How Can Astronauts Tell How Fast They’re Going?

Here on Earth, it’s easy to know how fast you’re driving. You get a good sense of it just by seeing trees and cows pass by. And of course you also have a speedometer that counts how many times your tires rotate per second and computes a speed based on their circumference. (Fun fact: Put bigger tires on your car and your speedometer will be wrong.)

If you’re flying over an ocean, of course, there’s no visual reference, so from inside it looks like you’re motionless. But airplanes can get their airspeed by using sensors to measure the rate at which air is passing over the wings. If there’s any wind, this won’t be the same as your speed relative to the ground, but you can get that by using GPS location data from orbiting satellites.

Now imagine you are flying to Mars ...