[amsat-bb] Re: Planet Pseudo-Keps

Stephen Melachrinos melachri at verizon.net
Sat Feb 27 07:37:14 PST 2010

Feb 27, 2010 08:43:51 AM, bruninga at usna.edu wrote:

>>> Since the planets revolve around the Sun,
>>> and not the Earth, there can't be any Keps. 
>>> Keps are based on the Earth being the 
>>> center of an orbiting object.

>> You're saying that if I lived on another planet 
>> or in another star system then my satellites 
>> wouldn't have keps and I couldn't use my 
>> software to track them?... That sounds rediculous.

>Yep. "keps" (as we use the term here regularly) either in AMSAT format or in NASA two-line format
>are just inputs to an "Earth orbit" propogator that predicts where a satellite is around Earth based
>on the laws of physics.

>If you lived on Mars, and had a "Mars Orbit Propogator" then you could download "Martian Keps" 
>from the Martians and track Martian satellites. But the Martian tracking program would not be able 
>to track earth satellies, just as ours Earth based programs cannot track theirs. Because the size 
>and mass of Mars is different from Earth, so the propogators have to be different... or something 
>like that. I'm on the limit of my meager knowledge here...

Bob (and Nigel) -

That's pretty accurate. Although the laws of physics apply universally around all the planets, the specific implementations of orbit mechanics software are location dependent. You gave one example--size and mass of the Earth vs Mars. Other examples are the oblateness of the planet (how flattened at the poles is Mars vs Earth) and the effects of other objects such as the moon and other planets. (The effects of the earth's moon on an earth-orbiting satellite will be different from the effects of Deimos and Phobos on a Mars-orbiting satellite.)

Although the equations are all the same, most programs (that are publicly distributed) "assumes" the earth as the core body, and embeds those assumptions into the software. I think STK (Satellite Tool Kit) by Analytical Graphics may have the ability to analyze non-Earth geometries, but it's hardly within the realm of amateurs. (A full-up version costs something like US$50,000.)


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