Orbiting Satellites - Orientation/Attitude question

[SirJodelstein]

So, i've come here because of a discussion with some coworkers today we couldn't settle due, and i've come here to find good scientific explanations (preferrably with references) on who is right or wrong:

Assume a sattelite in a circular orbit around earth (ignoring atmospheric effects). At one point in time, the satellite cancels all local rotational moments in relation to earth (with gyros/RCS/KSP timewarp). From that point on (provided there are no further attitude control actions from the satellite), does the orientation of the satellite stay fixed in relation to the universe (A) or does it rotate and constantly faces earth with one side?

Please do not discuss how KSC simulates this, i'd like to discuss real physics here. Topics covered in our heaty discussion today touched tidal locking, force fields, water-in-a-bucket-swung-on-a-rope, ISS active attitude control and Nerf gun fights, but no side could prevail. Do satellites automatically tidal lock to earth? Do they need active attitude control to keep facing earth? Or do they float around their orbits with a precisely-calculated fixed rotation speed that keeps them oriented towards earth without further force/torque input? What happens when they stop this rotation once? Does it depend on orbit height and/or vessel mass or size?

I have a very clear opinion on those matters (which i am not going to post to not bias this discussion in a certain direction), i just couldn't find references for it.

[check]

If we're talking about a special case, spherical satellite (with even mass distribution), then I think the answer is A. But the gravitational gradient will, according to wikipedia " tend to align its axis of minimum moment of inertia vertically."

Edited October 16, 2013 by check

[WestAir]

I'm almost positive A is the answer you're looking for. I'm fairly certain attitude control systems keep our satellites facing Earth during their orbits.

As an aside, I'm also fairly positive KSP provides "real physics" in this regard.

[check]

As an aside, I'm also fairly positive KSP provides "real physics" in this regard.

It doesn't simulate gravitational gradient torque. So no.

[Kryten]

Originally gravitational forces and vectors were done seperately for each part-which led to gravity gradient effects arising organically. That stopped when gravity started being calculated from CoM in one of the first rounds of optimisation.

[WestAir]

It doesn't simulate gravitational gradient torque. So no.

Shows what I know.

[Nikolai]

Translational velocity is relative. Rotational velocity is not.

If you could cancel all rotational velocity, you'd orbit as in Diagram A. However, many spacecraft in orbit set themselves to rotate with a period as close as possible to their period of revolution around their primary, so in practice, most satellites orbit as in Diagram B.

Any satellite left on its own for long enough -- where "long enough" is an amazingly long time -- will eventually be tidally locked with the heavy end pointed towards the center of mass of the primary.

[Nibb31]

Real satellites add a rotation factor that is synced with the orbital period, just like the Moon.

[Nuke]

i wonder if they design satellites to be bottom heavy so they have a natural alignment tendency. seems it would be easier/cheaper than just doing everything with reaction wheels and thrusters (you would still need them but their power and fuel draw would be noticeably less).

[Drunkrobot]

I read the post, understanding what you were asking (as far as I would know, A is the true case), but I read the second to last paragraph, and mis-read part of it. I'm now imagining a Nerf gun fight on the ISS, something I would do unspeakable things to see happen.

[SirJodelstein]

Thanks for all the answers, i just saw this amazing video on gravitational gradient stabilization

from what i gathered, this effect happens for any satellite, but is usually too weak to stabilize the satellite unless you have special devices (long tethers/booms with mass at the end) to deliberately utilize it?

so if i put a car into orbit and cancel its rotational movement it will float like in "A" (but will stabilize over the course of many years/orbits), but if put two cars into orbit connected by a several kilometer long rope the system will self-stabilize and orbit like in "B"? (And most satellites use the "add synced rotation factor" method to continually face earth?)

[Person012345]

Purely on tidal locking it happens because gravity acts on each body to deform it, which then causes the gravity to pull more strongly on these distortions. It will speed up a non-rotating satellite and slow down a quickly rotating one "automatically". The same effect is happening to the earth right now from the moon (although IIRC we will only tidally lock to it long after the sun expands to engulf us all anyway so it's irrelevant). That brings me on to the next point, that this process takes millions or billions of years as far as I'm aware, giving the satellite plenty of orbits to complete in an unlocked state.

Edit: Wait, what do you mean by "satellite"? Artificial or like a moon? I was assuming you mean a natural satellite.

[SirJodelstein]

i was talking about an artificial satellite in Low Earth Orbit

[Nibb31]

i wonder if they design satellites to be bottom heavy so they have a natural alignment tendency. seems it would be easier/cheaper than just doing everything with reaction wheels and thrusters (you would still need them but their power and fuel draw would be noticeably less).

No, you just need to give it a small jolt to start the rotation at a rate of one revolution per orbit (like in diagram and it will keep on spinning at the same rate without drawing any power. There's no drag or friction to slow down the rotation so it will just spin forever. It just needs a bit of adjustment from time to time, but that's nothing a reaction wheel or small thrusters can't handle with very little power.

Edited October 17, 2013 by Nibb31

[Person012345]

No, you just need to give it a small jolt to start the rotation at a rate of one revolution per orbit (like in diagram and it will keep on spinning at the same rate without drawing any power. There's no drag or friction to slow down the rotation so it will just spin forever. It just needs a bit of adjustment from time to time, but that's nothing a reaction wheel or small thrusters can't handle with very little power.

Also "bottom heavy" wouldn't do anything.

[shand]

No, you just need to give it a small jolt to start the rotation at a rate of one revolution per orbit (like in diagram and it will keep on spinning at the same rate without drawing any power. There's no drag or friction to slow down the rotation so it will just spin forever. It just needs a bit of adjustment from time to time, but that's nothing a reaction wheel or small thrusters can't handle with very little power.

No = Very little space, espeically LEO isnt empty. there is no clearly defined "whelp, thats the end of the atmosphere" altitude. in fact ISS skims through a particularly "thick" bit of atmosphere, they have to feather their solar panels to reduce drag! but yes, this is where the "bit of adjustment" comes in

Generally A) is the relivent pattern, with induced spin creating a like pattern and larger structures able to use gravity gradients to lock in

[Nuke]

the general idea was to exploit gradient torque for alignment purposes to increase efficiency and there for operational life of satellites (i was thinking geosynchronous, not stuff scraping the atmo in leo). it seemed obvious like it would be something already in use.