By Marco Langbroek - Dutch Meteor Society, Holland <marco.langbroek@wanadoo.nl>
 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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