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tgoolsby2
2011-Mar-28, 03:30 AM
Since speed can only be determined relative to something else, what do spaceship report as their speed?

The escape velocity on the surface of the Earth is approximately 11.2 kilometers per second. So if a ship were trying to break free of Earth's gravity, it would be practical for the ship to report that it's speed is slightly greater than 11.2 km/s. But once the ship has left Earth, it becomes impractical to say the ship is moving at the former speed.

So what should we say about how fast the ship is moving?
Should a ship's speed be in relation to it's origin? How do you define it's origin?
Maybe it should be in relation to the Sun?
Or maybe it should depend on the context of the situation?

How do they do it today?

Jens
2011-Mar-28, 03:40 AM
Or maybe it should depend on the context of the situation?

How do they do it today?

I think the way they do it today is precisely your last suggestion, that it depends on the context of the situation. When the Space Shuttle is launching, they give speed from the ground. But when it is approaching the ISS to dock, obviously they are going to measure their speed vis-a-vis the ISS. I'm sure that when the lunar lander was preparing to land on the moon, they were calculating their speed relative to the surface of the moon.

I think this is probably true generally, but on the earth we have the special situation where we mostly travel from destination to destination in a state where all the destinations are attached together. There are exceptions, though. I'm pretty sure that when planes are flying in formation in an airshow, the planes following the lead plane are concerned about their speed vis-a-vis the leader rather than the ground.

CaptainToonces
2011-Mar-28, 03:40 AM
Rockets don't launch at a speed of 11.2km/s. Maybe that's the speed you would need to fire a cannonball to escape Earth's gravity. A rocket burns fuel to accelerate over time, so it's constantly changing speed.

To measure speed in coordinates you do need an origin for that coordinate system. For space vehicles either the Earth or the Sun could be used or maybe even the Moon or Jupiter if you were in the vicinity of those bodies.

danscope
2011-Mar-28, 04:04 AM
You would measure relative velocity ; relative to your destination , and also record your relative velocity from Earth.
It's all part of rocket science. " So.... this is where Earth is going to be by the time we get back...." Hmmm....

Best regards,
Dan

WayneFrancis
2011-Mar-28, 05:23 AM
depends on the context and who is doing the talking. Basically it would be a dominant body you are travel from or to. Leaving the solar system it would be the solar system, basically the sun. Going to and from the moon then your speed relative to both are about the same. Sometimes you'll see something like the space shuttle's speed given in terms of it travelling with respect to the ground but when the space shuttle is docking that speed is just of news articles...The shuttle is more concerned with its relative speed to the space station which is very slow.

kamaz
2011-Mar-30, 04:58 PM
Since speed can only be determined relative to something else, what do spaceship report as their speed?

It is possible to measure velocity of the spaceship relative to a ground station, by measuring Doppler shift of the ship's transmission. It is possible to measure relative velocity of two spaceships using the same mechanism.

Or maybe it should depend on the context of the situation?

Indeed. Assume that you want to rendezvous with the ISS. You want to get near the ISS and bring your speed w.r.t. to the ISS to zero, so you don't crash into it. So the ISS is the only reference you are really interested in.

But, this maneuver is equivalent to matching your ground speed with the ground speed of the ISS. So, if for some weird reason, you couldn't measure your relative speed w.r.t. to ISS, but had reliable measure of your ground speed and ISS's ground speed, then you could use that.

It is also worth remembering that velocity is a vector. If you have one spacecraft in a circular 400km equatorial orbit, and another one in circular 400km polar orbit they both have the same orbital speed of 7.67 km/s (w.r.t. center of the Earth).
But velocity of one spacecraft w.r.t. to another is 7.67*sqrt(2)=10.81 km/s. Also, if you use a ground station to measure their velocities, then you will get different readings, because the ground station rotates together with Earth.

If you want to understand how spacecrafts actually operate, then I recommend you download the free Orbiter simulator and fly the ISS approach scenario yourself using this tutorial: http://smithplanet.com/stuff/orbiter/orbitaloperations.htm A few evenings spent playing this will give you more understanding then weeks of studying theory.

danscope
2011-Mar-30, 05:23 PM
Hi tgoolsby2
The true answer to your inquiry lies with inertial navigation systems , how they work and why we employ them. You will find them on Submarines , some ships (I served on Observation Island, which had the initial prototype Sins (ship's inertial navigation system ), the guidance systems for Polaris missiles , and on and on through systems like STS .
I hope this guides you in your search.
Best regards,
Dan

astromark
2011-Mar-30, 08:10 PM
Just touched on but previously discussed is the 'speed' and 'velocity' They are not the same thing.

