# How does one measure velocity in space?

Just what the title states.

Velocity, by definition, is the distance covered in a defined amount of time. How does one measure the velocity of a spacecraft that is not in Earth Orbit? E.g. Beagle

EDIT: I qualified the location as outside of Earth orbit so as to clear the frame of reference used sometime to measure velocity.

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You need a frame of reference to measure velocity. It doesn't mean anything without a frame. Please try to be more precise--- I have a rock floating through space, and a spaceship, and I want to measure the velocity of the rock relative to the spaceship. You can do this by bouncing light off it and measuring the Doppler shift. – Ron Maimon Jun 23 '12 at 19:50
@RonMaimon: Just what I was looking for. Could you post it as an answer instead of comment please? – Everyone Jun 23 '12 at 19:56
I am sure that this is what you already suspected, you just wanted confirmation. I don't think this is the best use of the site--- you should ask things you don't already know the answer to. I don't like the question, but I did as you asked. – Ron Maimon Jun 23 '12 at 20:02
@RonMaimon: I stand rebuked. At the same time, I do wish to present my thoughts behind this question. Space is mostly empty, or so one reads. Given the vast volumes of empty space I found myself wondering about the practicality of using doppler shift to measure velocity – Everyone Jun 24 '12 at 15:46

You need a frame of reference to measure velocity. It doesn't mean anything without a frame. Please try to be more precise--- I have a rock floating through space, and a spaceship, and I want to measure the velocity of the rock relative to the spaceship. You can do this by bouncing light off it and measuring the Doppler shift.

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But is this also used in real life? Since all current satellite not orbiting earth would also have to be tracked on earth. So would the satellite use a laser to determine its relative velocity and distance to known celestial bodies and beam this to earth. Or can they also be tracked from earth? – fibonatic Mar 29 '14 at 13:54

Velocity is still measured the same way wherever you are. It doesn't need to have any connection to Earth's orbit.

We can't measure an absolute velocity anyway - should we measure against a point on Earth? well, it is rotating, and Earth is orbiting the Sun, which is orbiting the centre of the Milky Way, which is ... and so on.

Just use whatever reference point is useful, and measure the relative velocity.

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Sorry for being so thick - could you explain please? – Everyone Jun 23 '12 at 19:48
Why do you think velocity should be connected to Earth? The definition doesn't mention Earth at all- Definition: Velocity is a vector measurement of the rate and direction of motion. – Rory Alsop Jun 23 '12 at 19:49
The definition certainly does not mention Earth. I only included Earth in the question because, for instance, optical markers, radio beacons(GPS/Beidou) may serve to provide a known distance. Such an option may not be available to a satellite outside of Earth Orbit – Everyone Jun 23 '12 at 19:55
Okay - so just measure relative to whatever you are wanting to use as a reference point. If you want it relative to you, you could use a laser doppler measuerment. If you want Beagle's speed relative to a point on Mars, just measure it the same way you would on earth: triangulation – Rory Alsop Jun 23 '12 at 19:57

I'll qualify this by saying it should be a comment to the question so please don't shoot me based on that...

taking the extended commentary of the question into account regarding using doppler shift, you would be measuring relative to the object or emmissions that produced the doppler shifted particles, which is a longer explanation of the accepted answer... without a reference, there is nothing to measure against, and distance (and perforce velocity) become irrelevant.

assuming dispersion of some reference from your spacecraft you could measure any change in velocity from the point of dispersion, but I'm not sure that's what you were after.

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