please see below
If we apply Newton's law, the force due to gravity will be
The satellite moves in a circular orbit, and the centripetal force must be provided by the gravitational force. So for a stable orbit
So if the velocity is too high, then
The force of gravity is insufficient to maintain the satellite in a circular orbit and the satellite will "fly off into space" unless it can be directed into an orbit of lower radius.
Or seen another way, rearranging the above equations gives:
If velocity is too low:
The force gravity exceeds the centripetal force required. Unless the orbit can be moved to a larger radius (reducing the force of gravity until the above are in equilibrium), the satellite will spiral in towards the earth.