# Thread: Sci-Fi question on determing speed

1. ## Sci-Fi question on determing speed

In a spaceship that loses all internal electical power and has no working instruments. Could the crew determine the speed of the vessel if it wasn't within a solar system? Oh and obviously no outside observer could contact them. In what way, if any, could they determine there speed or would the correct term be velocity?

2. Possibilities:
1.) The simplest method would be to assume no acceleration or deceleration, and take their last known velocity reading and run with it, so to speak.

2.) Depends on exactly what instruments are and aren't dependent on the ship's power. If they're going at relativistic speeds, a simple spectroscope could give them a good idea of how fast they're going, or they could measure the displacement fo nearby stars. If not...well, things get more complicated.

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I think Romanus is right. There last known velocity when everything shut down would be accurate unless they hit something right?

"If they're going at relativistic speeds, a simple spectroscope could give them a good idea of how fast they're going"

Do you mean by this; measuring and calculating the redshift of known stars if an external view of space is possible?

4. Would this be a kind of Space Sextant we're talking about? Hmmm that might be neat for a Dr.Who episode.

5. in sci fi, when the engines die, the ship generally comes to a complete stop.. so, the velocity would be zero.

in sci fi, when the engines die, the ship generally comes to a complete stop.. so, the velocity would be zero.
Not in real SCI-fi.

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Originally Posted by LotusExcelle
Would this be a kind of Space Sextant we're talking about? Hmmm that might be neat for a Dr.Who episode.
Would not the ability to calculate redshifts be like a space sextant?

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Originally Posted by LotusExcelle
Would this be a kind of Space Sextant we're talking about? Hmmm that might be neat for a Dr.Who episode.
More of a log than a sextant: a sextant measures position; you needed a log (of the sort you heave over the side) to measure velocity.

Grant Hutchison

9. Ah. Yes I'm not up to *speed* on nautical equipment. Speed. Get it?

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Originally Posted by LotusExcelle
Ah. Yes I'm not up to *speed* on nautical equipment. Speed. Get it?
I'm in no position to comment.

Grant Hutchison

11. Just to clarify this would be real scifi - the ship would keep moving and in the scenario I'm thinking of the velocity was not known-known to a computer of course which is now shut down.

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their velocity with respect to what? You cannot determine a velocity without a reference point.

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## Distant Pulsars

First, several awards:

Oxymoron of the week to m1omg for 'real SCI-fi.'

Dry humor of the week to Grant for 'I'm in no position to comment.'

Relativity grasp of the week to What Max for 'their velocity with respect to what?'

More seriously, distant pulsars have been proposed as fixed beacons for navigating. If you know what the apparent pulse rates were when you left, and what they are when you lose power, and their directions, then you should be able to calculate both where you are and how fast you are moving with respect to your departure point.

Regards, John M.

14. ah interesting, thanks

15. All of these methods seem to rely on one of two things being true, however. Someone either has a spectacular memory, or there are hard copies of pulsar data or other stellar information you plan to use on the ship. If you don't know it, you have to look it up someplace.

For the spectacular memory thing, there is the "Pulsar Geek" option. This is like the sports geek around today. Somewhere out there is a person that knows Sammy Sosa's batting average against left handed pitchers, on natural turf, under artificial lights, above 2500 feet in elevation, after a 2 day down time. I know there is. Then you could have the tension build as the rest of the crew doubts this person, who might be wrong after all.

For the hard copy, you would need to have some mention of it, probably being redundant and a waste of mass. This would make for a really good "Told you so" moment.

16. Originally Posted by Hans
In a spaceship that loses all internal electical power and has no working instruments. Could the crew determine the speed of the vessel if it wasn't within a solar system?
It's easy. You lick your finger, stick it out the window, and feel which way the wind is coming from.

17. Photo plates. That's the only thing I can think of (its early, I've had no sleep, etc) that could really do it manually. Maybe with some kind of parallax? I'm really not up to speed on this but maybe a forward and backward looking photoplate then look at red/blue shift. Not sure what the speed scale would look like for something like this.

Would that work?

18. I don't think so. In order to figure parallax, you have to have a known distance. Since the distance from the first measurement to the last is the main unknown, we would need the distance to the star that seems to move more than the background ones. That would require knowing what that star is, and the position of the ship.

(Possible double post)

19. Originally Posted by Jens
It's easy. You lick your finger, stick it out the window, and feel which way the wind is coming from.
Not that far off really.

The best answer depends on the scale of speed that the ship is moving, and the extent to which "all instruments" are unavailable.

If, for some reason, the speed is non-relativistic, but still much higher than the speeds of our current space probes, you could use a spectrograph if you had it available. You could measure the pulses of pulsars if you had the right instrumentation. But, the OP said no instruments... That pulsar thing would be difficult even for McGuyver without instruments.

But you might be able to make a reasonable estimate of the speed relative to the interstellar media simply by noting the temperature of the outer hull on the leading side, vs, trailing side of the spacecraft.

20. This thread calls to mind ancient sailing technology. Even before sextants. Almost a Norse level tech but in space. Fascinating and leaves a lot to the imagination. Someone want to write a short stroy based on this? I think it would be neat.

21. Originally Posted by LotusExcelle
Someone want to write a short stroy based on this? I think it would be neat.
I'm sort of going on the assumption that Hans is, unless he says otherwise.

