ECSS E 10-04 v0.10 - European Cooperation on Space Standardization ...

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<strong>ECSS</strong>-E-<strong>10</strong>-<strong>04</strong>B (draft v0.<strong>10</strong>) 13/03/2008
Space Environment
Galactic cosmic rays (GCR) are high-energy charged particles that enter the solar system from
the outside, the flux of which becomes modulated in anti-correlation with solar activity due to
the solar wind. They are composed of protons, electrons, and fully ionized nuclei. There is a
continuous and isotropic flux of Galactic Cosmic Ray (GCR) ions. Although the flux is low, a
few particles cm -2 s -1 , GCRs include energetic heavy ions which can deposit significant amounts
of energy in sensitive volumes and so cause problems to spacecrafts' electronics and humans in
space. As for Solar particles, the Earth’s magnetic field provides a varying degree of geomagnetic
shielding of near-Earth locations from these particles
9.1.2.4. Geomagnetic shielding
The Earth’s magnetic field partially shields near-Earth space from solar energetic particles and
cosmic rays, an effect known as geomagnetic shielding. However, these particles can easily
reach polar regions and high altitudes such as the geostationary orbit. Geomagnetic shielding of
protons is computed on the basis of their trajectories in geomagnetic B, L space.
9.1.2.5. Other planets
The above environments are common to planets other than the Earth. Jupiter, Saturn, Uranus
and Neptune have strong magnetic fields inducing severe radiation environments in their radiation
belts. Mercury has a small magnetosphere which may lead to transient radiation belts. The
other planets (Mars, Venus) have no trapped radiation. Missions to them are only exposed to
GCR and SEP.
9.1.2.6. Neutrons
Neutrons are ejected by the Sun. They decay rapidly in the interplanetary medium, and only a
few can reach the Earth. They are important for missions close to the Sun.
When highly energetic charged particles strike the earth’s upper atmosphere they create secondary
particles throughout the atmosphere including very significant fluxes of neutrons. Of
these, some are emitted back into space as atmospheric albedo neutrons of between 0,1 and 2,2
cm -2 s -1 , depending on the geomagnetic latitude and the phase of the solar cycle, and these are
significant for LEO spacecraft including ISS. Model results for albedo neutron spectra are
given in Annex I.
For some planetary environments, such as Mars, the secondary neutrons from cosmic ray and
solar proton interactions with the atmosphere and regolith become the dominant radiation, in
particular for manned missions.
9.1.2.7. Secondary radiation
Secondary radiation is generated by the interaction of the above environmental components
with materials of the spacecraft. A wide variety of secondary radiations are possible, of varying
importance. The <strong>ECSS</strong>-E-<strong>10</strong>-12 standard deals with these sources of radiation. Secondary
neutrons are important for manned missions and also play a role in generating background in
sensitive detector systems.
9.1.2.8. Other radiation sources
Other sources of radiation include emissions from on-board radioactive sources such as in instrument
calibration units, Radioisotope Thermo-electric Generator (RTG) electrical power
systems and Radioisotope Heating Units (RHU). Any use of reactor power sources will provide
intense fluxes of neutrons and gamma rays.
9.1.2.9. Effects survey
The above radiation environments represent important hazards to space missions. Energetic
particles, particularly from the radiation belts and from solar particle events cause radiation
damage to electronic components, solar cells and materials. They can easily penetrate typical
spacecraft walls and deposit doses of hundreds of kilorads (1 rad = 1 cGy) during missions in
certain orbits.
Radiation is a concern for manned missions. The limits of acceptable radiological dose for astronauts,
determined to ensure as low as reasonably achievable long-term risk, is indicated in