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Re: (meteorobs) Fwd: How fast do Meteors go?

Hi John and others,

There is a way to accurately determine meteor speeds and that is by high
accuracy photographic triangulation. That is a mouth full, but it works like
this: you have two photographic camera's, which you put up some 40 to 50
miles away from each other, making sure that they point to the same part of
the atmosphere at roughly 90 km (that is, about 60 miles) altitude (which is
the altitude at which meteors appear). In that way, you try to photograph
the same meteor with these two cameraas. If you succeed, you then have two
pictures of the same meteor against a starry background taken from two
different locations. If you look at these pictures, you'll note that the
meteor on both pictures appears to have a slightly different path across the
starry background. This is due to the effect we call "parallax". You can
best understand what I mean by putting up a finger at arms length, close
your right eye and look where your finger appears to be with respect to the
objects in the background (your mum and dad: your couch: the wallpaper):
then open your eye and close the other eye: do you note how your finger
appears to move (or better: shift) with respect to the objects in the
background? This is parallax, and the amount of shift of your finger as seen
between your right and left eye is a measure of the distance and position of
your finger with respect to your two eyes.
Now: with the meteor example, the two cameraas 40 miles apart are just like
your two eyes: it takes that 40 mile distance instead of the inch or so
between your two real eyes to see the parallax of a meteor because it is so
far away from you (if your finger would be as far away as a meteor, your
eyes would have to be 40 miles apart to see it shift too!).
Thus: the meteor appears to have shifted with regard to the starry
background on the two photographs, just like your finger when seen by both
your eyes apart. With the help of the two photographs, we can measure how
much the meteor appears to have shifted with respect to the background, the
stars in this case (which are so far away that their parallax is so small
you can't see it, even when your "eyes" (the two cameraas) are 40 miles
apart).
With the help of some very difficult calculations, we can use the shift
measured to calculate the exact positions of both the starting point of the
meteor and the end point of the meteor in the atmosphere. That gives you not
only an idea of the trajectory in the atmosphere which the meteor travelled,
but if you know the location of the starting point and the ending point you
then can also measure how long that trajectory was in miles. That can be
tens to hundreds of miles, usually.
Now, the next step (yes, this is a very complicated story, but hold on!) is
to get an idea of how long the meteor was visible. For, if you can somehow
measure that it was visible for 1.5 seconds and from the earlier parallax
measurements you know it travelled a trajectory of  say 45 miles long: then
you know that it travelled 45 miles in 1.5 seconds, which is similar to a
speed of 30 miles a second (45:1.5=30).
So, in order to know the speed you should not only know (through parallax
measurements using two cameraas 40 miles apart) the length of the trajectory
of the meteor, but you should also know it's duration in seconds. That is
possible to determine by putting some kind of  wide bladed "propellor" in
front of the camera lenses spinning at very high speed. The blades of the
spinning propellor thus periodically cover the camera lens during the
exposure. This also happens during the 1.5 seconds (in our example) that the
meteor is visible. We use "propellors" (we call them "rotating shutters")
which cover the lens 50 times each second: so with a 1.5 second duration for
the meteor, it's trail on the photograph will be chopped up in 75 (1.5 x 50)
little pieces by the blades of the propellor moving in front of the lens.
Just count into how many pieces a meteor trail image on your photograph is
chopped up, and if you know how many times your propellor covered the lens
per second, you can determine how long the meteor was visible.
Well, to end this long and difficult story: you then know how long the
meteor trail trajectory was in miles; and how long the meteor was visible.
These two  measurements together allow you to calculate how fast it was...!

There are now a few thousand of these kind of measurements on meteors. The
Dutch Meteor Society, of which I am a member, made over a thousand of such
high accuraccy measurements. The fastest meteors measured by us had a speed
of  71 kilometers per second, that is about  44 miles per second or 160000
miles per hour, just as Lew already told you. These speeds are very
accurately determined: they are accurate to within just a few miles per
second or better.

If you want to take a look at some results and pictures of the equipment we
use for such measurements, check out http://www.dmsweb.org

Best wishes and good luck!

Marco Langbroek
Dutch Meteor Society
Holland





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