# Thread: The physics of an artifical earth orbit in layman's terms

1. Newbie
Join Date
Dec 2007
Posts
9

## The physics of an artifical earth orbit in layman's terms

Dear board,
I'm currently calculus and scientifically illiterate, so please bare with my ignorance of the subject matter in trying to explain my question on how an orbit works.
The way I perceive it is as follows: You basically need to get an object moving "parallel" to the surface of the Earth so fast that it can't fall to the ground, and yet slow enough so it doesn't leave the Earth entirely, due to the fact that the surface is circular. The "parallel" velocity is fine tuned to a sweet spot where the object's distance never changes from the ground (assuming a circular orbit). And, since the object is in a vacuum, no further energy is necessary to power it since it's in a frictionless environment.

Would such ramblings be a correct interpretation?

2. Established Member
Join Date
Nov 2004
Posts
887
Correct, it is a sweet spot.

3. Order of Kilopi
Join Date
Dec 2004
Posts
11,218
All correct, though it could be interpreted as sounding a bit misleading.
Saying that it is "fine tuned to a sweet spot" makes it sound as if it were
very unstable -- that is, if the orbit deviates at all from a perfect circle,
it will rapidly deviate farther and farther until it either re-enters or flies
off to who-knows-where. What actually happens, of course, is that
the orbit becomes slightly elliptical. Because of perturbations to orbits
by the gravity of the Moon and the Sun, the gravity of Earth's equatorial
bulge, and drag from Earth's upper atmosphere, no Earth orbit is totally
stable, but for dense satellites in orbits that don't go too high or too
low, they are pretty long-lived -- pretty stable. Elliptical orbits will
slowly precess (change orientation) because of the perturbations, but
unless they go near the Moon or dip into the atmosphere, they will stay
pretty much the same ellipse shape for a very long time.

Here's a webpage I made on orbital speed.

-- Jeff, in Minneapolis

4. To echo what Jeff said, and put a different spin on it, any "sideways" motion is going to result in an orbit, as long as the satellite doesn't run into the body (the surface or the "thick" part of the atmosphere) that it is orbitting. As the OP says, no further energy is required.

All that is necessary is enough speed to get beyond the body by the time the body's gravity pulls it down to that level. Such an orbit might not be circular.

5. Yes, although an elliptical orbit will be going faster (possibly much faster) than a circular orbit at the altitude of the elliptical orbit's periapsis (point of closest approach).

6. Originally Posted by cjl
Yes, although an elliptical orbit will be going faster (possibly much faster) than a circular orbit at the altitude of the elliptical orbit's periapsis (point of closest approach).
And slower than a circular orbit at the altitude of the elliptical orbit's point of farthest approach!

That was what the OP and alainprice meant by sweet spot, I figured.

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