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(meteorobs) A rethinking of the upcoming Leonid meteor rates

    Those who have read my Leonid paper in WGN 27:3/4 will have noted how 
through orbital simulations I had integrated material from the great Leonid 
storm in 1833 forward to 1866.  What I discovered was that the orbital 
separation between the Earth's orbit and this 1833 material had widened to 
almost 0.007 a.u. by 1866.  Also, I integrated material from the 1966 forward 
to 1999 and found a separation between Earth's orbit and this material of 
nearly 0.003 a.u.  

    My initial supposition was that such a difference between 1866 and 1999 
would result in 1999 seeing much higher hourly rates compared to the 6000 to 
10,000 rates of 1866 (as noted in Icarus 138, 287-308 by Peter Brown).  

    Last week however, I had a chance to e-mail David Asher and inquire why 
he left out the meteor trail map for 1866 from the Armagh Observatory 
internet site.  He kindly -- and very quickly -- responded by adding the map 
to the site, which indicated that a meteor trail shed by 55P/Tempel-Tuttle in 
1733 was primarily responsible for the 1866 display.  Over this past weekend, 
I performed an integration for 1733 myself and found that this material 
passed ~0.001 to 0.002 a.u. from Earth's orbit in 1866, pretty much 
confirming what David had indicated. 
      
    As a result of this, I am toning-down my original "guesstimate" of Leonid 
activity to 2,000 to 6,000 per hour.  In WGN 27:2, David and Rob McNaught 
suggest a peak of 1,500 per hour -- a rate that is quite close to the 
lower-end of my revised range.  

    I should, however, point out something that is not-often cited in 
predicting Leonid rates and that is the propensity of meteor activity during 
a storm apparently coming in "waves" or "surges."  The oft-quoted "40 meteors 
per second" from Dennis Milon's team at Kitt Peak Observatory in 1966 was 
arrived at after a consensus, but was probably the ABSOLUTE UPPERMOST LIMIT 
of observed activity.  Indeed, there were others who claimed to have seen 
similar, if not even higher rates (James Young at Table Mountain Observatory 
suggested 50 per second!) but again, this may have only been an extreme upper 
limit.  As I comment in a footnote in my WGN paper, many other witnesses of 
the 1966 storm noted lower rates of "only" 10 to 30 per second.  A very 
telling description came from Dana K. Bailey of Boulder, Colorado, who 
commented in the January 1967 Sky & Telescope that " . . . no fewer than 10 
new meteors were appearing each second, for many minutes, yet  sometimes the 
rate was double or triple that . . . "

    Another suggestion of meteor activity apparently coming in surges, comes 
from a description of the 1866 Leonids as reported in The London Times:  

"The spectator had soon counted half a dozen; then he felt sure he had seen 
thirty; then six or seven in a minute . . . Then there came two or three 
together; then not less than a dozen of all kinds."  

    In essence . . . if indeed, a maximum of 1,500 per hour occurs this year, 
that's 25 per minute or one meteor every two or three seconds.  It would not 
be surprising however, if -- based on the above description -- there come 
brief intervals when the rates may actually reach several times this rate.  
You've heard of major storms that, for example, produced " . . . sustained 
winds of 50 miles per hour, but with occasional gusts to 80."  Well . . . in 
1999, it could very well be that we'll hear of overall sustained 
single-observer rates of say, 2,000 per hour with occasional bursts ("gusts") 
to 7,000!

-- joe rao         
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