[meteorite-list] Meteorite-list Digest, Vol 36, Issue 28

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Fri, 8 Dec 2006 01:59:14 -0600
Message-ID: <006901c71a9e$c17677b0$a925e146_at_ATARIENGINE>

Hi, All,

Chris said:
> I don't know if anybody has worked out the
> likelihood of that happening- very, very rare...

    I called my oddsmaker in Vegas (or was it Vega),
and here's what he said...

    The problem is essentially the same as the likelihood
of being smacked by a one-time long period comet; it
falls in from the back of nowhere , slingshots around
the Sun, and zaps back out.

    It's completely random; it could come from any
direction -- the Oort Cloud is a sphere. So, imagine
that the radius of the orbit of the Earth defines an
inner sphere surrounding the Sun, through which
the object will have to pass in order to swing around
the Sun and back out.

    The surface area of that sphere is about two billion
times the cross section of the Earth itself, so the odds
of being hit by the incoming comet is one in two billion,
and the odds of being hit by the outgoing comet is one in
two billion.

    Overall, the odds are about one in a billion for both
coming and going. There is a good sized (10 kilometer
diameter and up)* long period comet almost every year,
so we will get comet-whacked every billion years or so.
    [* Comet Hale-Bopp was 40 MILES in diameter.]

    On average...

    "Little" long period comets (1 kilometer to 10 kilometers
diameter) are 5-10 times more common, so expect a medium
comet whack every (couple of) 100,000,000 years or so.

    Of course, being gob-smacked by a long period comet is
just about the worst. I hate when that happens. The comet is
going at the solar system escape velocity (almost); the Earth
is going at its orbital velocity. What the vector total of those
two?

    Answer: Too much. The kinetic energy goes up by the
square of the velocity, so maybe 4 to 6 times the energy of
the impact of an asteroid of the same mass. That's going
to leave a mark, as they say.

    Just to prove that the Universe isn't a sporting
proposition, a long period comet coming from the Oort
Cloud isn't likely to brighten enough to be detected by
visual comet finders until it's near the orbit of Jupiter,
which would give us about 2-3 weeks of warning of
an incoming encounter -- hardly enough time to get
drunk, have a last fling, and say your prayers.

    Of an outgoing encounter, we'd have 4-5 weeks of
warning time. That's some improvement but not much.
Not, for example, enough time to move several billion
people to the side of the planet away from the impact
point. Hmm. How many frequent flyer miles you got?
You feel like a long vacation?

    Of course, if the comet was just from Far Kuiper
County, with a period of 3000-4000 years, we'd have
months (instead of weeks) to get ready. You'll be ready
in 4-5 months, won't you?

    Since the Leonids are retrograde and the Earth prograde,
the encounter velocity is the vector sum of the two, but the
angle of incidence between the Earth and the Leonid stream
varies from year to year; when it's 180 degrees, or face-on,
the encounter velocity is the oft-quoted 71,000+ mps. At
lesser angles, it's somewhat less but still hefty. Nice that
they're mostly just pea gravel and sand sized bits; very pretty
and they don't leave marks.



Sterling K. Webb
-----------------------------------------------------------------------
http://adsabs.harvard.edu/abs/1997neo..conf...67W

"The long-period comets pose a unique problem for the impact hazard problem.
Because of their very long orbital periods and generally large distances
from the Sun, they cannot be surveyed and catalogued in the same manner as
the near-Earth asteroids and short-period comets. They appear at random,
uniformly distributed on the celestial sphere. Current technologies can
detect long-period comets at distances of approx. 5 AU, giving somewhat less
than a one year warning time for potential Earth impactors. The mean impact
probability for a long-period comet crossing the Earth's orbit is 2.2 to 2.5
x 10-9 per perihelion passage. The mean impact velocity is approximately 52
km sec-1 but the most probable impact energy is characterized by a velocity
of 56 to 58 km/sec. The estimated current impact rate for cometary nuclei
large enough to create 10 km diameter (or larger) craters on the Earth is
between 5 x 10-7 and 2.8 x 10-6 per year, with a bed estimated value of 1.0
x 10-6 per year. Nuclei large enough to initiate global climatic
disturbances strike the Earth on average every 16 Myr. The impact frequency
may be increased substantially for brief periods of time during cometary
showers, initiated by major perturbations of the Oort cloud. Improved
technologies are needed to detect approaching long-period comets at large
heliocentric distances so as to increase the warning time for potential
impactors. "
------------------------------------------------------------------------------------
----- Original Message -----
From: "Chris Peterson" <clp at alumni.caltech.edu>
To: <meteorite-list at meteoritecentral.com>
Sent: Friday, December 08, 2006 12:02 AM
Subject: Re: [meteorite-list] Meteorite-list Digest, Vol 36, Issue 28


> Comet Tempel-Tuttle, the parent body of the Leonids, is in a
> low-inclination, retrograde orbit. We encounter the debris at 71 km/s,
> and our own orbital speed is 29.6 km/s. Subtract that out and you get
> the orbital speed for Leonid meteoroids: ~41.4 km/s. The solar escape
> velocity at the Earth is 42.1 km/s. That's why the Leonids are as fast
> as any periodic meteors can be- faster meteoroids would leave the Solar
> System. Of course, a sporadic meteor could be produced by a body that
> would escape the Solar System if it didn't encounter the Earth- either
> because it originated outside the Solar System, or because it picked up
> enough energy through momentum transfer during some sort of slingshot
> around another body. I don't know if anybody has worked out the
> likelihood of that happening- very, very rare I'm sure.
>
> Chris
>
> *****************************************
> Chris L Peterson
> Cloudbait Observatory
> http://www.cloudbait.com
>
Received on Fri 08 Dec 2006 02:59:14 AM PST