of extragalactic origin and their acceleration mechanism is still under debate.

Solar particles compose the solar wind and are mainly composed by low energy protons and electrons (kinetic energy less than 100 MeV) coming from outer shells of the Sun (the chromosphere) at speed around 400 km/s. Usually solar particle are not of great hazard, but particle events associated with solar flares of coronal mass ejections could occasionally cause a sudden increase of particle flux with possible risks for astronauts safety.

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Solar flare of 13 February 2011. Credit: NASA/SDO

These two components can be found even outside earth magnetosphere and must be taken into account when planning missions outside Earth orbit.

The third radiation component can be found only in Earth orbit. Charged particles are trapped by the magnetic field in the radiation belts (Van Allen belts): the inner belt is mainly composed by protons and the outer one is composed by electrons. The outer belt is crossed by geostationary satellite orbits used for telecommunications. Solar storms cause an increase of particle flux and could harm or damage these satellites. The inner belt is crossed by the orbit of the Space Shuttle of the space stations.

Near the Earth the cosmic rays are almost totally deflected by the magnetic field that acts as a shield. These particles could be channeled along the field lines near the poles originating the fascinating effect known as aurora. These auroras are visible also on other planets.

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The orbit used for long term missions is the result of a compromise between a stable orbit (out of the atmosphere to avoid friction) and a safe environment for the astronauts (far away from the center of the inner radiation belt).

Previous posts:
Cosmic rays and human exploration of space
ALTEA- An Italian experiment onboard the International Space Station
The effects of cosmic rays on astronauts: the Light Flash phenomenon

Further readings and sources:
Raggi cosmici e missioni spaziali (in Italian)
A special thanks to Riccardo ‘Unreal’ Rossi for pictures suggestions (via Facebook)

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3 01 2012
What if you are hit by a cosmic ray? « ALTEA Space

[…] Cosmic rays are completely ionized atomic nuclei (without any orbiting electrons) that travel through space at very high speed. We already talked about how they are distributed in LEO orbit (as Space Station and Shuttle orbits) where their intensity increases near the magnetic poles and that there is a trapped component of protons in the inner radiation belt that is crossed by LEO orbits in the South Atlantic Anomaly region. […]

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