- Space is not the benign environment that was once thought.
It is traversed by small pieces of matter (meteoroids) and also by a
large and variable radiation flux. The radiation field in Earth
orbit comes from three sources, galactic cosmic radiation, trapped
radiation belts (the Van Allen belts) and solar radiation. The first
two sources are particulate radiation, mostly protons and electrons.
Solar radiation is both electromagnetic and particulate (atomic and
sub-atomic). Galactic cosmic radiation consists mostly of very high
energy protons that form a constant low level background radiation
source. These particles are not particularly significant for
communications satellites, but present a possible hazard for long
duration manned spaceflights to other planets. Occasionally they may
be responsible for memory upsets in communication satellites.
Trapped radiation belts are low energy particulate radiation that must be considered for satellites that spend significant time in medium altitude orbits. The Van Allen belts are in fact responsible for the bimodal distribution of satellites. Orbits below about 1500 km are mostly below the radiation belts, whereas geosynchronous orbits lie above them. Satellites in semi-synchronous orbits (eg GPS satellites) must employ radiation hardened components (particularly in the computer memory area) to survive for many years. So far, Molniya type satellites, with very elliptical orbits, are the only comsats to spend much time in the Van Allen belts, and even these transit the danger region fairly quickly on their way from perigee (where they are non-functional) up to their apogee where they spend most of their active life. Solar radiation is extremely variable. The background solar ionising radiation consists of low level X-rays and a small particle component we term the solar wind. However, during explosive events that occur on the sun (flares and coronal mass ejections), the flux of X-rays may increase several orders of magnitude, and the energy of this radiation increases (becomes "harder"). Electrons and protons may be ejected in large numbers, and in rare events some particles may be accelerated to very high energies (even in excess of 1 GeV). It is these solar energetic particles (SEP's) that can cause damage to spacecraft. SEP's may cause direct radiation damage to spacecraft components such as large solar cell arrays. Some spacecraft have had the efficiency of their solar cells reduced by over 30% in a single large solar particle event. This effectively reduces the lifetime of the spacecraft by several years (and denies the owner several million dollars revenue). Even large numbers of lower energy electrons have caused damage to satellites, in one case resulting in a total spacecraft loss (a Canadian Anik geosat). Other effects of particles on satellites are related to vehicle charging. In cases where a spacecraft has surfaces which have a small radius of curvature, and in particular several surfaces that may be insulated from each other, differential surface charging can occur. This may induce deep dielectric charging in the space-craft's circuit boards. If the charge builds to a high value, a sudden discharge may occur with resultant damage to electronic components. Single particles may also deposit sufficient charge inside the space-craft near memory cell components and result in a "bit-flip", changing the state of the memory cell from a zero to a one (or vice versa), causing an error in a system program or data. These are temporary effects referred to as single event upsets or SEU's.
A potentially dangerous condition can arise for geosynchronous satellites (geosats) when solar wind and interplanetary magnetic field conditions create sufficient pressure to push the boundary of the Earth's magnetosphere (called the magnetopause) to a lower altitude than the satellites orbit. Normally the magnetosphere, the region where the Earth's magnetic field controls the motion of particles in space, provides a degree of shielding and protection to satellites within its borders. If a satellite finds itself outside the magnetosphere, effectively "in space", then it will be exposed to a massive increase in solar particulate radiation when on the sunward side of the Earth. In addition, older satellites also rely on the Earth's magnetic field to maintain correct orientation. When a magnetopause crossing occurs, these satellites will lose their orientational reference.
- IPS -