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Re: (meteorobs) Meteor shower prediction

Robert,
The velocity of an object at some specified distance from the Sun
defines the orbital period of that object.  Thus, the dust ejected from
a comet at say, perihelion, produce a range of velocities that are
slightly different from the parent comet.  The resulting range of orbital
periods cause the dust to extend into a trail that extends both in front
of and behind the comet.  In the standard ejection models, smaller
particles are likely to be ejected at higher speeds and thus tend to be
in orbital periods more different from the comet.

The next stage to this this process is the effect of solar radiation
pressure on orbital period.  The "push" sunlight gives to a particle
slightly counteracts the gravitational attraction.  As these effects
are in exactly opposite directions, the particles still move in conic
sections (ellipses being the relevant motions here), but move more
slowly around the orbit.  This results in mass differentiation, that
in the Leonids amounts to around a 34 year orbit (rather than 33 years)
for visual sized particles resulting in the biggest ZHR storms occuring
after the comet has passed perihelion.

These two aspects highlight the difference between two approaches to
predicting Leonid activity.  David and I use the standard ejection
model which allows dust to be ejected at speeds of many tens of m/s.
The dispersion of velocities, modified by solar radiation pressure,
causes the trail extension.  Lyytinen and van Flandern use van
Flandern's satellite model of cometary nuclei in which dust orbits the
nucleus and "leaks out" at low velocities as the comet approaches the
Sun.  The dust then elongates into a dust trail under the effects of
solar radiation pressure.  Of course, both these effects could be
happening!

These dust trail theories are more successful than other methods as
they specify the geometry of the dust trail and examine the overall
dust trail density as a uniform entity (eventually becoming non uniform
as each revolution goes by).  The various theories define paramaters
for the dust trails and fit these to the observed ZHRs.  David and I
and also Lyytinen and van Flandern use the ZHRs from Peter Brown's
historical analysis of Leonid activity.  Peter Jenniskens uses his own
derived historical ZHRs which are often much lower.  David and I also
"adjusted" some ZHRs we suspected of being poorly defined (like 1833
when the maximum was estimated at dawn, but may well have occurred
some 45 mins later).

It is not immediately clear to me that the different approaches taken by
David and I and by Lyytinen and van Flandern can be easily separated
by ZHR estimates.  Some forms of physical measurement are probably
required, although the oscillations in ZHR during the 1999 storm may be
indicative of jet activity from a rotating nucleus.  But one thing is
certain; Lyytinen and van Flandern have done an excellent job in
fitting the ZHR data to their model parameters.

Cheers, Rob

On Mon, 4 Dec 2000, Robert Gardner wrote:

> Gradually I have formed a picture of how the meteor showers are
> predicted.  I gather that the cloud of material that form the shower are
> created mostly at or near periapsis passage of the parent comet.
> However I don't understand what are the forces involved that move this
> cloud of material around to different positions in the orbit.   I
> presume they still remain very nearly in the same orbit or we would
> never see them again.  Is it light pressure or solar winds or other
> forces?  Though the orbit is independent of the mass of the object,  the
> lower mass would be more sensitive to any perturbations.


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