The concept of zero gravity or weightlessness experienced by objects at a certain altitude above the Earth's surface is a common misconception. In reality, the force of gravity never completely disappears at any distance from the Earth. Newton's law of gravity still applies, but its strength diminishes with distance according to an inverse square law.
The sensation of weightlessness that astronauts and objects experience in space or during freefall (such as in an airplane during parabolic flight) is actually the result of falling towards the Earth while also moving horizontally with sufficient velocity to maintain a curved path. This state of continuous freefall creates the illusion of weightlessness.
When an object is in orbit around the Earth, it is constantly falling toward the Earth due to the gravitational pull, but its forward velocity prevents it from getting any closer to the surface. The combination of the object's horizontal velocity and the curvature of the Earth's surface creates a continuous state of freefall, giving the perception of weightlessness.
As an object moves higher above the Earth's surface, the force of gravity weakens, but it is never completely absent. However, at distances of 30-50 kilometers above the Earth's surface, the force of gravity is still significant, although it is slightly weaker compared to the surface. So, there is no precise altitude at which gravity suddenly becomes zero.
It's important to remember that weightlessness is not the absence of gravity but rather a state of continuous freefall that counteracts the sensation of weight. The force of gravity still acts on all objects in space, including satellites and astronauts, but they are in a perpetual state of falling, which creates the perception of weightlessness.