[meteorite-list] Fwd: RE: Cyclic meteorite streams?
From: E.P. Grondine <epgrondine_at_meteoritecentral.com>
Date: Tue, 26 Feb 2008 19:01:12 -0800 (PST) Message-ID: <756735.25190.qm_at_web36901.mail.mud.yahoo.com> --- "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; ____________________________________________________________________________________ Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it now. http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ Received on Tue 26 Feb 2008 10:01:12 PM PST |
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