You Asked:

How does a spacecraft measure its speed in space? — Andru, Bucharest, Romania

Marshall Brain Answers:

When a car measures its speed, it does it by counting how many times its wheels rotate and sending that information to its speedometer. When an airplane measures its speed, it uses a pitot tube to detect its movement through the air.

But something in space is moving through a vacuum, and there is nothing material against which it can measure its speed. So it has to use other techniques.

One way is to use an inertial guidance system. Accelerometers and gyroscopes measure every turn and acceleration that affects the spacecraft and a computer uses uses this information to calculate the current velocity.

If an object is in orbit around earth, one way to calculate its speed is to measure its altitude, the shape of its orbit and the amount of time it takes to complete one orbit.

If a spacecraft has an atomic clock on board, it can send the current time to earth in a radio transmission. The amount of time it takes for the signal to arrive can be used to calculate distance. Looking at change of distance over time can reveal speed. The GPS satellites use atomic clocks to reveal distance like this.

Other techniques use radio waves. For example, the shift in radio wave frequencies (known as Doppler shift) from the spacecraft when they are received on earth can be used to determine speed. A spacecraft can also use radar reflecting off a moon or planet that is moving at a known speed.

Using techniques like these, scientists discovered what is now known as the Pioneer Anomaly – a very small perturbation in the speed of the Pioneer probes that have left the solar system.