Resonances:  Properly timed gravitational “pushes”
		• Like someone pushing kid on a swing – Timing of pushes just as important as force used • Pushing at random times has little effect • Pushing at just right point in each cycle can produce big effect
			• Resonances are same sort of effect – for orbits • If orbital periods have ratio 1:2, moons pass each other in same place each orbit, so gravity of one on other badly disturbs orbit
						Io : Europa orbital periods 1.7 days  :    3.4 days 1      :        2