Satgen435  Perseid's Propects   by GM4IHJ  (BID SGEN435)   26 July 97

It is that time of year again, when the Earth's orbit around the Sun
takes us through the trail of debris left by Comet Swift-Tuttle ,
producing the Perseids meteor shower. For the past 6 years the
Perseids has given an excellent sustained shower of meteors delighting
both Radio Communications enthusiasts and Experimenters, and those
other hardy souls who brave the cold night air for fleeting visual
glimpses of these celestial messengers.

There is no one peak to the Perseids. The first peak is expected this
year at an hour or two either side of 0730 UTC on 12th August 97, with
a second broader but less intense peak, which seems to emanate from
material following a slightly different older orbit , due from,
between 1800utc on the 12th to about 0300 utc on the 13th.

This midweek event will be watched much more carefully this year by
IHJ. Noting that the presence of the AF9Y Fast Fourier Transform
Digital signal processing, adds several new dimensions to radio meteor
scatter observation. Notable amongst these new features is the way in
which both the original starting doppler shift of the signal can be
recorded , together with its subsequent developement , with many
bursts exhibiting distinctly non linear changes in doppler as the
burst progresses.

In addition to the above, it is also possible to see meteor returns
originating from the same transmitter, displaying doppler shifts which
can differ in original frequency by up to 50Hz on a 50 MHz original.
Sets of meteor returns which appear near the frequency at which the
same originating transmission is propagated by Sporadic E are the most
common. But less frequently there appear to be a quite different
family of burst which differ from the more common variety by having an
original frequency up to 50 Hz different.

Back of the envelope mathematics suggests that this indicates a
difference in relative velocity of about 300 metres/sec, but just what
is going on here is by no means clear, with these different sets being
present during random meteor events and, during well known showers, as
if the less frequent group was a permanent rather than a transient
feature. A topic which just happens to have surfaced as astronomers
are being asked to believe that the earth is being hit by a continuous
bombardment of loosely packed snowballs , or as some reports call them
" stealth comets", (Sky and Telescope Aug 97 refers). 

Frankly it has to be admitted that a somewhat sceptical astronomical
jury is still out , arguing the possibililities and evidence for these
stealth comets. But one thing is sure , new computer aids now becoming
a regular feature in many Digital Age Ham shacks seem to be telling us
something new about meteor scatter.