Name: James A. Murray
Date: Around 1999
What is the principle behind the "gravity assist" that they used to
get the Galileo probe to Jupiter? It seems that one would lose all the energy
one gained falling by climbing away from the gravity source. Or is the net
energy gained by the probe only taken from the planet's momentum (and
therefore no energy is gained by using the sun)?
You have the idea exactly. It works only if the body you're using is
moving in the general direction you want to go.