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(meteorobs) Xi Draconids
Rainer mentioned the possibility that George's and Bob's meteors might be
spurious. Although this is always a possibility with newly found minor shower
radiants, my interest in minor streams always pushes me to continue pursuing
the possibility of the stream's existence until I start hitting deadends.
I decided to check Bob's list of probable Xi Draconids that he sent out. These
give the beginning and end positions of each meteor on his plots. Although a
few long-trailed meteors do exist, I was struck by the number of short-trailed
meteors (one was about a degree long, and several are less than 10 degrees
long). I think the short-trailed meteors act as a good pointer to the actual
radiant, and the estimates I come up with are in very good agreement with the
estimates of Bob and George. In addition, the fact that George also plotted and
arrived at a similar radiant adds further support to the existence of the
radiant.
I am also struck by the existence of not only previous visual evidence, but,
more notably, the photographic and radio-echo evidence. Now that I have finally
got my meteor orbit program to work again, I hope to isolate the most probable
Xi Draconids out of the photographic and radio-echo candidates I have found. I
hope to have a list of these available by tomorrow.
George Gliba asked if a hyperbolic orbit was possible. Of the thousands of
meteor orbits I have investigated over the years, there have never been any
convincing hyperbolic orbits. The 1961 catalog of McCrosky and Posen which
gives over 2500 orbits from photographic double-station meteors lists many
hyperbolic orbits. This study did not attempt to generate precise orbits, but
instead generated rough orbits for statistical purposes. A few studies have
been conducted which have generated precise orbits for some of the same 2500
meteors. Their careful re-measuring of the photographs and precise calculations
revealed that most of the hyperbolic orbits were actually elliptical. It has
been conjectured that many of the remaining hyperbolic orbits were derived from
double-station meteors that were not particularly well-suited for orbital
computation.
Why are hyperbolic orbits for meteors unlikely? Objects moving within our solar
system are constantly undergoing gravitational battles with one another. The
smaller an object's mass, the more easily affected it will be. By the time an
interstellar stream reached Earth, or a distance of one AU from the sun, the
sun's pull should be great enough to have easily altered, or bent, the object's
path. It would most likely be locked into an elliptical orbit by the time it
encounters Earth.
When you start dealing with objects as small as meteors there is another factor
that comes into play--the solar wind. The feature that would identify an
interstellar stream would be its velocity. We all know that satellites orbiting
Earth must maintain specific speeds to remain in specific orbits. The drag of
the atmosphere slows them down, which causes them to drop into smaller orbits.
Dust within meteor streams can undergo the same process when they start hitting
the solar wind (or vice versa :-) ). The closer meteoric dust gets to the
sun, the stronger the solar wind gets, and the more likely the possibility of
being slowed down. Also, the smaller the particle, the more likely it will be
slowed. As the velocity drops, the orbit will be converted from a hyperbolic
orbit into an elliptical orbit. We sort of see this process in action when we
look at comet tails always pointing away from the sun.
Gary W. Kronk