you see a lot of fast space crafts on movies and games, but really,how the hell would one gather its own speed in deep space ?

off course they have smartphones and an app for that!

If you know your ship's mass, how much force the engines are generating, and for how long, then you can determine your speed. Plus there's also known distance divided by time. I'm sure there are other ways, such as the change in the red shift or blue shift of distant objects compared to the Doppler effect when at rest.

DEEP SPACE ! like star trek they traveled long distances without any object in close distance......and measuring your speed by how long the engines are generatin is kinda silly ? how long would be too long ?

I'm not sure what you are asking, but if you are trying to go from point A to point B, the only distance that really matters is your distance to point B, and you could probably measure that as it changed, just as we can measure it now. All that really matters is how fast you're traveling relative to the other objects, and we can see very far away now, so it shouldn't be a problem to measure the relative speed.

Once we understood the Milky Way well enough, we could figure out where we were based on the positions of the stars, know where a place that we want to get to would be given out travel distance, and then set a course for that set of coordinates at a given speed - relative to other objects and their movement around the galaxy as it spins - and get there at the right space and time.

If you just wanted to measure speed through the galaxy, it may not really make sense to do that since we're constantly moving anyway, and don't have a reference for an absolutely stationary object. It's all about acceleration and relative movement.

Distance between stars on your star chart app.

that actually makes sense.......y-you NERD.

But i'm still not satisfied...can an object actually be stationay in space ? like when kirk say full stop ? there is always momentum, no ?

I think the only thing that you might be able to measure against would be the speed of light, since it's a constant. But in most other cases, it really doesn't matter if it's not relative, because if you were in an empty universe traveling at a constant rate at a million kilometers per hour, or two million, or ten, you would not know the difference. No forces would be acting on you, so there would be no way of knowing how fast you were moving, in what direction, or even If you were moving.

Not moving and moving at a constant rate are the same thing, unless you factor in some other object to measure against, and if there were more than one other object, and if they're moving relative to each other, your measurements will be different when compared to each.

Thank you Arthonon

What velocities, distances, time periods, and locations are we talking about?
Within the solar system you use star trackers to maintain your frame of reference. Then you measure the angle towards planets, and given that the ephemerides are known and tabulated you can triangulate your position; from multiple triangulations comes a course, and a velocity.

In deep space with no planets around there are two options - doppler shift movement of your target star's spectrum if you're fast enough and if you have a good enough spectral analyser. Or you are so slow in relation to your destination that you can't use that, but then you probably don't need an exact measurement of your velocity anyway.

If there's a nearby object with which you want to dock, use radar or some equivalent.

Cant ya use the Space Needle ?

That would be perfectly ineffectual, as the whole point of
relativity is that c is measured as the same no matter who
does the measurement and how they are moving.

If you want to know your position and relative motion well
in a space environment, you need to have a really good
computer model of that environment, with the gravitation of
all nearby bodies of any significant size well charted,
and an accurate history of the accelerations applied to
your craft. The computer system will then crunch the numbers
and determine how your craft will move through that environment.
The computation will then have to be tweaked at intervals
using observations of surrounding objects, including timing
of communications if you're close enough to anything for that
to be practical, in order to compensate for the inevitable
small errors, mostly in the acceleration magnitude and duration,
but with the large distances and velocities involved, these errors
will accumulate otherwise. When navigation through the solar
system becomes commonplace ( ), we can expect to
see navigation beacons placed around the system to assist craft
in determining their location, just as systems on the oceans of
earth, from lighthouses to LORAN to GPS developed to aid ships.

I was thinking more measuring your speed as a percentage of the speed of light, since things start to change as you approach it.

Wut?
The speed of light stays the same, but the frequency changes, known as Blue- and Redshift. The Doppler effect. So if you know the spectral signature of a target star (or any other light-emitting source with a complex light emission and absorption sturcture) you can compare the frequency shift ans as such know with a precision of just a few kilometer per second how fast you're approaching it (or flying away).


No. It's sufficient to know the relative position of stars that are near your intended flight path. Then you can measure their parallax shift while you're moving, and from that triangulate your position. Sure, the stars aren't fixed in space, but assuming that your travel velocity is fast enough for interstellar travel the parallax shift will become large enough to be easily detectable.

