Okay, numbers crunched.
Any body (like, the Earth or Moon) has internal gravitational forces that hold it together.
Now, you all know about tidal forces, right? It is the phenomenon that creates our ocean tides, and has the moon locked so only one side ever faces us on Earth - tidal forces arise from the fact that the strength of a gravitational pull drops off with distance. The Earth pulls on the near side of the Moon more strongly than it does on the far side of the moon. The difference produces a stress on the Moon.
So, if the tidal forces on a body are as strong or stronger than the internal forces that hold it together, in general, you expect the thing to get ripped apart. There are some special cases and orbits you can imagine where this doesn't happen (see the work of Dr. Forward, previously mentioned), but in general, there's a limit on how close you can get two bodies together before one or the other gets shredded.
The point at which this happens is called the "Roche limit", after Édouard Roche, who first calculated the thing.
If we assume (as Roche did) that the bodies are rigid (they aren't, but are close enough for our purposes of the moment), the calculation of the minimum acceptable distance between them is simple. And, if I have my numbers right, in all cases, if the planet-moon distance is the same as the distance from Earth to our own Moon, we are okay - you can put Mars where the Moon is now. Or you can put Earth in an orbit around Jupiter at that distance, and nothing would get shredded. Deformed and tidally locked, perhaps, but not destroyed.
Any body (like, the Earth or Moon) has internal gravitational forces that hold it together.
Now, you all know about tidal forces, right? It is the phenomenon that creates our ocean tides, and has the moon locked so only one side ever faces us on Earth - tidal forces arise from the fact that the strength of a gravitational pull drops off with distance. The Earth pulls on the near side of the Moon more strongly than it does on the far side of the moon. The difference produces a stress on the Moon.
So, if the tidal forces on a body are as strong or stronger than the internal forces that hold it together, in general, you expect the thing to get ripped apart. There are some special cases and orbits you can imagine where this doesn't happen (see the work of Dr. Forward, previously mentioned), but in general, there's a limit on how close you can get two bodies together before one or the other gets shredded.
The point at which this happens is called the "Roche limit", after Édouard Roche, who first calculated the thing.
If we assume (as Roche did) that the bodies are rigid (they aren't, but are close enough for our purposes of the moment), the calculation of the minimum acceptable distance between them is simple. And, if I have my numbers right, in all cases, if the planet-moon distance is the same as the distance from Earth to our own Moon, we are okay - you can put Mars where the Moon is now. Or you can put Earth in an orbit around Jupiter at that distance, and nothing would get shredded. Deformed and tidally locked, perhaps, but not destroyed.
