The Leonids Meteoroid Shower occurs annually, each November. Every 33 years or so, throughout recorded history,
it is particularly significant and spectacular. This year the event is centred
over Europe. This set of synthetic
images help to explain what is happening and the possible effect on spacecraft
that are exposed to the stream of meteors.
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The spectacular show every 33 years is linked to the return visits of the parent
comet of the Leonid meteor showers, the periodic
Have a look at the video (comet.mpg 1.3MB)
As the comet nears the sun, the heat of the sun boils off debris particles
from the comet on the sun-facing side which forms a trail in the wake of
the comet. This last happened for Tempel-Tuttle in spring 1998 with the closest approach to the sun 14 February 1998.
Have a look at the video (comet_approaching_sun.mpg 1.2MB)
The trail of debris, made up of very fine particles of ice and material,
is travelling extremely fast, at a speed relative to Earth of 71 kilometers
per second. This debris trail is several earth diameters across. On
17 November the Earth crosses the comet's wake, so scientists predict that
extremely high numbers of meteors will be visible.
Have a look at the video (high_speed_comet.mpg 1.6MB)
The trail of debris move in the opposite direction to the Earth's Orbit, therefore the speed relative to Earth is so high.
Have a look at the video (orbit.mpg 1.9MB)
The meteoroids impact the Earth's atmosphere which enables us to determine
the number of meteoroids from a count of trails seen from a particular
position on Earth per hour. Rates as high as 150000 per hour were seen
for the last Leonids event in 1966. These trails are the result of burn up
in the atmosphere of the larger particles. Of course
there could also be many very small particles which do not get counted
because they are too small to make trails, but could still penetrate spacecraft
and create significant amounts of plasma.
Have a look at the video (shower.mpg 1.1MB)
These very small particles can impact any area of a spacecraft facing
the stream. The velocity of the stream is so high that any surface would
be penetrated. ESA's Olympus spacecraft, shown here, was probably impacted
by the yearly Perseids in August of 1993 on its solar arrays, which
are generally the largest exposed area. Normally the solar arrays would
be almost parallel to the Leonids and if penetration is in a critical
area it can do significant damage. There is also a very significant production
of plasma, whose potential current is proportional to the velocity of impact
to the power of 4.5, a very large value. This plasma may enter the electronics
and cause difficulties. In the case of Olympus, the plasma probably entered
via the launch umbilical and through a sun sensor. In this clip the plasma
results in Olympus moving off station.
Have a look at the video (plasma.mpg 4.5MB)
In order to protect its mirror, the NASA/ESA Hubble Space Telescope
will be turned away from the stream and the area of the exposed solar arrays
will be minimised. For communications satellites, it is desirable to minimise
the solar array cross-section and shield any sensitive equipment by orientation
of the spacecraft for approximately two hours at the peak of the event.
Of course, these actions may interfere with normal services which in some
cases will not be possible. Impacts and effects are a result of probabilities
of impact which is proportional to the stream rate.
Have a look at the video (hubble.mpg 2.3MB)