Having a record of your ship's acceleration vectors and durations certainly helps but cannot account for the influence of strong gravity pulls as it's all experienced as free fall. It's like knowing the wind but not the current on an ocean. You can apply dead reckoning and it's reasonably accurate for short to medium distances but now and then you just need a triangulation fix to work out the accumulated estimation error.

All that counts is where you are and where you're going to.

"Full stop" in Star Trek is nothing more than naval-era terminology applied for dramatic effect that is easily understood by mass audiences.




The Jedi Master

Yes, I guess that's true - Doppler can be used these days to detect
speed down to under 1m/s, vastly better than when I was last paying
attention to such things - doing measurements by hand with a photometer
output from a spectroscope, determining full width half height centre
frequency, trying to detect changes of 1/10th of an Angstrom.


And have a library of their spectra as measured from a frame
you've defined as your base for velocity measurements. That
and the library of stellar locations amounts to a "really good
computer model of your environment".

I wasn't thinking about interstellar travel, I was thinking about
motion in space in general, no matter the speed. You really have
to be doing some considerable travelling to get a change in stellar
parallax, and again, you need a great library of extremely accurate
data to check against to detect your motion. But I was thinking not
only of measuring instantaneous motion, but of how we currently know
the velocity of spacecraft - generally, we know pretty well because
we've plotted it all out ahead of time, taking in to account all the
gravitational influences and accelerations we apply from thrusters.
Then we take measurements, mostly of time delay in communication,
but also of the positions of relatively near objects, like planets,
as seen from the spacecraft, to tweak our expectations against the reality.

But none that requires a precise knowledge of all the gravity wells around each star. In fact, we're just talking about a catalog of stars with their coordinates and spectral signature. Sure, over the course of your travels each and every measurement of the locations of known stars their 3D coordinates will become ever more precise so that over time the "really good model" will eventually be a natural consequences of all your navigational activity. But it's less of a prerequisite for space travel but rather a consequence of it.


Ah, but the original question of this thread mentions "deep space". To me that suggests anything beyond interplanetary distances, a situation where you're so deep inside a star's gravity well that all your trajectories are noticeably curved, and where you have a more or less precise knowledge of the planetary bodies which would be the prime references for all triangulation.

In deep space you can't rely on planetary movement to determine your location and velocity. The next best choice are the stars themselves.

It's also a matter of your ability to perform precise measurements. The bigger the error in your triangulation the more distance you need to travel to figure out where you are. But we can already measure the parallax shift of the stars closest to our solar system, out to about 10 light years, with a base of just under 300 million kilometers (2AU), which is well within the order of magnitude of interplanetary travel.


Absolutely, but our methods of space travel are still rather rudimentary. We're operating with a strict observation of fuel and reaction mass limitations (the few operating ion drives are the first exception to that rule, but since their thrust is so tiny they are useful only for multi-year travels into the outer solar system). We simply cannot afford course corrections and make up our course as we fly. That's not "space travel", it's "space coasting" or "interplanetary drifting". In short, it's cheaper to plan everything ahead and to rely on precision navigation than to burn fuel.

A little OT question but space related.

If the stars in a galaxy are all moving at the same rate why is the galaxy spiral shaped?

I understand that the stars nearest the galactic centre travel at the same velocity as stars at the edge of the disc, not dropping off speed with the distance from the centre as first was thought.

I know that mass spinning in space can drag spacetime around with it and most if not all galaxy's have a black hole at the centre, is this the reason why they are spiral shaped, would that dragging be enough?

I had reasoned if the stars all revolved around the galactic centre at the same rate the galaxy would resemble a flat wheel of some sorts, with spokes radiating to the edge.

Thanks,

Mick.

Answer to the original question: The speedometer

I think that you are mistaking velocity with rate measured in degrees per year (or however you want to measure it). Observe:



In Fig. A, the stars are moving around the centre point at the same rate in degrees per increment of time, but with different velocities; in Fig B, the stars are moving at the same velocity, but the one nearest the centre point will move in a full circle far before the one on the outside of the galaxy. Given that the velocities are the same, Fig. B will be the one reflected in reality, and thus the spiral shape for the galaxies that you describe.

For more graphic representations of this, play Kerbal Space Program; also, take a look at The Space Engine.