system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how does a spaceship know how fast it's going?

We know how fast Voyager 1 is going, is that because it has a device on it to measure its speed? Or is it because we visually measured its speed compared to background objects, and have implied it?

A tire is the object that rotates and measures a vehicle's speed. Air speed passing by a sensor determines a plane's speed. We don't know what dark matter is (and presuming we don't have sensors to measure it), so how do we know how fast a vehicle in space is going?
gpt
We know how fast the spaceship is going because we knew have fast it was going, exactly where it was heading, and how long it's been travelling, as well as everything near it that matters (the sun, basically).  If you know all those factors you can figure out what speed it will be moving in the future.

In addition we can tell how fast it's going based on the signal that it's sending to earth.  This is also how we can tell how fast other galaxies are moving away from us.

But in all these cases the speed is actually **speed relative to something**.  Voyager 1 is listed as travelling X mph away from the sun, but it's exactly as accurate to say the sun is moving at X mph away from Voyager 1.