Gravity well

There simply is an endless supply of map-ish xkcds by Randall Munroe. this one does a beautiful job of illustrating gravity wells. The "title-text" at the actual webcomic says:

This doesn't take into account the energy imparted by orbital motion (or gravity assists or the Oberth effect) all of which can make it easier to reach outer planets.

Click on the image to enlargify.