How do you measure velocity in the void of space?

Question

Speed is measured relative to a stationary object. Velocity is determined by calculating the time it takes to travel a known distance from a stationary object. A cars speedometer measures speed relative to the stationary ground it is traveling on. An airplane measures speed by the same thing, by how long it takes to travel between points of a known distance on the ground.

So when it is said that a spacecraft is hurtling at 46,000 miles per hour beyond the orbit of Pluto towards the next star, what is that speed being compared against? In that part of space, there is no stationary object to calculate velocity. Is it still being compared to a stationary object on the surface of the Earth? At that distance, the Earth is traveling through space in orbit around the sun, which is traveling in orbit around the galaxy, which is traveling through space around who knows what.

So, what is the spacecraft traveling at 46,000 miles per hour from or towards that can be measured?

Answer 1

kind of a weird question, but actually when you are measuring relative to a "stationary" object such as the earth, you arent really....right now i am stationary, but am still moving fast enough to move completely around the earth in one day, and even if i measured my speed relative to the axis of the earth, even the axis is rotating around the sun so that it completes one rotation every year....that is really pretty damn fast if you think about it....truthfully they can figure the distance between the earth and the planet (pluto in this case) and assume the velocity constant so they measure the time it takes to get there....it is pretty accurate to consider the velocity constant because most of the time the velocity in outer space is due to inertia with a few adjustments to counteract the pull of planets...
EDIT: you can't use your so-called light doppler effect if the object is not emitting light...that was used to determine that solar systems were moving away from us....pretty cool concept tho

Answer 2

In the case of the spacecraft hurtling at 46,000 miles per hour beyond the orbit of Pluto, that is speed is measured against the Sun.

You are correct that speed is relative. For example, a geostationary satellite has speed 0 as measured from the ground. But it is really orbiting Earth at almost 7,000 miles per hour as measured against a hypothetical non-rotating Earth. And since Earth is orbiting the Sun and the Sun is orbiting the galaxy core, it can be really confusing. Normally NASA reports speeds with respect to the Earth or the Sun, which ever makes more sense in each particular case. For space craft orbiting other planets, like Mars or the Moon, they will usually report speeds with respect to those bodies.

Answer 3

Please note that Speed and Velocity differ:

Speed is the measure of distance per time,
velocity is a Vector, containing two scalar quantities Speed and Direction.

Technically there is no 'void of space' becuse the universe is full of stuff, unless an object is in a Relativistic Bubble, that is it is travelling at the speed of light because it has -
1) zero mass
2) infinite energy
3)that it is sufficiently far enough from the next closest object in the universt that the very space that it is in is expanding at the speed of light.
These conform with the relativistic E=M*C^2 and the universal expansion theory speed=constant*distance.

Things that have zero mass generally don't sit still very long.
Things of infinite mass are theoretically as hard to handle as they are hard to find, and the same with objects outide our "Hubble Bubble".

Another nifty relativistic Idea is that everywhere is the 'center of the universe' (I've actually been there). Anywhere can be a reference point. Generally, when an object's speed is measured in space, it's referenced against the instantaneous rate of speed that it separates or hurls toward the Earth, or another Source or Target - whichever is most convenient for subsequent analysis.

When a comet hits Jupitor, the rate of speed is measured between Jupitor and the comet, and collision energy can be calculated. The rate of approach or separation between the comet and any other object that the comet is not involved in has no interest except thoeretical such as "if it hit earth at that speed".


A rocket zooming past pluto might have a solar reference, as it is the most influential and stable body of reference.
Since the Earth revolves around the sun, our average velocity is zero with respect to the sun or the velocity of the sun with respect to another reference that is set as a still point. (A full circle of instantaneous vectors points to zero and therefore earth's average velocity, relative to the sun is zero, while seasonal instantaneous speed can be between
+(2*PI*93*10^6 mi/year) and -(2*PI*93*10^6 mi/year) (about 66,705 mi/hr - earth's instantaneous Speed))

The actual measure in speed of a satelite may be a measure of red shift in communication signal, which is relative to earth and it's seasonal position/velocity. Since a satelite beyond Pluto is thought of a leaving the Solar System, earth's velocity component paralell to the satelite should be subtracted to obtain a solar reference velocity. Approximatly (66,705*cos(dayOfYear/356.25-theta )) where theta is the angle
between the Earth at New Year's Day and the Satelite.
The rocket's path is not likely to be rectilinear, but a parabola or possibly a hyperbola, but may appear to be a straight path if it is sufficiently fast and far enough from large gravitational forces. .

Over a sufficiently long period time period, all things have an average speed of zero or nearly zero - either they return to the same spot periodically (exactly zero), though the sameness of that spot may be questionable in Zeno's world (Zeno's paradox), or Things will stop moving at all - a drop in kinetic energy or relativistic bubbles.

If this answers questions, good
if this raises more questions better.
if this does nothing at all - perhaps best.

~S~

Answer 4

There are multiple reference points that are nearby, and accounting for their change in position over time, the key here is referred as "paralaxis" in that as an object moves through time-space the different in "perspective" relative to the assumed positions of the other objects, creates a reference of "position" and therefore speed...

This can also be substantiated nia radio wave signal reception to "some extent" and I say that because of Einstein's theorys of relativity... the watches begin to move more slowly as velocity increases. Paralaxis is the most acurate way that I know of to determine realtive velocity... I'm sure those a NASA do things in a much more sophisticated way.. Maybe you should ask a cosmologist or astro-physicist.

Answer 5

It is being compared to earth ground speed. The instruments there do not change when you go into space. They still measure in that way. Theoretically, they are travelling differently in air miles and "space" miles, but no apparatus (As far as I know) measures that. Since everything has the same acceleration in space, unless you go at light speed when changes occur, you can calculate mph like that.

Answer 6

Using triangulation you can measure the distance between 2 points from a single point in space in an instant where it can be said to be stationary, then determine the time it takes to move between those points and you have yer speed, + direction and you have velocity.

Answer 7

By the displacement of the lines of the light spectrum to the red while observing a bright reference point, The higher the displacement to the red, the higher the speed. Something like the Doppler effect.