Question

How do astronomers determine the distance between Earth and Mars at different times in their Orbits?

Answer 1

Using LIght Detection And Ranging (LIDAR), Mars surface points are illuminated. Using reflectors in telescope that flashes light, the faint rebound signals are detected to determine distance.

Explanation:

(To and fro time /2 ) / ( velocity of light ) = distance
This method fails when the space object is a source light that is flashes.
So, parallax method is used for stars.