(meteorobs) Question about cometary fragments and meteoroid distribution

[RainerArlt]

David,

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

It is not tidal forces which disintegrate the comet into
meteoroids. It is the evaporation of the volatile parts
of the comet on the sunny side, which also pulls dust particles
from the surface. The particles each have an individual,
small ejection velocity which adds to the orbital velocity
of the comet.

These velocities are very small (a few m/s) compared to
the orbital speed (tens of km/s), but are enough to make
the meteoroids move on slightly different orbits than
the comet.

The velocity in a given point defines the semi-major axis
of the orbit. The meteoroids have slightly different
semi-major axes, and have slightly different orbital
periods. This is why the spread along (around) the entire
cometary orbit over time. For this to take place, no other
gravitating body except the Sun is necessary.

Note thereby, that the particles having a slightly larger
total speed than the comet due to ejection in a direction
close to the motion of the comet, hav a larger semi-major
axis, a larger orbital period, and return to perihelion
AFTER the comet does. Particles having slightly smaller
velocities being ejected into a sector more towards
the rear side of the comet (in terms of motion, not in
terms of sun), will have shorter orbital periods and
return to perihelion BEFORE the comet. This picture 
changes, however, if other forces come into play such
as radiation pressure which will make all particles
tend to have larger orbital periods.

There was one case, where tidal forces has disrupted a 
comet and produced a meteor storm, and that's Comet
Biela and the associated meteor storm of -- was it 1871?
I am not sure at the moment about the dates.


Best wishes,
Rainer

--
Rainer Arlt  --  Astrophysikalisches Institut Potsdam -- www.aip.de
Visual Commission - International Meteor Organization -- www.imo.net
rarlt at aip.de --  phone: +49-331-7499-354  --  fax: +49-331-7499-526