9.1.2                 Overview of energetic particle radiation environment and effects

9.1.2.1              Radiation belts

Energetic electrons and ions are magnetically trapped around the Earth forming the radiation belts, also known as the Van Allen belts. The radiation belts extend from 100 km to 65 000 km and consist principally of electrons of up to a few MeV energy and protons of up to several hundred MeV energy. The high energy particle flux in the radiation belts is dependent on the solar activity. The so­called South Atlantic anomaly is the inner edge of the inner radiation belt encountered in low altitude orbits. The offset, tilted geomagnetic field brings the inner belt to its lowest altitudes in the South Atlantic region. More information can be found in references [RN.10] and [RN.11].

9.1.2.2              Solar energetic particles

Solar Energetic Particles (SEP) are high-energy particles that are encountered in interplanetary space and close to the Earth. These particles are seen in short duration bursts associated with other solar activity. Solar Energetic Particle Events, as detected in Earth orbit, can last from a few hours to several days. The Earth’s magnetic field provides a varying degree of geomagnetic shielding of near­Earth locations from these particles. They consist of protons, electrons and heavy ions with energies from a few tens of keV to GeV ranges (the fastest particles can reach relativistic speeds) and can originate from two processes: energisation in association with activity seen on the solar disk e.g. flaring, or by shock waves associated with Coronal Mass Ejection (CMEs) as they propagate through the heliosphere. They are of particular interest and importance because they can endanger life and electronics in outer space (especially particles exceeding some tens of MeV).

9.1.2.3              Galactic cosmic rays

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-2s-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 near­Earth locations from these particles.

9.1.2.4              Geomagnetic shielding

The Earth’s magnetic field partially shields near­Earth 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-2s-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 ECSS-E-ST-10-12 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 on­board radioactive sources such as in instrument calibration units, Radioisotope Thermo­electric Generator (RTG) electrical power systems and Radioisotope Heating Units (RHU). Any use of reactor power sources 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 long­term risk, is indicated in ECSS-E-ST-10-12. There are many possible radiation effects to humans, beyond the scope of this document. These are described in . Heavy ions and neutrons are known to cause severe biological damage, and therefore these contributions receive a heavier weighting than gamma radiation. The “quality factors”, as they are called, are established by the International Commission on Radiological Protection [RD.13].

Energetic ions, primarily from cosmic rays and solar particle events, lose energy rapidly in materials, mainly through ionization. This energy transfer can disrupt or damage targets such as a living cell, or a memory element, leading to Single­event Effect (SEE) in a component, or an element of a detector (radiation background). These effects can also arise from nuclear interactions between very energetic trapped protons and materials (sensitive parts of components, biological experiments, detectors). Here, the proton breaks the nucleus apart and the fragments cause highly­localized ionization.

Energetic particles also interfere with payloads, most notably with detectors on astronomy and observation missions where they produce a “background” signal which is not distinguishable from the photon signal being counted, or which can overload the detector system.

Energetic electrons can penetrate thin shields and build up static charge in internal dielectric materials such as cable and other insulation, circuit boards, and on ungrounded metallic parts. These can subsequently discharge, generating electromagnetic interference.

Apart from ionizing dose, particles can lose energy through non­ionizing interactions with materials, particularly through “displacement damage”, or “bulk damage”, where atoms are displaced from their original sites. This can alter the electrical, mechanical or optical properties of materials and is an important damage mechanism for electro­optical components (e.g. solar cells and opto­couplers) and for detectors, such as CCDs.

For a more complete description of these effects refer to ECSS-E-ST-10-12.