[meteorite-list] Re: Meteorite-list Digest, Vol 26, Issue 30
From: Steve Schoner <schoner_at_meteoritecentral.com>
Date: Sun Feb 12 15:42:56 2006 Message-ID: <20060212.124205.23941.252823_at_webmail25.nyc.untd.com> As Sterling Webb wrote, if the reasoning he posited follows then there is no way that tectites came from the moon. The distribution on the earth, the ablation shapes, stretch forms, and lack of cosmic ray exposure pretty much eliminate the moon as the source. Steve Schoner IMCA #4470 Date: Sun, 12 Feb 2006 03:00:46 -0600 From: "Sterling K. Webb" <sterling_k_webb_at_sbcglobal.net> Subject: Re: [meteorite-list] Orbital debris watching radar To: <cynapse_at_charter.net>, "Meteorite List" <meteorite-list_at_meteoritecentral.com> Message-ID: <008e01c62fb2$d064d0f0$bb57e146_at_ATARIENGINE> Content-Type: text/plain; format=flowed; charset="iso-8859-1"; reply-type=original Hi, Darren, I gather from the phrase about having their orbits decay, that by "Earth orbit," you mean "in orbit about the Earth." Orbits around the Earth only "decay" because the orbit touches the uppermost atmosphere enough to cause drag which, however minute, reduces orbital velocity. It may seem logical that materials kicked off the Moon would easily and immediately end up in an orbit around the Earth, or at least some of them would. But the truth is that it is nearly impossible to get from the Moon to the Earth, and that lunar meteorites almost certainly do not arrive at the Earth that way, however illogical that sounds. The many simulations of transfer of materials around the solar system show the same result: impact materials from the Moon mostly go into eccentric solar orbits. a small percentage go into "co-orbits," that is, they enter solar orbits very similar to the Earth's orbit, sort of wandering along with us, and it is from that population that some get tangled up with the Earth's gravity and get pulled in. "Short" transit times are 10,000 years. When a lunar shows no cosmic ray exposure, that only means that it was less than 25,000 years. The reason why it's so hard to get from the Moon to the Earth is this: any object that falls to the Earth from a "great distance" achieves escape velocity by the time it gets very near to the Earth. And escape velocity is just that: you escape. No orbiting for you... There is a point, between the Earth and the Moon where the gravitational pull of the Earth and the Moon balance each other. Since the Earth is heavier than the Moon that point is closer to the Moon than the Earth. The point that lies in a straight line between the Moon and the Earth is the first LaGrange Point, BTW. But there are a multitude of points in every direction where equal force vectors from the Moon and the Earth meet: a sheet of zero gravitational potential. If an object is ejected from the Moon's surface toward the Earth without enough velocity to reach the zero sheet, it falls back toward the Moon. If it arrives at the zero sheet with just a smidge of velocity more than zero, it will fall toward the Earth, ramping up to escape velocity or near escape velocity at its closest approach then roar on out into the solar system. If it arrives at the zero sheet with a good deal of velocity more, it will fall on an Earth-influenced path and probably ramp up to a lot more than escape velocity... So, you see, stranger, thar ain't no way to get thar from here... Sterling K. Webb Received on Sun 12 Feb 2006 03:41:28 PM PST |
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