6.What is the Roche limit?

The Roche limit was first described by Edouard Roche in 1848. The Roche limit is the closest distance an object can come to another object without being pulled apart by tidal forces. If a planet and a moon have identical densities, then the Roche limit is 2.446 times the radius of the planet. A large moon orbiting inside the Roche limit will be destroyed. The Earth's Roche limit is 18,470 km (11,470 miles). If our Moon ever ventured within this Roche limit, it would be pulled apart by tidal forces and the Earth would have rings. The four gaseous outer planets do have their rings systems inside of their respective Roche limit. The Roche limits for the gaseous planets are:

Jupiter - 175,000 km (108,000 miles)
Saturn - 147,000 km ( 92,000 miles)
Uranus - 62,000 km ( 39,000 miles)
Neptune - 59,000 km ( 37,000 miles)

On July 7, 1992, Comet Shoemaker-Levy 9 broke apart in 21 pieces due to tidal forces when it made a close approach of Jupiter which was within the Roche limit. On the subsequent pass, each of the pieces of the comet impacted Jupiter.

from Frequently Asked Questions about Saturn's Rings, by Ron Baalke, Jet Propulsion Laboratory