When it comes to the planets in the solar system, astronomers don’t have reflectors to return pulses of light. Instead, they use radar. Before radar was available they used the speed of light and the Parallax method to calculate the distance of planets. Today, however, they send a pulse of radio waves towards a planet, and wait for the faint echo to return after the waves have bounced of the planets rocky surface. Radio waves travel at the speed of light, so the calculation is the same as for measuring the distance to the Moon.
Radio astronomers have picked up radar reflections from all the planets with rocky surfaces – Mercury , Venus and Mars – and even from the rings of Saturn, which are made of billions of tiny lumps of ice. They cannot detect a radar echo from Saturn itself, or Jupiter, because both consist of gases and do not reflect radar.
Picture Credit : Google