[dalke]

The issues is that, technically, orbits aren't stable in the sense of being described perfectly as ellipse with a point mass at one focus.

That approximation can describe an orbit for a while, but after time the position will differ from the predicted orbit, until at some point you have very little idea of where it will be.

The time it takes for this to happen is called the Lyapunov time. For example, the Solar System is chaotic that way in the ~100 MY time scale. See http://en.wikipedia.org/wiki/Stability_of_the_Solar_System .

A Rosetta goal now is to figure out the comet's mass distribution, so that the orbital predictions can be better. The orbit won't be "stable" but it will be more "predictable."

Even then, we expect Rosetta to release gas, dust, and water vapor which affects the orbit. I expect there will be orbital corrections over the next few years.