Tim Furniss/LONDON

THE UK'S DEFENCE Evaluation and Research Agency (DERA) has produced "...conclusive evidence that many satellites are at risk from electrostatic discharges while in orbit".

The threats result from the "surprising" interaction of the solar wind with the Earth's magnetic fields as they occur near the period of minimum activity of the Sun during its 11-year solar cycle.

The research team, led by Dr Gordon Wrenn, says that the electrostatic discharges cause status changes, often termed "phantom commands", on spacecraft and can ultimately cause other failures similar to those experienced by the Canadian Anik E1 communications satellite, which lost its primary momentum wheel-control system in 1994, and also lost half its power in March this year.

Wrenn says that, since 1992, "...numerous other geostationary-orbiting [GEO] communications satellites have suffered from these operational anomalies" during the dinal phase of the Sun's minimum period of activity.

More events of a similar nature could occur in 1997-8 before the Sun approaches the start of a new period of maximum activity, says the DERA. The build-up of energetic electrons is linked to the 11-year solar cycle because coronal holes are usually more long-lasting during the declining phase approaching the solar minimum.

These generate fast streams of hot plasma - electrons - forming a more powerful solar wind, which subsequently interacts with the magnetic fields surrounding the Earth, to critically disturb the spacecraft environment.

The electrons penetrate through thin coverings of the satellites, such as thermal blankets, and produce dangerous charge build-up within highly insulating materials and on isolated conductors inside the satellites. The process is referred to as internal dielectric charging.

Wrenn says that his team has identified a particular circuitry - in the attitude-control system aboard a spacecraft-bus series (which the DERA will not name) - with a "relatively high susceptibility to switching".

By monitoring the circuit over a period of time, it has been possible to correlate recorded events with measurements from electron detectors carried aboard GEO satellites and to track the growth of the threat. "It is a very touchy subject among satellite manufacturers," admits Wrenn.

Source: Flight International