[meteorite-list] Largest Crater in the Sahara Desert and LDG

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Mon Mar 6 03:13:02 2006
Message-ID: <007b01c640f5$c5d4f170$86e08c46_at_ATARIENGINE>

Hi, Mike,

    The hypervelocity impact has always been
in the running for The Tektite Source, off and on.

    The worse case would be a comet with an
orbital eccentricity of .99 in a retrograde orbit.
In other words, a first-timer falling out of the
Oort Cloud from 10,000 AU or 20,000 AU
out that's traveling like the proverbial bat out
of hell by the time it gets to the inner Solar
System that would then smack into the Earth
when we're headed right for it. All the bad
luck in the world.

    A comet's still bound to the Sun's gravity,
so we can figure its velocity at the Earth's orbital
distance, sum up all the bad-luck velocities, and
get a maximum possible impact velocity of about
73.4 km/sec. That for any object gravitationally
bound to the Sun (not an alien spacecraft under
power), but comets are the only likely ones.

    The "average" impact velocity we'd get from
a NEA would be 15-20 km/sec, so the retrograde
comet hit would be about four times faster. Back to
the bad luck department: the energy depends on
the square of the velocity, so the bad-luck comet
hit would deliver sixteen times the energy per pound
than the usual mass-extincting disaster. Ouch x 16.

    So, it's really nice that the odds of a collision
with a long-period comet are so low... (To figure
those odds, calculate the area of a sphere with
a radius equal to the Earth's orbital distance,
divide by the cross-sectional area of the Earth's
disk, and you have the odds of any one long-period
comet, which could come from any direction,
hitting the Earth.) When all is said and done, the
chances are from one hit per 2,000,000,000 years
to one hit per 500,000,000 years, depending on
how frequent long-period comets are (argument
in progress).

    Really nice, because such a comet could just
as easily be up to 100 miles across or more, or
like the recent Hale-Bopp. (Was it "only" 40 or
60 miles across?) Getting hit by an object this size,
at any speed, makes the Big Kill All The Dinosaurs
Asteroid (10 miles across) look like a kid's fire-
cracker, a puny firecracker at that. Getting hit
by a 100 mile object is just plain unthinkable.

    Are there other big objects? Chiron, the
Centaur (which is both Comet 95P and Minor
Planet 2060) between Saturn and Uranus and
is about 160 miles across, was deflected there
by Jupiter (most likely). All the Centaur objects
(all pretty big, over 100 of them!) are orbitally
unstable on a 100,000 year time scale and have
about a 20% chance of escaping "inward"
(and 80% "outward"). Root for them to head
outward, please.

    But comets in general have more eccentric
orbits than asteroids, which means higher impact
velocities in general, so their impacts are likely
to be more energetic (by the square, a mere 44%
increase resulting in double the punch).

    But even mildly big objects, 500 meters or
1000 meters, are not bothered by our atmosphere
one bit, even at a 30 degree angle. Air provides
great protection against small and medium bodies
but once you get up to the large size impactor,
it doesn't even slow it down. A 500 meter body
that would achieve 15 km/sec might "only" lose
less than 1 km/sec of that velocity, not a big help.
A 1000 meter (and up) body would lose even less
speed. A 10,000 meter body wouldn't slow down
at all.

    There are other reasons for suspecting a comet-
tektite connection, but I'm saving it for another post
to the List.

    Stay tuned.


Sterling K. Webb
-----------------------------------------------------

----- Original Message -----
From: "Mike Fowler" <mqfowler_at_mac.com>
To: <meteorite-list_at_meteoritecentral.com>
Cc: "Mike Fowler" <mqfowler_at_mac.com>
Sent: Saturday, March 04, 2006 11:11 AM
Subject: [meteorite-list] Largest Crater in the Sahara Desert and LDG


>> This is actually a more general point: there are lots and lots of
>> impact
>> craters but very few tektite producing ones; why?
>>
>>
>> Sterling K. Webb
>
>
> Why not very high velocity comet impacts, at a near vertical angle.
> Maximum cometary velocities would be about 10 times more than average
> asteroidal impacts. Near vertical would reduce the atmospheric
> column that the explosion has to punch thru to the minimum.
>
> Looked at from this point of view, perhaps only 1 in 100 crater
> producing impacts would qualify, which might explain why there are
> many large craters, but few tektite strewn fields.
>
> Mike Fowler
> Chicago
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Received on Mon 06 Mar 2006 03:12:55 AM PST