How to detect if you are in orbit?

[Vertigo]

Let's see if I can explain what I'm after here.

Imagine a moderately large habitat style spaceship, a rotating cylinder, say something like a 1000m in diameter and several km in length. Assuming no access to any kind of external sensors and no windows, is there any way you could tell if you were in orbit around a planet or just coasting through interstellar space?

I'm pretty sure I read something a while back that described a way of doing this.

Also how would such a thing orbit? Would it orbit end on, so that the cylinders axis points at the centre of the planet, or lengthwise so the axis is pointing along a tangent to the orbit?

I was wondering if you could do something like this:

The orbit radius is dependent upon the speed; faster speed, bigger radius. If the ship is orbiting end on then the orbit radius would be based on the speed of the cylinder's centre of gravity. So if you floated a balloon at one end of the cylinder would it then naturally drift towards the centre to get it's (the balloon's that is) orbit radius right for it's speed.

If it was orbiting lengthwise, would there be a measurable difference in the centrifugal pseudo-gravity when the spin of the cylinder places you closest to the planet as oppopsed to when it places you farthest away from the planet?

Hope that makes sense!

[Bowler1]

The first is easy enough, assuming you were standing in the middle of the habitat, I don't think you'd be able to tell if you were coasting through deep space or not. 7km Is a long distance visually. The sky would be low if there was one. Only at the ends of the tube then outside space would be visable. Think how far away is your nearest shop, your distance to work. A lot of people would stay within 7km of their home most of the week!

The rest of the question was difficult for me to follow. But, I think with a small radius habitat, if you dropped a weight it might look to fall a little to one side, because you would have moved with the structure while the object dropped/centrifugul force takes hold. A structure with a bigger radius then this would be harder to see/measure - assuming 1g gravity. Thats what I think, I'd like to have that confirmed or debunked myself.

I don't see why the axis of the habitat would matter in relation to a nearby planet.

Good question.

[RJM Corbet]

Very simply, it would appear that in a state of weightlessness, with no windows or sensors of any kind, there's be no way to tell. Imagine being completely weightless and completely in the dark, for instance? What would you know?

But ... let's wait for Crispen

[Metryq]

There's no reason for the habitat to change orientation as it orbits the planet—doing so would use up power. Hold a pen between thumb and index finger so that it is horizontal. This becomes a model of the cylindrical habitat. Now sweep the pen in a wide circle, keeping it horizontal. Relative to its own reference frame, the habitat does not change orientation, although different sides of the structure face the planet as it orbits.

(ISS generally maintains the same orientation relative to Earth with gyroscopes, but does change orientation on occasion.)

Could some simple test of dynamics, like a bearing rolling down a ramp or maybe a pendulum, tell the difference between orbit and coasting freely through space? I doubt it. The rotation of the habitat on its axis would likely overwhelm any tiny effects caused by orbiting. (Like the urban myth about water spiraling down drains in different directions between north and south hemispheres of Earth, there are other factors that completely overwhelm any Coriolis or other effects.)

[Vertigo]

Thanks for the responses!

Bowler - You can't see out of the ends of the cylinder, so you have no way of see outside at all. That is the problem my protagionists face. They have been told they are cruising to another star but they now believe (as is the case) that they are actually in orbit around a planet. What I need is some way for them to prove it.

RJM - I am certain there is some dynamic way of confirming it. In both cases the cylinder's speed is constant however when in orbit your velocity is constantly changing (ie the direction of your motion) whereas when cruising it is not.

Metryq - I wasn't talking about changing the orientation of the habitat whilst orbiting but just wondering which orientation would be the logical one to use for an orbit. I suspect it would be the lengthwise orientation. Actually now I come to think about it without applied power I suppose you would actually slowly roll (relative to the planet) through 360 degress each orbit. In order to maintain the same orientation to the planet would you actually have to have a very slow rotation? In the same way that the moon has a slow rotation of one revolution per orbit.

[RJM Corbet]

Originally Posted by Vertigo

... RJM - I am certain there is some dynamic way of confirming it. In both cases the cylinder's speed is constant however when in orbit your velocity is constantly changing (ie the direction of your motion) whereas when cruising it is not ...

I'm sure there must be. The weightlessness in orbit is a 'falling' weightlessness, the cruising weightlessness is 'no gravity' weightlessness?

