What Defines Safe Orbital Distances for Planetary Bodies?

[Insanity]

Just curious as to how someone may determine what orbital distances are "safe" or "stable" for planetary interactions?
or
How close could two planetary bodies get before the gravity interaction destroys one or the other?

i.e. Jupiter's Io's orbit places it close to Jupiter, and most of Io's geological activity is thought to be due to tidal stresses from this orbit. How much closer could the orbit be before Io was destroyed?

By looking at the orbital, & mass data from the numerous moons in the solar system, I was wondering if Io represents a limit, as Io is the closest "spherical" moon, all the other interior moons are irregular.

I am not sure if Newton's law of gravitation is appropriate?
using this formula, Io experiences the highest gravitational force among the Galilean moons, and most of Saturn's moons. In fact a few in order from highest down in the sol system, Jupiter 4.59E+23, Io 6.41E+22, Venus 5.59E+22.

I had used excel to calculate these, using the mass, and the periapsis orbit data from wikipedia.

I am sure there is a structural limit depending on the material composition, but there should be an average to use based on the overall density I assume.

Any help?

 

[D H]

You are talking about the Roche limit. Wiki: http://en.wikipedia.org/wiki/Roche_limit

 

[Insanity]

Thanks, I knew the name for this with black holes was the event horizon, but couldn't think of the equivalent for planetary interactions.

So i.e. Earth and Jupiter, I figured the Roche limit is 5.45E+07 meters, which means Earth would be inside Jupiter's atmosphere before being torn apart.

Any speculation as to what may happen to Earth's atmosphere prior to entering Jupiter's?