# Celestial Mechanics

1. ### a.a

128
The greater the altitude of an Earth satellite, the longer it takes to complete one orbit. Why is it impossible for any vehivle to go aroung Earth in less than 80 minutes?

2. ### mgb_phys

8,809
Is this a homework question?
You have to at least try and answer it, describe what you know and any ideas you have about the solution

3. ### Danger

9,663
It's not impossible, but the energy expenditure would be ridiculous. If you're referring to ballistic orbits, I suspect that it's because an orbital period of less than 80 minutes would put the object inside the atmosphere. (Although, I haven't actually heard of that limit before.)

edit: Sorry, Mgb; didn't mean to step on your toes here. I started this response before seeing yours. It didn't occur to me that it might be a homework problem.

Last edited: May 21, 2008
4. ### michalll

44
Because the centrifugal force would be greater than gravitational, unless you compensate it some other way. Computing this you get the minimal period something about 84 minutes.

5. ### a.a

128
Its not a homework question. It was a question that arose in class and I didnt really understand the explaination and as a result didnt take notes.

Im sorry michalll but i didnt quite get why the centrifugal force would be greater than the gravatational force.

6. ### mgb_phys

8,809
Surely the definition of an orbit is that centrifugal force equals the gravitational.

If you calculate the height necessary for an 84min orbit and compare it to the height of the atmosphere / density of the atmosphere at that height you will see why a satelite orbiting at that height isn't going to last long.

you could orbit the Earth at sea level if you had an air free pipe to avoid air resistance.