Orientation of an object in orbit

[Camacha]

Allright, this might be a simple question, but I Googled quite a bit and I could not find any satisfactory answer. My question is how your orientation behaves when in orbit. Since it is a bit hard to explain I made a nifty schematic to clarify.

Scenario A - red triangles on the left: In KSP you always face the same direction - not touching the controls of course. Since you are orbiting a planet, this means the position of the planet seems to move around you and returns to your starting point after one rotation. In KSP this is most visible in the navigation ball. Your perceived orientation moves all the time, ending up where you started again.

Scenario B - blue triangles on the right: This is how I always imagined it. It is more like begin attached to a string and flung around. Looking down on the planet I would be facing the planet the entire orbit. Looking at space stations this also seems to be the logical way to go, but I might be entirely wrong. When I check movies from the ISS however, it seems to be true.

What is accurate for real life? And more importantly, why? Is it indeed because of centrifugal forces like an object on a string would experience?

Edited April 21, 2013 by Camacha

[Camacha]

Silly me, I forgot to assign a category in the title.

[Awaras]

The red triangles (and the way it is in KSP) are accurate. The ISS spins at exactly one rotation per orbit (which needs constant correction) to get the effect from the video.

[Camacha]

The red triangles (and the way it is in KSP) are accurate. The ISS spins at exactly one rotation per orbit (which needs constant correction) to get the effect from the video.

Do you know why it works like that? I know, not the most refined question, but I'm trying to understand the basic mechanics. Any sources on this subject are also welcome.

[Awaras]

Any object remains motionless unless a force acts upon it. If you point the hubble space telescope towards a specific star, it will remain pointed towards it even though it orbits the Earth.

There are no forces acting on objects in orbit that would force them to always orient them self to face the planet they are orbiting (well, tidal forces DO act that way, but the tidal forces acting on ships/sattelites in orbit are so small to be negligible - it would take decades for tidal forces to spin up a ship or sattelite in this way) so they keep their orientation.

[SolarLiner]

The "Gravity-gradient torque" may help you staying pointed towards the Earth. If you leave an object long enough in LEO (like a few days), you'll see that one side of the object will face way longer and the other side. If you wait even more (like few months or years, depends on the object), that side will remain facing the Earth.

But the ISS is way too massive to stay aligned. They uses CMG to help orientating the station along Earth's horizon.

The fact that your ships in KSP remains motionless is because of the "on-rails" simulation. I think if you stay long enough in 4x physics warp you will see the ship rotating. Or the gravity-gradient torque is simply not (yet) implemented in KSP.

[Awaras]

Actually, the on-rails simulation does not appear to save orientation data. Many times I would orient my space station in some manner (oriented north/south or so its solar panels are facing the sun for example) and the next time I load the station it is oriented completely differently....

[K^2]

Either of the two pictures are possible, and it has nothing to do with tidal interactions. If you give an object a bit of a nudge, it will just keep on spinning, its orientation constantly changing. If you give it just the right amount of push, it's rate of turn can match its orbit around the parent body, giving you the second picture.

Of course, with real objects, you do have tidal interactions and axis tumbling, which will make the whole thing way, way more complicated.

[Camacha]

Thanks for the answers, that all helped a lot.

The "Gravity-gradient torque" may help you staying pointed towards the Earth. If you leave an object long enough in LEO (like a few days), you'll see that one side of the object will face way longer and the other side. If you wait even more (like few months or years, depends on the object), that side will remain facing the Earth.

But the ISS is way too massive to stay aligned. They uses CMG to help orientating the station along Earth's horizon.

The fact that your ships in KSP remains motionless is because of the "on-rails" simulation. I think if you stay long enough in 4x physics warp you will see the ship rotating. Or the gravity-gradient torque is simply not (yet) implemented in KSP.

I was thinking something along these lines, though with a different train of thought. The problem with the ISS appears to be its flatness and not its mass per se, although its relative mass to a device or part making use of the gravity gradient might be. Alignment is plausible, but not something that is easily achieved.

I think gravity in KSP drops off dramatically and as such, gravity driven schemes are a bit useless, most of the time.

[FaceDeer]

Gravity-gradient methods can be greatly enhanced by using a long tether with a counterweight on the end to enhance tidal effects. A steeper gravitational gradient (as we see with these tiny high-density planets in KSP) also enhances tidal effects.

I think we could afford to "fake" a lot of this stuff, though, since controlled rotation is generally pretty easy in KSP. Maybe some sort of stabilization component that you set to point in a particular direction along a gravitational gradient and then it uses ASAS-like torque control to keep things that way.

[Camacha]

Gravity-gradient methods can be greatly enhanced by using a long tether with a counterweight on the end to enhance tidal effects. A steeper gravitational gradient (as we see with these tiny high-density planets in KSP) also enhances tidal effects.

As long as you actually are within the gravitational field, this is true. I think I should remeasure Kerbin's field again, but I seem to remember it zeros out quite fast.

Edit: not quite zero at a 1000 kilometers, so if gravitational drag is implemented (which some sources suggest it is) using it in LKO to stay aligned might be feasable.

