Hi Rob -
"no" it is, based on the meteoritic data.
That's an answer that I can understand, and goodbye to
the fantasy of knowing what night a fall is expected
and then simply going outside and watching them come
in.
So many questions, so few good answers. I wonder when
the hell NASA is going to stop fooling around so much
with Mars, and use some of the money to place a really
good probe in the belt instead.
E.P. Grondine
Man and Impact in the Americas
--- "Matson, Robert D." wrote:
> Hi E.P.,
>
> > It's a wonder how we end up with asteroids
> fragmented into meteoroids
> > in the first place. Particularly from iron parent
> bodies.
>
> Oh, I think the impacts are fairly regular in the
> Main Belt; it's just
> that by the time the fragments of those impacts
> migrate into earth-
> crossing orbits, they're all completely decoherent
> with respect to
> each other and thus go unrecognized by us (at least
> dynamically) as
> having a common ancestor.
>
> > So assume a solar orbit for a meteoroid stream,
> and that it is not
> > so spread out. Then the intercepts occur at some
> interval of years,
> > and on both ascending and/or descending nodes.
>
> If a NEA gets impacted, then yes -- there is a short
> period of time
> during which those fragments disperse within the
> orbit, eventually
> spreading out to populate the entire ellipse
> (thousand years or so).
> But for the first couple hundred years, the
> fragments would all be
> bunched together in a short arc of the full orbit.
> So even if the
> orbit intersected earth's orbit, you wouldn't always
> get a "shower".
> The fragments would have to be at the location of
> the nodal crossing
> to produce a shower.
>
> Say, for example, their arc covered 15 degrees
> (1/24th of the orbit).
> On average, you would only expect a shower about
> once every 24 years
> (excluding special cases where the asteroid orbital
> period is very
> close to a small integer ratio of earth orbital
> periods). But you
> see: the case you're asking for is even more
> restrictive then the
> one in which fragments have spread out to cover the
> entire orbit.
> The probability of an episodic meteoroid stream like
> this is less
> than 1/100th that of a stream that has already
> spread to cover the
> entire orbit. This is because the lifetime of
> coherent streams is
> ~100 times the length of time it takes to go from
> NEA impact to a
> fully populated orbit of fragments.
>
> > Now are there meteoroid streams? Your answer is
> no, but I don't
> > know yet.
>
> It ~can~ happen; it's just that there's no evidence
> that one currently
> crosses earth's orbit. The necessary evidence would
> be two recovered
> falls occurring within, say, a week of each other
> (not necessarily
> in the same year) that shared a common radiant, the
> same petrologic
> type, and most importantly the same *young* cosmic
> ray exposure age.
> So far, there are no meteorites in the world's
> collections that
> meet these criteria. Indeed, only a handful of
> meteorites have
> CRE ages less than 1 million years old. --Rob
>
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