Hi,

 

Say the moon doubled in size magically, what would keep it from becoming part of the Earth again?

 

I guess I'm asking what would have to happen to stabilze the orbit of the two objects if one just suddenly became larger ? Would one have to spin faster? or conversely move away from the other ?

 

At present, the Moon has an average orbital speed of 1.015 km/sec. If it had twice the mass, its mean orbital speed at the same distance would only have to be 1.009 km/sec. Thus, if it suddenly doubled in mass (and nothing else changed) then it already would be moving faster that it needs to in order to maintain its existing orbit. To compesate it will change its orbital eccentricity to raise its average orbital distance. This adjustment will just happen as a consequence of the change in mass.

Thank you for responding so succinctly, it was a pleasure reading it and helped me understand things a bit better. Thx.