A satellite orbits the Earth with a fixed radius, what happens to the satellite if the speed is decreased to zero?

It falls to Earth and burns up.

This happens to most satellites, eventually.

If the speed of a satellite in orbit is decreased to zero, the satellite will no longer be able to remain in orbit and will start to fall towards the Earth. In other words, it will experience a change in its motion from a circular path to a straight-line path towards the Earth's surface.

To understand why this happens, we need to consider the forces acting on the satellite in orbit. In an orbit, there are two primary forces at play: the gravitational force pulling the satellite toward the Earth, and the centripetal force exerted by the satellite's motion that keeps it in a circular path.

These two forces are balanced when the satellite is in a stable orbit at a particular speed. This speed is determined by the radius of the orbit and the mass of the Earth. When the satellite's speed is reduced, the centripetal force decreases, causing an imbalance between the gravitational force and the centripetal force.

As a result, the gravitational force becomes stronger than the centripetal force, leading to a change in the satellite's trajectory. Without the necessary centripetal force, the satellite cannot maintain a circular orbit and will start to move in a straight line towards the Earth's surface.

It's important to note that if the satellite's speed is reduced gradually, it will gradually shift to a lower orbit before ultimately falling towards the Earth. In any case, without sufficient speed to balance the gravitational force, the satellite's orbit will become unstable, and it will eventually reenter the Earth's atmosphere or collide with the Earth's surface.