[meteorite-list] Fwd: RE: Cyclic meteorite streams?

[E.P. Grondine]

--- "Matson, Robert D." wrote:

Hi E.P.,
 
> > While many have taken a look at annual fall
> > patterns, I have often wondered if some meteorite 
> > falls are not instead cyclic, in other words come
> > from intercepting recurring debris streams from
> > asteroids?
 
So far there is no (strong) evidence for any
currently existing meteoroid streams that intersect 
earth's orbit, with the possible exception of
fragments from 2003 EH1 (more on that later).  

Meteoroid streams (and comet streams, for that matter)
have short dynamical lifetimes due to the combined
effects of gravitational perturbations by the planets,
solar radiation pressure, and Yarkovsky effect, and
therefore become decoherent on time scales of
10,000 to 100,000 years.  But the similarities
between comet streams and meteoroid streams ends
there; the mechanisms for producing the two different
types of streams are quite different.
 
Since it takes timeframes of longer than a million
years for an asteroid to migrate from the inner main
belt through a resonance into an earth- crossing
orbit, the impact events (which produce the meteoroid
streams)
must occur *after* the asteroid has become a NEA. 

The probability of impacts between NEAs is much, much
lower than the probability of an impact in the main
belt, so you begin to see why meteoroid streams are
not only short-lived, but rare.  To summarize, you
need three things to happen to have a meteoroid
stream:
 
1.  rare impact between two NEAs
2.  the NEAs are in orbits that actually cross
earth's orbit
3.  #1 has occurred within the last 10^4-10^5 years
 
> > Along those lines, why are meteor showers (comet
> > debris stream intercepts) annual, and not cyclic
on > > another basis?
 
Because the date of a shower is tied to when the
parent comet's orbit crosses the earth's orbit (nodal
crossing).  Perturbations will cause the comet's nodal
crossing point to slowly precess over the centuries,
so the date of each shower's peak does slowly drift,
but nothing noticeable on decade time scales.
 
It ~is~ possible for a comet (or an asteroid for
that matter) to have an orbit that crosses earth's
orbit in two locations:  an ascending node and a
descending node.  In fact, the most famous comet,
1P/Halley has two crossings (Eta Aquarids in early May
and the Orionids in late October.)  (I'm not aware of
any other comets that have two nodal crossings.)
 
Now, 2003 EH1 is an interesting case as it has been
tied to the Quadrantids.  2003 EH1 is in a highly
inclined orbit that does not currently intersect
earth's orbit.  The closest it gets is about
0.21 a.u. However, only a few centuries ago its
perihelion would have been inside earth's orbit. 

I still wouldn't call 2003 EH1 the source of a
"meteoroid stream", since its orbit is clearly
cometary
rather than asteroidal. In short, 2003 EH1 is an
extinct comet nucleus.
 
--Rob
 
P.S.  If you would, please forward to the Met-list;



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