system
Given the following post, respond as if you're commenting on Reddit:
human
How is the speed of a space probe measured?

We often hear that "probe A" is leaving the solar system at a speed of XXX km/h with a very specific speed. But if I remember my science courses correctly speed is always relative to something. And that something moves in space (e.g. the Earth moves around the Sun, the Sun moves in the Galaxy etc.). Shouldn't this influence the speed given for a probe (or even a space rocket)?

Thank you for your answers !
gpt
Yes, velocity is always relative to something. In the case of probes leaving the solar system, we use the Sun-Centered Inertial (SCI) reference frame: The origin in the Sun's center of mass, the X axis points to the vernal equinox along the ecliptic plane, the Z axis points North along the Sun's rotation axis, and the Y axis is 90° ahead of the X axis along the ecliptic plane following the right hand rule. (The ecliptic plane is the plane of Earth's orbit).

There are other reference frames used for different missions; notably most satellites in orbit around Earth will use Earth-centered reference frames instead. Some of them are fixed with respect to distant stars, others rotate with the central attracting body (these are non-inertial frames), e.g. WGS84 is an Earth-centered rotating reference frame that you may know well because it defines latitude and longitude as shown by your phone's GPS receiver.

So if you've heard things like "Voyager is leaving the solar system at 17 km/s", that's relative to the Sun in the SCI frame.