The source of gravity is the mass of the Earth. As you leave Earth, the amount of mass between you and the center of the Earth decreases, so the gravitational force acting on you decreases. The more massive the object, the greater the gravitational pull it exerts. As you leave Earth, you experience a progressive weakening of the gravitational force as you move away from the planet's center. This explains the sensation of weightlessness experienced by astronauts in orbit and the diminishing gravitational pull felt during ascent to higher altitudes.