Satgen327   Counting Comets  by GM4IHJ                        1st July 95

For most Radio Amateurs , Comets have two very desirable effects on the
Earths ionosphere. When the Earth crosses the trails of dust which Comets
deposit around their orbits, we get meteor scatter as the particles of
dust are vaporised by collision with the atmosphere, providing brief burst
of ionisation which reflect HF and VHF signals. In addition the smaller
particles of dust, too small to burn up by friction survive to drift
slowly down through the atmosphere . When this dust gets to the region of
the upper jet stream winds, at around 90 kms altitude, it is collected on
the outside eddies of bends in the Jet streams ,where it provides a base
for any ionised material to build up, thereby creating the phenomenon we
call Sporadic E ionisation , which can give excellent enhancement of HF
and VHF propagation.

Unfortunately none of this is good news for satellite communicators or
EME operators, because the Sporadic E SpE can block the passage of their
signals to and from their chosen satellite. Though there is the
compensation that interesting satellite experiments can reveal a great
deal about Meteor Scatter and Sporadic E. In fact the more we know about
Comet dust trails and their orbits the more we can record the past and
future histories of particular comets , and work out where they come from
and how other Solar system bodies such as Jupiter or Saturn may have
affected their orbits. Up to 1992 there were two theories about where
Comets come from , but not much evidence to prove anything positively.

Both theories were based on the suggestion that Comets are the icy debris
out beyond the orbit of Neptune, the biggest distant planet. Debris left
over from the great ring of matter which condensed to form the Solar
system 4.5 billion years ago. The Dutch astronomer Oort proposed that the
home of the Comets was 1000 times further from the Sun than Neptune ,
whereas a later proposal by Kuiper suggested that many comets are circling
the solar system just beyond the orbit of Neptune. 

In 1992 a few facts began to appear when astronomers using a large
telescope at high altitude in the clear atmosphere of Mauna Kea on the
island of Hawaii, found first one then two small objects orbiting beyond
Neptune. In 1993 and 1994 more objects were found beyond Neptune , but the
picture has now changed completely following an experiment using the
Hubble Space Telescope. Using Hubble astronomers took many CCD pictures of
a small patch of sky beyond Neptune. They then laid all the resultant
transparencies in a sandwich one on top of another and examined the
result. They found that as expected spasmodic sky noise provided a general
background but this was overlain by lots of brief tracks of light , all
typically from orbiting objects moving slightly between each picture shot.
Based on this our first real look at a portion of what is called the
Kuiper Comet belt, there are millions of these objects circling the Sun
beyond Neptune. So meteor scatter and SpE users need not fear a future
dearth of Comets, and if hopefully we can soon launch a replacement for
UNAMSAT , which failed to orbit recently, we will be able to see if we can
measure meteor velocities to separate those from the Kuiper belt and those
>From further out, ( or from other stars ).