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Re: (meteorobs) 2 Questions (Fireballs)

In a message dated 2/12/00 12:41:53 PM Eastern Standard Time, 
brodwcjj@integrityonline.com writes:

<< 1.   At what magnitude will a fireball cast noticeable shadows on a dark
 night ?
       (It was so bright it made me squint and I noticed the shadows cast
 around me)

    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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