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

[Chris Peterson]

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