Edit: So it follows that a cruising vehicle would be exposed to some slight gravity effects from the closest mass: a sun, a planet, an asteroid, etc?

So perhaps an object floating in the cabin of a crusing vessel would eventually be pulled in a certain direction, towards the gravity source? Instead of just hanging there as would be the case in orbit? Perhaps a floating water 'bubble' would assume a pear shape, with the narrow end towards the gravity source, instead of being spherical, and would gradually move in that direction?

Thus if floating objects tend to congregate in one area, you know you're not in orbit, and vv?

[Vertigo]

Yeah now the problem is that theoretically everything - spaceship, dropped objects etc - are all in the same dynamic situation. They are all in orbit or just cruising through interstellar space. If there is a body acting on them - planet, star, whatever - then it should be acting on them all equally.

The only difference I can see is in the orbit situation an object in different parts of the habitat (or different stages in the habitats spin) is actually in a slightly different orbit to that of the habitat's centre of gravity and so should experience slightly different forces. But I'm not sure how big that difference would be.

It's driving me bonkers this I might have to try visiting some astrophysics forums and see if anyone can help there!

[RJM Corbet]

Oh yes, true ... hmmmm

[Vertigo]

Killer, isn't it? The really annoying thing is that I'm sure I came across this problem (with a solution) in a book I read maybe a year ago but I can't remember it. I thought it might have been Bester's The Star's My Destination but I can't find it in there.

Arrrgh!!!!

[RJM Corbet]

Gully Foyle is my name
Terra my habitation
Deep space is my dwelling place
And death my destination ...

[Ursa major]

Originally Posted by Vertigo

Bowler - You can't see out of the ends of the cylinder, so you have no way of see outside at all. That is the problem my protagionists face. They have been told they are cruising to another star but they now believe (as is the case) that they are actually in orbit around a planet. What I need is some way for them to prove it.

This begs all sorts of questions, such as:

• who told them they were cruising?
• when?
• why aren't they being told about being in orbit?
• are the "who" still alive?
• are the "who" automated systems?
• if the "who" were still alive/functioning, would they know?
• how would they know?
• is there any way the people not in the know could get access to the part(s) of the ship where those that know/knew are/were?
• why do those not in the know think they may be in orbit?
• is it just a hunch (or a belief that the ship's be travelling about the right time)?

(I suppose that the reason they need to determine the ship's situation is because they mean to take some action, and the knowledge will help them persuade others to join in. If the latter, the demonstration that they're in orbit will have to be clear and conclusive.)

Intriguing situation.

[RJM Corbet]

Going into the literary agency business, are you, Ursa?

[Ursa major]

Didn't you know that I'm already a top literary agent (in the same way that I'm already a bestselling author, that is...).

[RJM Corbet]

Originally Posted by Ursa major

Didn't you know that I'm already a top literary agent (in the same way that I'm already a bestselling author, that is...).

Now you've got me wondering.

You've sure been around here a long time. Coming up for 10k posts ...

[Vertigo]

Basically most of those questions are what the (short) story is about! Essentially the story is:

Aliens arrive at post apocalyptic Earth. All life has been destroyed some time ago but not too long ago. Through recovered computer records they learn human history and Earth biology. They recover DNA and 'grow' a bunch of children, plants and animals on a habitat (Earth is still polluted/radioactive or whatever) using android 'adults', indistinguisable from real humans, to bring them up and educate them. Their objective is to give them as 'natural' an upbringing as is possible based on the human history they have learnt.

They don't want the children to know the truth (ie that they are the only humans in existence) until they are older. So they create the story that they are a human spaceship off to colonise another star system. In actuality they are still orbiting Earth.

As the children get older they begin to notice anomalies. Things like the habitat technology seems to be significantly beyond the technology in the history they have learned. Why is there no new communication with Earth. They are supposed to be under permanent (though very low) acceleration and then deceleration (ramjet) but this is easy to check using pendulums and the oldest children have already figured that they are not under acceleration. So they begin to speculate that they must either be cruising or in orbit...

So in answer to your final points the (well-meaning) aliens are deliberately keeping the children in the dark but the children are beginning to get suspicious. Once they, hopefully, figure out they are in orbit then they will begin trying to get a 'peek' outside.

Incidentally the reader will know none of that background until the end. The POV will be with the children.