Edited April 22, 2013 by Camacha

[Camacha]

I did some additional testing and gravity seems to zero out at about 1788K on the altimeter. That should leave plenty of room for using the gravitational gradient There seems to be some kind of strange drop off tough; at about 1 m/s^2 the graph sharply drops to zero. I guess you leave Kerbin's SOI around that point.

[Lazarus78]

Actually, the on-rails simulation does not appear to save orientation data. Many times I would orient my space station in some manner (oriented north/south or so its solar panels are facing the sun for example) and the next time I load the station it is oriented completely differently....

Because the planet is orbiting around the sun too, so the position of the sun is not a constant. If you waited half a year, the sun would be behind you from where you positioned yourself. The same effect you experience as you orbit the planet applies to the planet orbiting the sun, and the sun orbiting around the galaxy.

This very principle was used to prove that the galaxy was actually moving. They used the most accurate gyros ever created and put them in orbit for several months (Or years, i forget) and had them pointing at a known galaxy. After a while, they checked them to see if they were pointing at it still. If we were standing still, they should be pointing at the same exact spot. Turns out they weren't and were actually shifted ever so slightly to the side.

[Awaras]

Because the planet is orbiting around the sun too, so the position of the sun is not a constant. If you waited half a year, the sun would be behind you from where you positioned yourself. The same effect you experience as you orbit the planet applies to the planet orbiting the sun, and the sun orbiting around the galaxy.

Yeah, but it would take at least a few hours/days. I would orient the station towards the sun, return to the space center and return to the station within the next few minutes, and it would be oriented differently...

[Kerbface]

The "Gravity-gradient torque" may help you staying pointed towards the Earth. If you leave an object long enough in LEO (like a few days), you'll see that one side of the object will face way longer and the other side. If you wait even more (like few months or years, depends on the object), that side will remain facing the Earth.

This is the same thing as tidal locking that occurs with moons, right?

[K^2]

Sort of. With tidal locking, the deformation of the planet/moon tends to play a role. Though, Earth's Moon has very peculiar mass distribution, so it's actually a lot more like what you see with satellites.

[AlternNocturn]

it would take decades for tidal forces to spin up a ship or sattelite in this way) so they keep their orientation.

Wait, what if the Vanguard satellite is doing that now? It's still up there after all this time.

[Rune]

Yeah, but it would take at least a few hours/days. I would orient the station towards the sun, return to the space center and return to the station within the next few minutes, and it would be oriented differently...

It saves "some" orientation. I know because when you load the same save a lot of times (because, say, you keep on getting the suicide burn wrong, not that that's happened to me... ^^'), it will always start with the same orientation... which has nothing to do with the orientation you left the craft in.

Rune. File that as quirks of the game.

[Jason Patterson]

I did some additional testing and gravity seems to zero out at about 1788K on the altimeter. That should leave plenty of room for using the gravitational gradient There seems to be some kind of strange drop off tough; at about 1 m/s^2 the graph sharply drops to zero. I guess you leave Kerbin's SOI around that point.

I'd be more inclined to suspect the source of the data, honestly. There is clearly a non-zero gravitational field surrounding Kerbin out to the edge of its SOI (which is 84,000km and change.) If you drop your orbital velocity to zero, you will begin falling toward the planet, if nothing else. Sure, it's very gradual at high altitudes, but also definitely non-zero.

[Camacha]

I'd be more inclined to suspect the source of the data, honestly. There is clearly a non-zero gravitational field surrounding Kerbin out to the edge of its SOI (which is 84,000km and change.) If you drop your orbital velocity to zero, you will begin falling toward the planet, if nothing else. Sure, it's very gradual at high altitudes, but also definitely non-zero.

Well, maybe not zero, but for these intents and purposes zero. They were extreme small and erratic numbers (.ie not successive, but up and down). That does not explain the sharp drop off

I used TeleMachus to do my measurements, by the way.

[Rich]

Telemachus pulls the gravity reading directly from each of the sensors on your ship, which off the top of my head are only accurate to 2 decimal places. Better hope the KSC develops some better sensors . You can of course get a more accurate reading from the game internally, but it takes aways some of the realism if you magically pluck values from the ship completely bypassing any sensors you have installed.

Edited April 28, 2013 by Rich

[Camacha]

Telemachus pulls the gravity reading directly from each of the sensors on your ship, which off the top of my head are only accurate to 2 decimal places. Better hope the KSC develops some better sensors . You can of course get a more accurate reading from the game internally, but it takes aways some of the realism if you magically pluck values from the ship completely bypassing any sensors you have installed.

Indeed, that is no fun at all Still does not explain the drop off though, from 1 to 0 was pretty much a straight line down. I think I didn't mess with the sensors, which might cause a similar result.

[Rich]

Now that you mention it the gravity graph that Telemachus produces does exhibit a strange stepping behaviour, which none of the others do - they are much smoother. Will have to investigate.

[Rich]

Sorry for the double post, but I had to investigate this gravity effect you were experiencing (a mixture of curiosity and wanting to check there was not a bug in Telemachus). It seems like the gravity sensor has a limited range as demonstrated in the screenshot below.

[LuvSicGrl]

the ISS uses gyroscopes to maintain orientation.