How do you define speed in space?

cowman

cowman

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I'm very very slowly writing a sci-fi story that may some day make it to the status of a real book. Maybe maybe not, we'll see if my lazy butt can keep cranking out chapters. Anyway little questions like this keep coming up for me and I think between pilots and people with aerospace knowledge on here someone might have an answer.

So here on earth when we define speed it's usually ground speed right? How much distance we cover across the earth per some unit of time. We pilots also have airspeed- speed we're moving through the air and of course there's calibrated, indicated, and true airspeeds there too..

Ok but in space there's not a ground and there's no air. If I have my basic astronomy right, the earth is moving around the sun, the sun and solar system are moving around the galaxy, the galaxy is moving around in the universe, and the universe it's self is expanding. So... how do you know how fast you're going in space? There's no ground speed, there's no air or any other medium to go through. Your speed would have to be relative to something I think, right? What would you make it relative to? Maybe within our solar system you use the earth or the sun as a reference point but then when you go on an interstellar trip then what? What if you're too far from the sun to use it as your reference point?

Am I overthinking this? Seems like something people in the space program would have to have thought of....
 
Delta V, honestly. Change in velocity in three dimensions from a reference point of your choosing or starting velocities in all three.
 
I’m thinking you have 2 choices
1. Relative to destination. I.e. how long will it take to get there. This is probably the most useful and practical choice
2. Relative to speed of light. A theoretical use as a craft performance number.
 
Adding to #1. If the destination is also moving at speed significant enough compared to yours, you need to find a common reference point for your and its position change. Could be center of the system, or galaxy. Or even theoretical Big Bang center
 
Yeah I think for navigation purposes you'd have to be using something relative to the destination point or origin point.

But say you had private space pilots... and these guys were all hanging out in the space FBO and got into an argument over who's space plane is the fastest. Short of a race you're gonna need an agreed measurement kind of like true airspeed to argue that point.
 
If your vehicle is traveling at relativistic speeds (e.g., high sub-light speeds), you'll measure the speed to a universal inertial frame of reference. Because, relativity.

If you aren't going to be traveling at a significant fraction of the speed of light, then speed is referenced to either your point of origin or your destination. Generally, you'd assume that you start at zero speed at your place or origin.

Ron Wanttaja
 
Wouldn't you be able to measure acceleration relative to yourself and use that as an imaginary point in space from which to reference velocity?
 
Warp speed.

Seriously though, I’d try to keep it simple and use some fraction of the speed of light. Remember that light-years is a measure of distance, not speed.
 
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To get anywhere in space that's not (relatively) next door, you are going to have to travel at a high percent of the speed of light (C). The Star Carrier books get into the minutiae of that. Some of the Honor Harrington books, too. (Not that I've read many :) )
 
Crashnburn said:
To get anywhere in space that's not (relatively) next door, you are going to have to travel at a high percent of the speed of light (C). The Star Carrier books get into the minutiae of that. Some of the Honor Harrington books, too. (Not that I've read many :) )
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I may have read the entire Honor Harrington series plus most of the spinoffs... and a whole host of other sci-fi that deals with the whole FTL problem in varying ways.

I swear David Weber must just either really enjoy math or have a friend who is into physics... the level of detail he goes through for his fictional starship and missile performance numbers is mind blowing. I've kind of leaned towards a cop-out of just having my characters talk about time to destination mostly and not giving out enough numbers that you can check any of my math. Although I did build a spreadsheet of how long it would take to accelerate to different speeds at different G-forces and did some figuring of how long it would take to go from known points at said speeds. Just so everything comes out reasonable and we don't take a day of story time to go a distance that should take 6mo...

I was just thinking about this randomly and the various ways to measure airplane speed in my head and it hit me that any speed in space would have to be relative to something else if you wanted any accuracy. Made me wonder if there was already a "right" answer or it was like a lot of other stuff in sci-fi where we speculate and build a world around it.
 
"Speed" would be important when in the vicinity of an object and exiting / entering orbits / atmosphere. Or, as a fraction of light if you are getting that fast. Otherwise it's about orbits that get you from here to there and the actual speed is not a significant thing - you aren't going to have a speedometer somewhere on the panel.
 
In space travel, everything is about delta-v. If, say, you're in earth orbit and want to get to Mars, you have to accelerate to get out of orbit, head toward Mars, and then speed up (or slow down, it's the same thing) to match Mars' orbital speed. Then there's the huge amount of energy required to reach escape velocity just to get to orbit from the surface...
 
Meters per second, or any other suitable until for velocity. The OP is asking the wrong question. It isn't how you measure velocity. That's easy, you just have to select your units. What's hard is what you measure against.
 
Take a fix off four far off objects. Determine what the fix from those objects would be at your destination. Then calculate the distance and the time you want to be there and it will give you the speed you need to travel.
 
drjcustis said:
Says you!

I know a dude that made the Kessel run in 12 parsecs.


Sent from my iPad using Tapatalk
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Right, he did, he bent space to make the run shorter. Hand over your nerd card, dummy! :D
 
Whatever worked for NASA to correctly arrive at, orbit, land on the moon and then return.
 
nevermind the math; just give the readout.
It’s right there on the panel:

6912974136_14449ce3f3_h.jpg
 
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wanttaja said:
If your vehicle is traveling at relativistic speeds (e.g., high sub-light speeds), you'll measure the speed to a universal inertial frame of reference. Because, relativity.
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I thought relativity tells us there isn’t a “universal inertial frame of reference”.
 
