Re: (meteorobs) 2 Questions (Fireballs)
In a message dated 2/12/00 12:41:53 PM Eastern Standard Time,
<< 1. At what magnitude will a fireball cast noticeable shadows on a dark
(It was so bright it made me squint and I noticed the shadows cast
Venus is capable of casting faint shadows . . . so perhaps we might use a
magnitude of -4 at the lowest limit for a shadow-caster. Those objects that
cast "noticeable" shadows are probably brighter than -6.
2. Why would the "train" last for almost 50 minutes, (most trains I
read about are only 2 or 3 minutes at best)..... could the particles in
the train supply a condensation point source for atmospheric water
vapour to condense yielding a cloud. (I was in a clearing between two
storm systems, so the humidity in the upper atmosphere could have been
fairly high) Any thoughts ?
It is not likely that humidity has anything to do with the duration of a
meteor train. The trains appear at altitudes above 40 miles and what little
moisture there is at such altitudes (virtually none) would be negligible.
One possible reason for the long-duration of Leonid trains are their very
high entry speed into our atmosphere: 45 miles per second, the fastest of
any known meteors. In Zdenek Ceplecha's 1968 classification of the beginning
height of meteors, the Leonids ranked highest at above C2, meaning that most
start flaring into visibility at altitudes higher than 70 miles above the
Earth. Assuming that the largest Leonid particles are pea-to-marble sized
(though with the consistancy of cigar ash), such an object tearing through
our upper atmosphere at 45 miles per second certainly has the potential to
leave a very long-enduring trail (as compared to the "slower" displays such
as the Perseids and Geminids). There may very well be other factors that
contribute to the long-duration of the Leonid trains. Further study
(especially from the most recent 1999 display) will certainly help.
-- joe rao
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