(meteorobs) Question about cometary fragments and meteoroid distribution

[Wayne T Hally]

David,

One key point is, that if the particles are traveling at different 
velocities, they cannot by definition be in the same orbit. Those that are 
moving faster travel in a larger orbit, the slower ones smaller. The only 
place they come close to each other is near the point where they acquired 
different velocities. Since the S-L 9 fragments were in orbit around 
Jupiter at the time of their demise, the orbits passed nearest to each 
other at the point of impact.
	For our meteor showers, the point where they are ejected from the comet is 
when they acquire different velocities when closest to the sun at 
perihelion.. So the faster ones (ejected in the direction of the comet's 
motion) go into a larger orbit (and therefore have a longer period), while 
the slower ones (ejected behind the comet) have smaller, shorter period 
orbits. The orbits all only come very close to each other at perihelion, 
where they were ejected. For the Leonids, Perseids, etc, that is right at 1 
AU. i.e. earth's orbit. Halleys comet's (eta-Aquarids and Orionids) 
actually come closest inside of the earth's orbit so are actually a little 
dispersed when they cross out path.
	This is actually how meteor streams get populated throughout the gaps 
between the comet's perihelions (~33 Yr for Leo's Tempel-Tuttle, ~ 133 Yr 
for Swift Tuttle and the Perseids). The meteoroids do not get spread out 
along the comet's orbit, a common misconception that you will see in most 
textbooks, and even in Sky and Tel sometimes. Rather they each have their 
own individual orbits that take a differing number of years to return to 
perihelion, and hence the earth's orbet when we intersect them. So if  S-T 
particle takes 200 years, and another takes 50 years they will return to 
the earth in that amount of time, rather than the comet's 133 years. Over a 
number of orbits, all the years get filled in and we have an annual meteor 
shower, rather than one that only produces outbursts when the comet is 
near. The Leonids are kind of in transition, although due to it's shorter 
period it will tend to spread out less in a direction releavent to the 
earth, but that's another long discussion, which I just grasped from a 
recent article in WGN.

	Oh, and we never mind you asking more questions :-)

Wayne Hally
NJAA,NAMN

I hope you don't mind me asking some more questions.

Prior to its impact with Jupiter, in July 1995, comet Comet P/Shoemaker-
Levy 9 broke into a number of fragments and these fragment became
distributed in a long narrow line. I can only think that this is due to
a gravity-gradient acting on the loosely-bound components of the parent
body - a sort of tidal effect. Even as a single body, the component
parts closest to Jupiter were being subjected to a stronger pull and
moved ahead of the rest. Those furthest from the planet were subject to
a weaker pull. Over time the body is pulled into a chain of fragments.
Is this a correct explanation of the mechanism?

If Shoemaker Levy had missed Jupiter, would the component parts continue
to have moved apart? It seems to me they would, as those leading the way
were farther from the planet and so subject to a lesser pull. And those
lagging behind would have been subject to a greater pull.

Is this how cometary debris become distributed around the orbit and how
the meteor streams are formed?

In an idealised situation of this sort, with a single parent body and
ignoring all other influences (solar radiation pressure, planetary
perturbations, relativity etc.) would all the components of such a
meteoroid stream eventually pass through the same volume of space, no
bigger than the original component body, albeit at quite different
times? Although this seems to imply that the fragments are following the
same orbit, but with different velocities, I can't see how this can be
other than true.
--
David Entwistle


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