Poor, Doomed Phobos  
An application of tides.


Phobos is one of Mars' two small moons.
 
 
 
 
 

As with any moon, its gravitational field raises a (very tiny) tidal bulge on its parent planet.

Since Phobos lies closer to Mars than a synchronous orbit, Phobos orbits faster than Mars spins.

Because of friction, Mars' tidal bulge does not quite keep up with Phobos, leading to an effective drag on the moon.

As a result, Phobos is spiraling in towards Mars, and at its current rate (1.8 m/century), will hit the planet in about 50 million years.

But at some point it will reach the Roche limit, which we can calculate to be

How can this be? (Phobos: I'm not dead yet!)
 
The actual Roche limit depends on the size and strength of the orbiting moon. The Roche limit is smaller for small, rocky objects (like Phobos). Best estimate is that
 
 When Phobos gets this close, tidal forces will destroy the moon.