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(meteorobs) Xi Draconids' velocity



George Z.

Actually the Perseids and Xi Draconids could have very similar HELIOCENTRIC 
velocities, and, from the sample of photographic and radio-echo Xi Draconid 
meteor orbits at my disposal, this is confirmed. The heliocentric velocities 
are just over 40 km/s. But, as I said before, the Xi Draconids hit Earth from 
behind and the Perseids hit us head-on. The Xi Draconids will (and do) exhibit 
an apparent (observed) velocity slower than its heliocentric speed, while the 
Perseids exhibit a faster speed. 

Please do not take any of this wrong. You and Bob appear to be the first to 
point out this stream's existence, and that is a credit to you both for being 
such good observers. I'm just saying that your speed estimates must be too high 
because of the radiant you gave. If your radiant was about 40-50 degrees off, 
it could push it around to the front of our planet's direction of motion and 
the velocities you indicated would be reasonable. But, I seriously doubt that 
you and Bob made mistakes in plotting, so I am just suggesting that a speed of 
just under 40km/s is much more likely because of the orbit the stream is moving 
in and the direction with which its members enter Earth's atmosphere. 

Anyway, I got my orbit program working and have computed parabolic orbits for 
the radiants you and Bob determined, based on an average solar longitude of 
81.6 degrees. I will send these to the list in a couple of hours. These orbits 
assume parabolic motion (the fastest acceptable speeds) and confirm what I said 
above about the maximum observable velocity. The only way to attain an 
observable speed of 50 to 65 km/s is to assume extreme hyperbolic motion. 

Gary W. Kronk