Questions about orbits and space

[Hyomoto]

All this KSP got me wondering, is it possible to stop in space? Is there a place in the Kerbol system where you could theoretically reduce your speed to 0? I never really thought about it before, but if everything is in the SOI of another object, technically you can never be at rest in space.

Secondly, when you are doing a transfer orbit, should you enter the SOI from the outside, or the inside of the orbit? Which is more efficient?

[Callmedave]

Im sure someone will have a more technical answer, but when you mention speed, this is relative to an object, either approaching it or heading away from it. lets say you are in a orbit over kerbin of exactly 100Km, you are neither getting nearer or further away from it so your speed in relation to your distance from kerbin is zero, but you are still moving across the surface, a higher altitude (from memory 2,800 km?) will put you in a stationary position above kerbin, but you are still moving around kerbin just at the same speed, so that is relivevly zero. but you are still orbiting the sun.

I would say that is is possible to be travelling at zero, but it depends what mass you are measuring from.

Hope that helps, anyone please feel free to correct me if im wrong.

[Luis]

Einstein's great insight was the realisation that there is no fixed origin in the universe. Everything is moving relative to something else. You can be stationary relative to a planet or a star or a galaxy but you will be moving relative to other planets, stars and galaxies and there is no way to say that one frame of reference is more correct than another. Space itself is expanding, like a balloon being inflated. If you draw dots on the surface of the balloon to represent stars, they will move away from each other as the balloon expands. There is no point on the balloon that is stationary, because the whole thing is expanding.

In KSP, it is perfectly possible to reduce your speed to zero in your current frame of reference. So you can have a surface velocity of zero, or an orbital velocity of zero. But if you switch the display to show another frame of reference, your velocity won't be zero any more. You haven't actually accelerated or decelerated; you've just changed the reference point.

[RoboRay]

All this KSP got me wondering, is it possible to stop in space? Is there a place in the Kerbol system where you could theoretically reduce your speed to 0? I never really thought about it before, but if everything is in the SOI of another object, technically you can never be at rest in space.

No. It's not possible to "stop" on a planet, either. You can pick an arbitrary object and stop in respect to it by coming in contact with it, but that's the best you can do.

Even out in deep space, you're orbiting something. If you match velocities with a nearby craft, they won't stay matched. They'll gradually drift apart due to their slightly different orbits, unless they dock together.

Secondly, when you are doing a transfer orbit, should you enter the SOI from the outside, or the inside of the orbit? Which is more efficient?

It depends on if you heading to an outer or inner body. You generally want to be slightly on the same side of the body (outer or inner) as your departure planet passes it on.

[Hyomoto]

So, in space it is impossible to 'not be moving'. I do realize we can be not moving in relation to something else, I'm just wondering if that's the best that can be done. I take it, what you are saying is because there is no specific reference in the galaxy the idea of 'standstill' is completely arbitrary WITHOUT comparing it to something else as essentially, speed is just that. Movement of one object relative to another.

The follow up of course, being that KSP is not an accurately modeled system, is it possible to 'stop' somewhere in the Kerbol system? Has anyone tried moving exceptionally far from Kerbol and then coming to a stop? I'm fairly sure Kerbol isn't moving at all.

Edited May 21, 2013 by Hyomoto

[Pursuedtank]

Technically it is possible but it would, again, be stationary in relation to something, and not something else. It would be stationary in relation to Kerbol orbit but not in relation to, say, Kerbin which might e moving away from you and so you would not be stationary in relation to the planet.

[Vanamonde]

I think there are two stationary points in the game, but you can't reach them. The north and south poles of Kerbol should be rotating but not going anywhere. Since everything else moves around the sun and there are no external reference points outside the Kerbol system, all motions are relative to the sun, and it sits still with regard to itself. But of course, you hit an invisible barrier and explode at a certain distance from the sun, and so can't reach the surface at those points, or anywhere else.

[Hyomoto]

Well, consider my question answered. Thanks everyone!

[Morzak]

I think there are two stationary points in the game, but you can't reach them. The north and south poles of Kerbol should be rotating but not going anywhere. Since everything else moves around the sun and there are no external reference points outside the Kerbol system, all motions are relative to the sun, and it sits still with regard to itself. But of course, you hit an invisible barrier and explode at a certain distance from the sun, and so can't reach the surface at those points, or anywhere else.

Even then you're only not moving in the reference frame of Kerbol, in the reference Frame of Kerbin or Duna or any other object in the Kerbol System you still would have a relative velocity. Since there is no fixed reference point you're always moving in relation to something else. It's all about reference points.

[Akalaël]

To go further, if you could reach sutch a point in our universe, you will still be subject to the dilatation of the universe... No merci in our lives, we still have to move, that's it !

[NeoMorph]

Like Einstein says, "Everything is Relative"...

You can stop in relation to anything if you have the delta-V to do it. Stopping your ship relative to galactic central is impossible though... not enough info or even delta-V to accomplish it.