The following FWD messages are for archival purposes, and as a reference for the FAQ web page:
------------------------------ Date: Wed, 30 Oct 2002 19:25:43 -0500 From: "Lew Gramer" <[EMAIL PROTECTED]> Subject: (meteorobs) Meteoroid heating: Ram pressure, friction or both? I had the pleasure to browse an excellent page today by Dr. Phil Plait, Webmaster & principle debunker of the revered http://badastronomy.com . In particular, I followed a link found on Joe Rao's interesting site "10 confounding cosmic questions", at: http://www.msnbc.com/news/825959.asp?pne=msn (This is a great site, by the way, Joe! And as we'd expect from Joe, comets, artificial satellites, and meteor showers all figure among the "cosmic questions"!) >From there I got to Dr. Plait's "Top 5 Cosmic Myths": <http://www.space.com/scienceastronomy/top5_myths_020903-1.html> The Good News (:>) is that TWO of Phil's top 5 myths have to do with meteors and meteorites. The particular one that sparked my email today is Myth # 4, entitled "Meteors are heated by friction as they pass through the atmosphere". (Again, this is labeled a "Myth"!) In that page, Phil states, "In reality, it isn't friction, but *ram pressure* that heats meteoroids. When a gas is compressed it gets hot, like when a bicycle pump is vigorously used to inflate a tire. A meteoroid, moving at 33,500 mph (15 kilometers a second) or more compresses the air in front of it violently. The air itself gets very hot, which is what heats the meteoroid. That's fact, not friction." What a fine pun! :) But on to that QUESTION: I can recall reading in Oepik's "Physics of Meteor Flight in the Atmosphere", that the formation of a pocket of compressed air under a meteoroid critically de- pends on the SIZE (cross-section) of that particle: below a certain cross-section, the particle would in fact (per Professor O) NOT create it's own "air shield", and so would be directly impacted by air molecules throughout it's flight... And of course, we can assume that particles which are JUST large enough to form air shields, might still experience occasional but significant direct molecule impacts. So the question is, Which is really more important for the average *visible* meteor: the ram pressure Phil describes so well, or direct impact friction? Last but not least, I noticed that Phil specially mentions the figure "15 km/s or more", as the point at which ram pressure will cause rapid heating. Is this in fact some sort of theoretical cutoff? I.e., for meteoroids moving at 11-14 km/s (like those of Summer's "June Bootids"), is the physics different? (I know, I know: three weeks before the Leonids is not the BEST time to bring this up... But I was so curious - and I thought some of us could use a wee break from "Leonid fever" for a while, anyway. :>) Clear skies and happy pondering! Lew Gramer (GRALE), IMO, NAMN Webmaster: http://www.meteorobs.org Webmaster: http://www.visualdeepsky.org Assistant: http://www.namnmeteors.org The archive and Web site for our list is at http://www.meteorobs.org ------------------------------ Date: Wed, 30 Oct 2002 23:53:45 -0700 From: "Jim and Jan Gamble" <[EMAIL PROTECTED]> Subject: Re: (meteorobs) Meteoroid heating: Ram pressure, friction or both? May we entertain the theory that many meteors illuminate our skies in much the same fashion as good old lightning. Think about it, calculations as to how much visible energy can be released due to heating of a spec of dust no bigger than that of the head of a straight pin frequently don't match up with how much light is actually emitted. Can heat alone really cause a rock to burn brighter than our sun. Electrical discharges are among the brightest visible energy sources in the universe. This certainly is not to say heat is not involved, but is it heat ALONE? Regards, Jim Gamble El Paso, Tx Station-Sandia Meteor Detection Network All Sky Camera System 31.47N 106.18W [EMAIL PROTECTED] http://www.geocities.com/desert_lights The archive and Web site for our list is at http://www.meteorobs.org ------------------------------ __________________________________________________ Do you Yahoo!? New DSL Internet Access from SBC & Yahoo! http://sbc.yahoo.com ______________________________________________ Meteorite-list mailing list [EMAIL PROTECTED] http://www.pairlist.net/mailman/listinfo/meteorite-list