A gravity well or gravitational well is a conceptual model of the gravitational field surrounding a body in space. The more massive the body the deeper and more extensive the gravity well associated with it. The sun has a far-reaching and deep gravity well. Asteroids and small moons have much shallower gravity wells. Anything on the surface of a planet or moon is considered to be at the bottom of that celestial body’s gravity well. Entering space from the surface of a planet or moon means climbing out of the gravity well. The deeper a planet or moon’s gravity well is, the more energy it takes to achieve escape velocity.