22. Originally Posted by antoniseb
But you might be able to make a reasonable estimate of the speed relative to the interstellar media simply by noting the temperature of the outer hull on the leading side, vs, trailing side of the spacecraft.
How would that work?

Also, could the log thing work in some way? If I have a bottle of compressed air that vents at a known (or that someone with math skills can figure out) could I fire a projectile straight behind me, then measure the diff...

Wait, no. It would still give me the speed relative to the ship and I would need to see it from an outside frame.

I think I leave that thought process there though, just in case others had a similar thought, and for the fact, that I really did figure it out as I typed it.

23. Originally Posted by Tog_
How would that work?...
There are a few atoms (or protons and electrons) in every cubic cm of space between the stars in the galaxy. These things have an average velocity, and an average deviation away from the average velocity relative to the local stars. If the spacecraft is moving through this medium at a speed large compared to the average thermal speed of the particles, then the particles will be hitting the leading side of the spacecraft, but only rarely the trailing side. The energy absorbed by the collisions will mostly be used to heat the spacecraft's hull. Note, you might need to put plates outside the spacecraft for this purpose so that the thermal conduction from the spacecraft doesn't overwhelm the temperature comparison.

24. Originally Posted by antoniseb
The energy absorbed by the collisions will mostly be used to heat the spacecraft's hull...
I figured it was some friction based thing, but is the degree of heating something that would be known already?

I can see where it might be something that ship designers might need to take into account once travel like this is possible, but is the amount of heat transfer something that we could figure out without actually being out there to get the values?

25. Originally Posted by Tog_
I figured it was some friction based thing, but is the degree of heating something that would be known already?

I can see where it might be something that ship designers might need to take into account once travel like this is possible, but is the amount of heat transfer something that we could figure out without actually being out there to get the values?
It would just depend on how many particles per cc there are and their relative velocity compared to your vessel. It is easy to calculate the amount of power being transferred by these particles, and also the rate of energy dissipation for a given temperature of material. The two must be equal.

Some of the values will be estimates.

26. I see. Thanks.

On the drive home, I think I figured out a way to get both speed and position (well, roughly) using stuff I actually have in the house right now.

It does need a few assumptions, though.

1. The ship needs to have both a "front" and a fixed horizontal. No spinning like a foot ball or bullet.
2. Distances to some very well known objects would have to be known in much better detail than we have now. In the book I have, distances are limited to 2 significant digits.
3. Objects like the Trapezium and the Lagoon Nebula will be recogizable from any angle. I really think the Trapezium will stand out well enough that if it can be seen, it can be ID'd.

Here's my proposal. (Accuracy of measurements enhanced because, we're talking about interstellar travel.)

We can find the X,Y,Z positions of anything we can see from Earth, Polaris is the Y axis, so we just need to find an X and Z. First Point of Aries is where RA starts, so that's as good a spot as any from Earth. Once we have the X,Y,Z plotted, we can find the bearing and distances to any other item we have plotted. If we move the Earth 1 LY along the X axis, the angles to everything (except the far ends of the X axis) will change a little bit. By measuring the amount of that change, the new position should be easy to calculate. The interval between those calculations should give us the distance traveled over time.

Day 1 take angular measure of both the trapezium and a star in M8. There should only be one position in the universe where those objects will appear at those same positions.

Day 2 take a second reading and compare them. Math it out and the result should give a rough idea of both speed and position. As long as there is a manual way to measure the angles accurately, a way to get the trig functions accurately enough, and a simple star atlas with the accurate coords for the two objects used, It should be pretty easy.

No electronics at all means my calculator would be useless, but I did have a spreadsheet that I used to plot star positions on a 3D CAD file. By changing where, 0,0,0 was, I was able to plot the sky from other stars. With the charts and tables, I think I could do the math by hand, and that means it's really pretty basic. For measuring the angles, I'd use the digital setting circles on my goto scope in landscape mode, except, no electronics. That sort of hoses me. Now, if the ship had surveying equipment...

Would this work?

27. Originally Posted by Tog_
... Now, if the ship had surveying equipment...

Would this work?
Depending on speed and scale, it could work. Suppose you were making a one hundred year journey to Alpha Centauri when you lost your 'instruments'. The method you describe would be a little difficult to do, because the measurements to those specific places wouldn't yield numbers larger than the error in any hand & eye type instruments, especially over the course of a day, and because the Trapesium and Lagoon nebula are distributed light sources... but the principle you describe could work if you look at a few near by stars, and how they are moving related to the background.

Measuring the angle between Alpha Centauri and Proxima Centauri would tell you distance in an absolute sense. Likewise measuring the angular dispacement to Barnard's star, and Sirius, and Vega, would give you plenty of error checking on those figures, and you should be able to make good approximations as to speed and location every few weeks using relatively primative instruments.

28. I just picked those as examples of things that should be easy to identify since there was no mention of scale in the OP. I understand that closer is better for something like this. I just wasn't sure if there was something that I wasn't considering. Math really isn't my friend most of the time.

29. Originally Posted by Tog_
...Math really isn't my friend most of the time.
Your solution was a good one and practical... Now, if the journey were going much faster and further on some generation ship where they had lost their knowledge of the current local stars... Well, they'd be in real trouble.

30. Assuming the travelers had knowledge of any local very close binaries and could observe their relative motions, then a velocity relative to them would be somewhat easy to calculate. [This is borrowing from Galileo's idea to determine longitude using the positions of Jupiter's moons, plus adding a dash of Romer into it. ]

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