[amsat-bb] Re: info on satellite placement

Jason White jason at jason.white.name
Tue Oct 31 10:38:54 PST 2006

Thanks for posting these responses Bob, your explanations are quite
clear.. I wish my high school physics professor had been nearly as
clear. Your answer made me think.. if the ellipse crosses the path of
the earth (or a path through enough atmospheric resistance that
sufficient energy is lost) it will  not continue to orbit. Simple
enough. What I'm interested in, then, is how do you determine if an
object has sufficient energy to escape orbit entirely? I can comprehend
that the gravitation between two objects is inversely proportional to
the square of the distance between them.. that at least "stuck" back in
school.. so as near as I can figure, if you double the distance of the
orbiting object from the center of the earth, it would take 4 times less
speed to escape. Is that even in the ball park? I'd imagine, from that,
that given a measure of gravitational pull on a measurable mass you
could derive a speed at which the forces would no longer be equal, and
the ellipse would never return back to the original spot.

I'd once set a goal for myself of being able to do at least the
rudimentary math involved in how the Apollo missions were able to orbit
the Moon, even assuming the two objects were stationary, but since I
never learned Calculus to me it's all just squiggles. I appreciate plain
language explanations like yours.

Thanks for the elucidation,

Jason - N1XBP

BTW, if anyone else is as curious as I am, I found the ARRL Extra class
study guide to have an easy to read and understand section on Kepler's laws.

Robert Bruninga wrote:
>> Obviously the further out, the longer it takes to do a 
>> revolution. I imagine at some point earths gravity will not 
>> be sufficient to hold an object of given mass in a circular orbit?
> Exactly,  If  you are at a given altitude and you are going to
> slow, then your satellite will "fall" towards earth..  But as it
> falls, it speeds up as it approaches its lowest point (on the
> other side) and that makes it go higher to arrive back where you
> are on this side... In otherwords, the circular orbit becomes an
> elipse with a high side and a low side.  

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