Events on the sun have a surprisingly wide range of important
effects on earth and in space. One very interesting and important
effect is on the operation of satellites. The space environment,
through which satellites travel, is a harsh place filled with
charged particles and radiation from the sun. At times, the flow
of charged particles can increase dramatically and affect satellite
operations.
The flow of charged particles can produce a build-up of electric charge on the
surface of a satellite (called spacecraft charging) and this can trigger
electric circuits controlling the craft. In many cases, these "phantom"
commands have no significance but occasionally they can initiate something
important such as causing the craft to point away from the earth
direction. Spacecraft have been lost due to this effect - e.g. the
loss of one of the Canadian Anik satellites several years ago.
Flows of electrons arising from coronal holes on the sun (IPS Solar Geophysical
Summary, November 1994) have been found to affect satellites and for this
reason IPS provides forecasts of times when a large flow of
electrons is expected.
On October 7, 1995 there was an outage of the Intelsat 511 which is in a
geostationary orbit above longitude 180 degrees East. This satellite
is used for communications between Australia and the USA. The following
report from Intelsat describes the event.
"The exact cause of loss of earth lock has now been determined as an
ESD (Electrostatic Discharge) event. The ESD event fired manual
thruster causing a large attitude disturbance. Due to the magnitude
of the disturbance it was necessary to perform the satellite into a safe
sun acquisition mode. The anomaly occurred shortly after 6pm satellite
local time. Recovery from sun acquisition at this satellite local
time requires the longest outage due to the configuration of the
sensors on the satellite and the earth sun geometry.
As you are aware earth acquisition was regained at 1624 UT and the
satellite is in a satisfactory state of health at this time."
The outage caused some inconvenience but thankfully no lasting damage. However,
it does serve to illustrate the importance of solar-related events
on our technology.
The graph below shows the flow of electrons recorded during the anomaly. The
peak electron flux, occuring between 6 and 9 October, corresponds to the time
when the anomoly took place.
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