Would an orbit with eccentricty 0 ever degrade?

Question

Let's assume that an object is orbitting a planet of 6 x 10 (21) tonnes, and the eccentricity of the orbit is 0. Would the objects orbit ever degrade? Assuming that the planet didn't have an atmosphere.

Answer 1

While you do assume a perfectly circular orbit and zero drag force, your phrasing of the question does *not* rule out tidal effects.

This depends on the planet's oblateness, known as its "J2 moment". Most planets are not perfect spheres, but slightly squashed along the axis of planetary rotation. If the planet is oblate and its rotation axis is tipped to the axis of the object's orbit, then there will be tidal forces as the object experiences a net torque between the near and far sides of the planet. Combined with lag from material stresses, there can be a transfer of rotational momentum...remember, the moon used to be much closer to the Earth and Earth rotated much more quickly a few billion years ago.

In general, this effect causes orbits which are prograde (orbit direction same as planetary rotation direction) to slowly migrate outwards, and retrograde orbits (orbit direction opposite to planetary rotation) to migrate inwards. This is one theory for the creation of Saturn's rings: that a retrograde moon drifted inwards until it was ripped apart from increasing tidal stresses.

Answer 2

As long as there is no other gravitational force acting on the orbiting body, mathematically the orbit would not degrade.
However, there are billions of gravitational sources in space (other planets, other stars, etc.) that will have a tiny but non-zero effect on this planet's orbit. While the effect may be so tiny that it can't be measured at one time, over time the effects can add up enough that the orbit will change and no longer have 0 eccentricity. Could take millions of years, but it's inevitable.

Answer 3

Depending upon the conditions of the thing that is orbiting and the object being orbited, whether in space, friction, extra-systemic forces, etc. If one considers the Theory of Entropy, everything degrades to a less ordered state.

Answer 4

Orbits don't ever degrade if there is no tidal forces at work, I don't think it matters what the eccentricity is. Of course there is always going to be some tidal forces which will cause an orbit to increase or decrease in size in tiny amounts.

Answer 5

The eccentricity is irrelevant. Any particular orbit has a particular total energy associated with it regardless of its e value. Assuming no interaction with any other masses (including space dust) and no tidal effects on either body that would turn energy into heat, the orbit would not degrade.

Answer 6

If the velocity remains constant, then there is no reason for the orbit to degrade. If the object loses or gains velocity through a collision of some sort, than the orbit may degrade to a more stable orbit for that velocity.