Itchy satellites

Most often, people think about high energetic protons, space debris and altitude loss from an expanding atmosphere as the most important threats to the survival of earth-orbiting satellites. However, also the abundantly present electrons can constitute a lethal danger to the satellite's life. Many satellites such as Telstar 401 and Galaxy IV did not recover from electron induced effects, so two of them will be sketched here.

The first effect is called "surface charging". During night time, electrons have the tendency to charge the satellite's surface negatively. In sunlight, the satellite's surface becomes positively charged. Of course, satellites have an irregular shape, so parts of the spacecraft will still be in the shadow, leading locally to different charges. This may result in an electrical discharge on the surface. These are comparable to tiny "lightning strikes" like when you touch a door knob after having walked for some time on tapestry.

The discharges tend to occur around local midnight, due in part to the immersion or exit from shadow to light acting as a triggering mechanism for the discharge, but also and very significantly due to the increased plasmasheet electron fluxes in the midnight sector.

Though these discharges are generally not life threatening to the satellite, they may result in phantom commands and glitches in e.g. data telemetry, which are of course a nuisance. Surface charging related discharges can also occur on spacecraft solar arrays. These can have a significant effect on the performance of the solar arrays and the overall power capability of the spacecraft.

Electrons that have a somewhat higher energy, i.e. more than 0.1 MeV (mega electron volt; a unit of energy) can have a different effect. These particles can travel through several mm of aluminium shielding, and thus deposit themselves in critical electronic components in circuit boards and coax cables underneath the satellite's surface. As these particles accumulate, charge can build up until a threshold is reached. The result may be an electrical discharge, which may destroy the electronic components. This "deep-dielectric charging" manifests itself especially in high altitude spacecraft which pass through the Earth's radiation belts.

The effects of these discharges are more pronounced as the number of particles and their energies increase. These particle streams from coronal holes, plasma clouds and substorms (accelerated particles from the Earth's magnetotail) can contribute significantly to the charging of the satellite and its components. Alerts are usually given when the flux of electrons having energies of more than 2 MeV exceeds 1000 pfu (particle flux units), with hazardous conditions if this level is sustained for several days. Over the last month, several active periods were noted, such as 24-28 June, 1-5 July and 10-14 July. These were mainly due to the passing streams of coronal mass ejections and coronal holes. Satellite operators noted a significant increase in anomalies during these periods, thought to be due to repeated electrostatic discharges. Fortunately, all spacecraft survived and are in good health.



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