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

From: Chris Peterson <clp_at_meteoritecentral.com>
Date: Fri, 8 Dec 2006 07:48:03 -0700
Message-ID: <014301c71ad7$ddb44c20$2721500a_at_bellatrix>

I was discussing the probability of encountering a meteor with a
velocity greater than the Sun's escape velocity at the Earth. The
likelihood of that happening should be much lower than the likelihood of
simply encountering a stray object kicked out of the Oort Cloud. There
are only a few ways an object can pick up additional energy by momentum
transfer, and an infinite number of ways it can avoid doing so. And I
wasn't particularly thinking of ruin-your-day sorts of events; even very
rare high speed meteoroids should follow a power law size distribution,
so a pebble should be much more common than a boulder (even if neither
has happened more than a handful of times since the formation of the
Solar System).

Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com


----- Original Message -----
From: "Sterling K. Webb" <sterling_k_webb at sbcglobal.net>
To: "Meteorite List" <meteorite-list at meteoritecentral.com>
Cc: "Chris Peterson" <clp at alumni.caltech.edu>
Sent: Friday, December 08, 2006 12:59 AM
Subject: Re: [meteorite-list] Meteorite-list Digest, Vol 36, Issue 28


> 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
Received on Fri 08 Dec 2006 09:48:03 AM PST