My aircraft has a air speed indicator. That it measures the velocity of my craft through the air.

Not its across the ground speed.

As Danscope has said. Global Positioning Systems triangulate your position in relation to multiple points.

Everything is in motion and to measure velocity you must have a reference point.

That any and all of your reference points are in motion is where some delicate mathematics is required.

A docking space craft measures against its target. while both are in motion regarding the earth.

The solar system, the Galaxy.

So 'my' speed through the air is not my actual velocity. I hope not to have confused you.

Jeff Root
2011-Mar-30, 10:58 PM
It could also be productive to search on the terms "delta v"
(meaning "change in velocity" or "velocity difference"),
"range and range rate", and "integrating accelerometer".

-- Jeff, in Minneapolis

tgoolsby2
2011-Mar-31, 10:30 PM
Why don't we make a 3d star map of the galaxy, or maybe just our immediate vicinity,? If we find the center of gravity of all the stars in that group, then we can find our velocity (our speed and direction) relative to the center of mass of that group. Is this better than doing it the way it is today?

Hornblower
2011-Apr-01, 01:30 AM
Why don't we make a 3d star map of the galaxy, or maybe just our immediate vicinity,? If we find the center of gravity of all the stars in that group, then we can find our velocity (our speed and direction) relative to the center of mass of that group. Is this better than doing it the way it is today?

Why should it be better? What is wrong with what we do now, which is to reckon a spacecraft's velocity relative to whatever body is appropriate for the task at hand?

cjameshuff
2011-Apr-01, 01:39 AM
Why don't we make a 3d star map of the galaxy, or maybe just our immediate vicinity,? If we find the center of gravity of all the stars in that group, then we can find our velocity (our speed and direction) relative to the center of mass of that group. Is this better than doing it the way it is today?

It still doesn't mean anything more than our velocity relative to the center of mass of an arbitrary set of stars.

There simply isn't any one universal reference frame that's preferable to all others. This is even the case for Galilean relativity, with plain old Newtonian mechanics. What's of interest in navigating a spacecraft is its velocity with respect to a number of different bodies that affect its trajectory...you just can't give a single number for "how fast it's going". Even if you define a standard reference frame, the resulting number just isn't of any particular use.

Van Rijn
2011-Apr-01, 02:13 AM
Why don't we make a 3d star map of the galaxy, or maybe just our immediate vicinity,? If we find the center of gravity of all the stars in that group, then we can find our velocity (our speed and direction) relative to the center of mass of that group. Is this better than doing it the way it is today?

When it comes to interplanetary travel, the galaxy isn't terribly relevant. The most relevant thing outside the solar system might be guide stars used in navigation, just because they're far enough away to be considered "fixed" for the distances and timescales of interplanetary travel. However, they aren't required for navigation, just convenient.

ToSeek
2011-Apr-01, 09:28 PM
I've worked on a lot of spacecraft and none to my knowledge have any sensor or derived reading remotely resembling a speedometer. They keep track of their attitude and rotation rate, primarily. Many of the more recent ones do have their own GPS receivers on board, which could be used to determine speed. But obviously that only works in low Earth orbit.

Grashtel
2011-Apr-02, 05:25 PM
Why don't we make a 3d star map of the galaxy, or maybe just our immediate vicinity,? If we find the center of gravity of all the stars in that group, then we can find our velocity (our speed and direction) relative to the center of mass of that group. Is this better than doing it the way it is today?
In addition to the points that the other people have brought up in response to this its also worth bearing in mind that even the local stars aren't moving at the same velocity relative to us or the galaxy as a whole. For instance Barnard's star is moving at upwards of a hundred kilometers a second relative to us.

publiusr
2011-Apr-02, 06:24 PM
pulsar positions and identification via spin rate would be a good deep space Stardate/GPS. Just put some light year markers and Burma shave signs by them.