[meteorite-list] Mammoth Stew, just right

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Tue, 18 Dec 2007 22:13:23 -0600
Message-ID: <037901c841f5$7f85fea0$b842e146_at_ATARIENGINE>

[Sorry if this appears twice; it never showed up for me.]
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Hi, EP, List,

    EP wrote:

> ...the gravitational effects of the Earth+Moon system
> should draw items in, gradually changing their orbits
> from those passing near to ones which intersect.

    The problem with the near miss, the close
approach, the graze is that, while they will modify
the orbit of the object passing by, they can (and will)
change that orbit but will do so in any (and every)
direction. A close pass by a little asteroid may mean
you'll never seen it again or it may come back aimed
right at you. It's even hard to predict the exact results
of a close pass when you know the approach elements.
It's really too touchy.

    Look at the evolution of the predictions of what
everybody's favorite potential impactor, Apophis, is
going to do. First, it might hit us in 2029. Then, no,
it's going to miss us in 2029 and hit us in 2036. Then,
no, it all depends on the gravity "keyhole" and whether
Apophis goes through the keyhole in 2029, but even
then, we won't know for sure until we can observe it
after, and everybody walks away muttering "More data,
more data..."

    So we might end up with only 6.3 years in which to
do something because we don't know if we need to
until 2030... And depending on the state the world in
2029, we might even drop the football, asteroid,
whatever. Mount a mission, fail, have 14 months left.
You write the script.

    The point is, we only hear about those approach
situations where there any chance of another approach.
You don't heard about the approaches when the object
is gone for the next billion years! The number of objects
observed in approach and never seen again, even when
looked for, is quite large.

    There were three different orbits proposed for the huge
fireball object that was observed grazing the atmosphere
over Grand Teton in 1972. Now, there was a close approach!
Not only were the orbits different, but one proposed that
it would be back (now, when was that supposed to be?).

    Well, there's the Internet for you! I went to Google for
orbital data on the Grand Teton fireball, and found... me!
http://six.pairlist.net/pipermail/meteorite-list/2006-January/019513.html

    I'll just quote myself:

The best source of information on the 1972 Grand Teton
object is this excellent page:
    http://comets.amsmeteors.org/meteors/1972.html
    The object was detected by an Air Force satellite, which
makes altitude determination possible: "The object first became
hot enough to be detected by the Air Force satellite at a height
of 76 kilometers over Utah. Its closest distance to Earth was
58 kilometers, which occurred over Montana. As it continued
its passage through the atmosphere it finally cooled below the
satellite detection level at a height of 102 kilometers over Alberta."
The length of the luminous path was about 1500 kilometers.
    In 1974, an estimate of size and mass was published in
Nature, of 1000 metric tons and about 4 meters in diameter
(if iron).
    The astronomer Jacchia published an estimate of mass
based on observed luminosity of somewhere between 4000
and 1,000,000 metric tons, with a diameter of 13 to 80 meters
(if a stone, more likely). Jacchia, by the way, who was a meteor
expert at the Center for Astrophysics in Massachusetts, just
happened to witness the fireball from Jackson Lake Lodge
in the Grand Tetons! You go on a vacation, but your work
just follows you...
    In 1994, Ceplecha re-calculated Jacchia's orbit for the
object and predicted a return in 1997, which did not occur,
however, so it would seem the earlier orbit was correct (or
they were both wrong.)
    The Ultimate Authority, the Wikipedia, says 5 to 15 meters
in diameter but declines to offer a mass estimate...
    Even at the lowest mass estimate (1000 metric tons), the
object would have delivered a Hiroshima-sized punch if it had
been pointed a little differently and impacted. The plane of
its orbit's intersection with the Earth passed right through Los
Angeles, so if its earth encounter had been delayed a few
minutes, we could have had the ultimate Hollywood "special
effect!" (Roll'em!)

    Since it did not "come back" in 1997 (it would have been
detected, I think), it apparently was diverged into a differing
orbit by its spectacular close pass (unless it suddenly shows
up in 2022). The point is, the result of any one close approach
does not have a preferred direction, like always causing closer
approaches.

    There is one "rule," though. The closer the approach, the
more energy is transferred to the smaller object, a "slingshot"
effect we use on spacecraft. Jupiter is renowned for kicking
things out of the solar system by this method.


Sterling K. Webb
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----- Original Message -----
From: "E.P. Grondine" <epgrondine at yahoo.com>
To: <meteorite-list at meteoritecentral.com>
Sent: Tuesday, December 18, 2007 11:23 AM
Subject: Re: [meteorite-list] Mammoth Stew, just right


Hi Sterling, Larry, all -

I'm feeling a bit thick headed today, so I'm going to
argue for a lower Earth impact rate again.

It seems to me that another problem with all of these
crater models is their assumption that an impactor is
either going to hit or miss. It seems to me that in
the real world, the gravitational effects of the
Earth+Moon system should draw items in, gradually
changing their orbits from those passing near to ones
which intersect. This should mean multiple passes
before impact, many of them near the Earth, or in
other words, very near or onto the Moon.

good hunting all,
E.P. Grondine

PS - I think we all remember the unusual Canadian
meteorite laws, and the sorry situation which occurred
in recovering the Tagish Lake fall. This means an
adventurous time for anyone going after the mammoth
pepperer, though certainly nothing to produce as
exciting accounts as South America.
Received on Tue 18 Dec 2007 11:13:23 PM PST