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(meteorobs) Xi Draconids' velocity



Visual velocity measurements in km/s have always bothered me, but without 
having seen the Xi Draconids myself, I really can not come right out and say 
your estimates of 50 to 65 km/s are wrong. However, please keep the following 
in mind. 

The speed of a meteor as seen by observers on the ground is a factor of the 
velocity of the meteor and the Earth. If the Xi Draconids were moving in a 
parabolic orbit, their initial heliocentric speed (i.e. the speed with respect 
to the sun, which represents the actual speed in its orbit) is a little more 
than 40 km/s. Since the stream is approaching Earth from behind, it is playing 
catch-up, so the observed speed, or pre-atmospheric velocity, MUST be slower 
than its heliocentric velocity. (An exaggerated example would be a car moving 
at 50 miles per hour being hit from behind by a car moving at 55 miles per 
hour. The result would be damage equal to a 5 mile per hour collision). A 
stream like the Perseids is different. Earth is basically undergoing head-on c
ollisions with the Perseids. Subsequently, although the Perseids have an 
initial heliocentric speed very similar to the Xi Draconids, the observed 
speeds end up being over 20 km/s faster (Another exaggerated example would be 
two cars moving at 40 miles per hour and hitting head-on. The result is damage 
equal to an 80 mile per hour collision). 

If the observed speed of the Xi Draconids were exactly 39 km/s, the meteor's 
orbital eccentricity would be about 1.2. An increase of only one km/s or to 40 
km/s, would result in the meteor's orbital eccentricity increasing to about 1.4 
(please bear in mind that these are rounded-off figures). I won't even bother 
computing the resulting eccentricities for speeds in the 50 to 65 km/s range, 
because the 40 km/s velocity already produces an eccentricity unlike anything 
observed before, except for a few low-precision photographic meteor orbits. 

Just offering the facts,
Gary W. Kronk