Re: TLE's for WOTS!
Philip Chien (kc4yer@amsat.org)
Wed, 27 Mar 1996 09:57:06 -0400
>Hello,
>
>You maybe asking yourself what is WOTS?!? Well it is my acronym for Way
>Out There Satellites :) I was just thinking wouldn't it be cool to have
>TLEs for way out there objects such as the moon, or our friend the comet?
>Just something we could plug into out satellite tracking programs? Just a
>thought.. I know there are ways of doing this now (heck there may even be
>TLEs) but I just thought that there could be another "fun" thing to
>track.. :)
>
>You may now start the "Josh is an idiot" flames.. :)
well, as long as you insist ...
Most satellite tracking programs are designed to track satellites in orbit
around the Earth - not around the Sun or any other solar system bodies. So
it would be awfully difficult to use them to track a comet ...
In theory the moon could be tracked. However it's extremely large relative
to other satellites, and at a fairly far distance. So it's perturbed by
the Sun's gravitational pull (the moon is actually attracted more by the
sun than by the Earth) so the gravtiational models are fairly complicated.
It wasn't until recently that the moon's prediction could be predicted
accurately! One of the biggest conundrums for early astronomers was that
they could predict the locations of the planets with a fair amount of
accuracy, but not the moon!
You can actually use a stationary Earth model (as most satellite tracking
programs do) and a moving Sun. So in theory you could enter keps for the
Sun as it goes around the Earth. (in any moving system you have to make
some assumptions about what's your frame of reference. Even if you choose
an invalid assumption everything else can work out properly as long as you
keep your conclusions consistent with your initial assumption. One obvious
assumption most of us make each morning is that the 'sun rises',
consequently the Earth doesn't rotate with that assumption. It's a
reasonable assumption because at first glance stationary objects appear
stationary, with the Sun moving through the sky).
BTW - there are 'way out there satellites' which do orbit the Earth,
including Geotail and IMP-8. These satellites have very elliptical orbits,
with their apogees well beyond the moon's distance. They can't be tracked
accurately because their orbits are perturbed by the moon too much
(something which most microcomputer tracking programs don't take in to
account). In fact they actually use the moon's gravitational pull to keep
their orbits in check!
Philip Chien, Earth News - space writer and consultant PCHIEN@IDS.NET
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