FastEddieB said:
I thought relativity tells us there isn’t a “universal inertial frame of reference”.
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Yes, this is correct. You can pick *anything* as a reference to measure your speed against, there is no single "correct" choice. No place is universally "at rest".
So yeah, if you're traveling locally around the Solar System, it might make sense to pick the Earth or the Sun or your destination planet, depending.
If you're traveling to other stars, it's less clear what to pick, or if picking something is even useful or meaningful at all.

If you're moving at a constant velocity, you can't feel it, and physics says there is no instrumentation possible purely inside your craft that will tell you that speed, without referring to a choice of spot *outside* the craft. This is called being in an "inertial reference frame". So space people generally talk more about accelerations (changes in speed), which can be felt by the occupants -- "delta-v" will make an astronaut spill her coffee (or die).

There is no "center of the Big Bang" because every point was once the center... but you *could* measure your speed relative to the Cosmic Microwave Background if you were really really far out there.

Relationships between time, space, and velocity get more complicated with relativity. If you travel at any appreciable fraction of the speed of light, there's time dilation and length contraction and those sorts of things...

Edit: space isn't a perfect vacuum (solar wind, interstellar medium, etc.), so you could measure your speed relative to those things, to get a kind of space version of "airspeed".
 
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I think if you are writing a science FICTION mover you have the unique opportunity to create your own unit of measure.
 
As a further aside, it’s always puzzled me when the “size” of the universe is discussed.

For instance, “At 10 to the minus 125 seconds* after the Big Bang, the universe was roughly the size of a basketball, the temperature was...”

Easy to picture someone “standing” just “outside” the expanding universe and comparing it to a basketball. But that’s absurd, because there was no “outside”, and any conceivable basketball had to be contained in that expanding universe.

I guess one can express the “size” of the universe by the amount of time it would take light to travel from one “side” to the other. But the whole concept of comparing it to something seems “off” - the universe is always exactly the size of the universe.


*Made up number, but you get the idea.
 
SoonerAviator said:
Warp speed.

Seriously though, I’d try to keep it simple and use some fraction of the speed of light. Remember that light-years is a measure of distance, not speed.
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if man ever travels more than the speed of light it would be interesting to know if they called those speeds WARP.
 
cowman said:
Your speed would have to be relative to something I think, right?
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FWIW: If you want to keep the science in your SF, find something here: https://www.space.com/36273-theory-special-relativity.html If you want to expand the fiction side, when I wrote SF short stories years ago, I would take the english words like speed, miles per hour, etc. and translate them into other languages to see if it produced a unique variant for the text.
 
cowman said:
I'm very very slowly writing a sci-fi story that may some day make it to the status of a real book. Maybe maybe not, we'll see if my lazy butt can keep cranking out chapters. Anyway little questions like this keep coming up for me and I think between pilots and people with aerospace knowledge on here someone might have an answer.

So here on earth when we define speed it's usually ground speed right? How much distance we cover across the earth per some unit of time. We pilots also have airspeed- speed we're moving through the air and of course there's calibrated, indicated, and true airspeeds there too..

Ok but in space there's not a ground and there's no air. If I have my basic astronomy right, the earth is moving around the sun, the sun and solar system are moving around the galaxy, the galaxy is moving around in the universe, and the universe it's self is expanding. So... how do you know how fast you're going in space? There's no ground speed, there's no air or any other medium to go through. Your speed would have to be relative to something I think, right? What would you make it relative to? Maybe within our solar system you use the earth or the sun as a reference point but then when you go on an interstellar trip then what? What if you're too far from the sun to use it as your reference point?

Am I overthinking this? Seems like something people in the space program would have to have thought of....
Click to expand...

I don't have an answer to your question, but are you planning on self publishing, or are you going to try to get trade published?
 
Clip4 said:
if man ever travels more than the speed of light it would be interesting to know if they called those speeds WARP.
Click to expand...

One would hope so, just to appease the sci-fi crowd, lol. Just make sure to set phasers to "stun".
 
cowman said:
Your speed would have to be relative to something I think, right? What would you make it relative to?
Click to expand...
I think you're over thinking it. The universe is expanding and everything in it is moving but a stretched out mile long piece of bailing wire is still a mile long even if its stretched out in the vacuum of space. And your space cruiser is going to have to fly past a whole bunch of stretched out mile long pieces of bailing wire if it wants to get anywhere. Miles per hour, or probably miles per second is probably all you need. Doesn't have to be relative to anything except the imaginary atoms all lined up end to end that are flying past the windshield or rather the atoms that would be flying past the windshield if the vacuum of space was filled with atoms all lined up end to end.
 
FastEddieB said:
As a further aside, it’s always puzzled me when the “size” of the universe is discussed.

For instance, “At 10 to the minus 125 seconds* after the Big Bang, the universe was roughly the size of a basketball, the temperature was...”

Easy to picture someone “standing” just “outside” the expanding universe and comparing it to a basketball. But that’s absurd, because there was no “outside”, and any conceivable basketball had to be contained in that expanding universe.

I guess one can express the “size” of the universe by the amount of time it would take light to travel from one “side” to the other. But the whole concept of comparing it to something seems “off” - the universe is always exactly the size of the universe.


*Made up number, but you get the idea.
Click to expand...
Bear in mind that when humans say the universe, we merely mean our universe.
 
Just as in aviation, the means of measuring speed will depend upon your purpose. In aviation, indicated airspeed matters for things like Va, Vne, Vs, etc. True airspeed matters for calculating ground speed, which matters for determining time en route and fuel requirements.

In space travel, you might need to determine speed relative to your original position for things like RF comms delays and antenna pointing. You would need to determine speed relative to a destination star or planet for orbit insertion. If docking with a space station, the speed relative to the station is important. And so forth.
 
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