Bodies are held in orbital equilibrium by the conflicting forces of gravity and the bodies' own momentum. Gravity, which works over long distances and is always attractive, works to pull objects down toward a central point. The objects' momentum, however, works to push them away from the bodies they orbit.
In order for an object to remain in orbit, the two forces must be balanced. A body that is relatively close to its point of orbit must maintain a high velocity to avoid being pulled into its parent body. Likewise, a relatively distant object cannot move too quickly without escaping the pull of its partner.