## How can we tell a spacecraft's speed?

As any spacecraft moves through space, it is communicating with Earth by sending off radio waves. These radio waves can tell us a spacecraft's speed in two ways. The first one is very simple. Since radio waves (like all other electromagnetic radiation) travel at the speed of light, we can figure out how far away something is by how long it takes for radio waves to travel to it, bounce off and travel back. If the waves take a longer time today than they did yesterday, we know how much farther the ship is, or how much it has traveled in a day. From that we can figure out the speed.

A much more accurate way of telling a ship's speed is by using the Doppler effect. If the craft is moving away from Earth quickly, radio waves from it will get spread out, and if it is traveling towards Earth, radio waves will be scrunched together. How much the waves are spread out or scrunched together depends on its speed. Using the Doppler effect is very precise. It makes it possible for the radio telescopes of the DSN to measure spacecraft speeds to within hundredths of a millimeter per second.