# Why is an sattelite's motion not circular?

1. Dec 14, 2008

### grizzlyjoker

Why is an sattelite's motion not circular? I mean if there is a force that adjust the orbit every instant shouldn't the orbit be circular?

2. Dec 14, 2008

### Danger

If a force is exerted, then an orbit can be confined to circular.
Most orbits, however, are ballistic.

3. Dec 14, 2008

### cepheid

Staff Emeritus
I don't think I understand the question. Not circular meaning what? Are you thinking of a specific example you encountered? In any case, most bound orbits are elliptical (of which a circular orbit is a special case). Unbound orbits are typically hyperbolic. This is a consequence of the strength of the gravitational force obeying an inverse square law with radial distance.

Friction can reduce the total mechanical energy of a satellite to decrease, causing its orbit to decay. This can cause an unbound orbit to become bound, and causes bound elliptical orbits to become more circular with time.

A geostationary orbit is certainly circular...and is used for telecommunications satellites.

4. Dec 14, 2008

### D H

Staff Emeritus
That orbits (in the Newtonian two body problem) are conic sections comes straight out of the mathematics. The only thing special about circular orbits is that they never occur in reality. There are always perturbing forces, and gravity is not Newtonian. Even in a purely Newtonian, two point mass universe, circular (and parabolic) orbits are a zero probability event in the sense that the set of all circular and parabolic orbits are a zero measure subspaces of the space of all orbits.

Geostationary orbits are neither circular nor geostationary. Very close, but not quite. The subsatellite point follows a figure eight. The non-spherical nature of the Earth's gravity and third body effects (moon and sun) make it impossible to have a perfectly circular orbit.

5. Dec 14, 2008

### Staff: Mentor

Uniform circular motion occurs when the centripetal force is always directed perpendicular to the velocity. Objects in space are not always constrained to move perpendicular to the gravity force.

6. Dec 14, 2008

### mgb_phys

Most orbits are almost circular, they are elliptical because to be a perfect circle would require a perfect balance of forces eg. the earth's orbit around the sun is only around 1% off a perfect circle.

However some satellites are deliberately put into high non-circular orbits, for instance a Molniya puts a satelite high and sow over russia for most of it's orbit then low and fast over the south pacific - this maximises the time the satelite is available over the country of choice.