Hi all - I just wrote a book on man and impact. It's called "Man and Impact in the Americas", and it's available through amazon. I'm tired now, so I'll keep this short.
The "experts" numbers for impact appear to be off by about factor of ten, in the impactors' favor, not man's. Over the last 6,000,000 years, we've come close to extinction several times. Impact rate estimates have been crippled for about 30 years, largely due to confusion spread by Dr. David Morrison over the role of comets in impact. While Morrison did pioneer ground breaking work with Shoemaker some years back, since then his use of the power he gained from that work has been to the detriment of the field, and the detriment of us all. We can deal with this now, with the technologies we have in hand, but only if we make a concerted effort. Think of it as the ultimate test of human intelligence. Will we pass? I don't know. I'm going to get some more coffee and cigarettes. good hunting, Ed --- "Sterling K. Webb" <[EMAIL PROTECTED]> wrote: > 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" <[EMAIL PROTECTED]> > To: <meteorite-list@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 > > > > > ______________________________________________ > Meteorite-list mailing list > Meteorite-list@meteoritecentral.com > http://six.pairlist.net/mailman/listinfo/meteorite-list > ____________________________________________________________________________________ Any questions? Get answers on any topic at www.Answers.yahoo.com. 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