How do scientists keep track of the satellites?

Question

How do scientists keep track of the satellites? How do scientists make sure the satellites are not lost in space?

Answer 1

It takes just a small number of parameters (Keplerian elements) to allow an orbit to be predicted accurately quite far into the future. Tracking measurements are made occasionally to see if any errors have accumulated, and the elements are adjusted accordingly. Yes, serious effort goes into knowing where they all are.

Answer 2

Much of the work today is done with quite sophisticated navigational aids and telemetry, however unless something goes really awry, simple physics will ultimately rule provided the particular locations and velocities of the satellite are known. It is highly unlikely for a satellite to become "lost in space" without some extreme case such as a collision or thruster malfunction

All (most) space junk is also tracked by the United States Space Surveillance Network, which is a network of tracking stations to keep track of everything flying overhead (which is quite a lot, the second link I posted illustrates this by rendering a 3D model of the Earth and a small fraction of the satellites currently in orbit).