I.1.2 Tailoring guidelines: orbital and mission regimes

In the following sections, attention is drawn to special considerations for various orbit types.

I.1.2.1. Geostationary orbit

Geostationary orbit is a circular orbit usually encountering an environment dominated by energetic electrons. This environment is characterized by strong time variations with many extended quiet periods of low radiation levels and many episodes of intense injections of energetic electrons which increase e.g. dose, sensor interference and electrostatic charging. Solar protons and cosmic rays have unrestricted access to this orbit. Solar particles make shortlived but important contributions to the total dose, interference and single event effects. They do not directly participate in charging processes. Cosmic rays provide a continuous source of singleevent effects and sensor interference.

I.1.2.2. MEO, HEO

These orbits encounter the electrondominated environment mentioned above, but in addition HEO encounters the inner, proton radiation belt. In HEO orbits, singleevent effects from protons and proton nonionizing damage need to be considered. These orbits often encounter more severe electron environments, near the peak of the electron belt (the location of which is also variable) than geostationary orbit and so electrostatic charging can be a more serious threat.

I.1.2.3. LEO

Currently manned activities are limited to low (< 550 km) and mediuminclination (<57.1°) orbits, however, this is going to change in the future. We refer to these orbital regimes as LEO. Missions in these orbits encounter the inner edge of the radiation belt. This region is dominated by the South Atlantic anomaly. Also important is the asymmetry in energetic proton fluxes from East and West. The lowaltitude environment is characterized by highenergy radiationbelt trapped protons. The deflection of charged particles from outside the magnetosphere by the Earth magnetic field (geomagnetic shielding) reduces the fluxes of cosmic rays and solar energetic particles, but the shielding is not total. Like polar orbits, LEO orbits also encounter outerbelt trapped electrons at highlatitudes.

I.1.2.4. Polar

Polar orbits are generally of less than 1 500 km altitude with inclinations above 80°. They encounter the inner proton and electron belts in the form of the South Atlantic anomaly and also the outer electron belt where the geomagnetic field lines bring it to low altitudes at “auroral” latitudes above about 50°. On the highlatitude parts of the orbit a spacecraft is exposed to almost unattenuated fluxes of cosmic rays and solar energetic particles. At low latitudes, geomagnetic shielding considerably reduces these fluxes.