[meteorite-list] Meteorite-list Digest, Vol 36, Issue 28
From: Chris Peterson <clp_at_meteoritecentral.com>
Date: Thu, 7 Dec 2006 23:02:24 -0700 Message-ID: <00d401c71a8e$6f051230$2721500a_at_bellatrix> 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 ----- Original Message ----- From: "Sterling K. Webb" <sterling_k_webb at sbcglobal.net> To: "Chris Peterson" <clp at alumni.caltech.edu>; <meteorite-list at meteoritecentral.com> Sent: Thursday, December 07, 2006 8:30 PM Subject: Re: [meteorite-list] Meteorite-list Digest, Vol 36, Issue 28 > Hi, Visual, Chris, List > > For the benefit of Listees following the question > of how slow a meteoroid can be... > > The orbital velocity for any body is maximally > the escape velocity divided by the square root of 2, > or 70.707070707...%. Can we just call that 71%? > Escape velocity is 11,263.04 meters per second. So, the > highest orbital velocity is 7964.17 meters per second. > > That's the orbital velocity at the lowest possible > orbit, skimming over the surface. The orbital velocity > gets less and less the higher the orbit, so that geo- > synchronous orbital velocity is positively pokey, > around 3000 meters per second. You have to go faster than that just to > get there, then slow down > to stay there. Crazy stuff, that gravity. > > The only orbit that can "decay" is one close enough to the top of > the atmosphere to be slowed > into re-entry. But (big but), the only way an object > from somewhere "not of this earth" can get to the > top of our atmosphere is to fall there, in the course > of which fall, it will acquire additional velocity, up > to escape velocity. > > Escape velocity is like taxes, in that there just doesn't seem to > be any way to wiggle out. > By the time an object gets to the top of the atmosphere, it will > have acquired all of escape velocity except that which it would (try > to) pick up in the last 50 miles. > > By even the Earth's escape velocity of 22,263 mps is quite slow > compared to the approach of most meteoroids. Leonids are among the > fastest (70,000 mps) in approach velocity (theirs and ours). Most > objects from the asteroid zone are going to intercept Earth at twice > our escape velocity or more. > > The "slow" fireball is a rarity, but the one most > likely to get something to the ground. The statistics > of meteorites (on the ground) are misleading: irons are much rarer > than their proportion on our collections. It's just that they can > withstand re-entry so much better than rocks and that they can persist > longer in an Earth environment than mere rocks do. In re-entry, > irons are better than rocks; slow rocks are better than fast ones; big > rocks are better than little ones. > A meteorite in the hand is better than 1000 in freefall. > > > Sterling K. Webb Received on Fri 08 Dec 2006 01:02:24 AM PST |
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