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Assume you want a satellite to constantly point its radio dish towards Earth while orbiting it, or its solar panels towards the Sun if it is instead orbiting it. Is any of the following true about what is needed to achieve this:

  • continuous change in its orientation,
  • once and for all giving it the right spin to begin with,
  • it happens naturally.

The last point is true for a toy car on a sloping road curve, it keeps the same side facing the center of a circular track.

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5 Answers 5

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Experts, I think you have possibly "over-answered" Local's question...

Local, when you put a satellite (say, a communications satellite or spy satellite) in orbit around the Earth, it DOES NOT "just happen" to sit in the handy facing-earth direction.

Your first two points are precisely correct, you hit the nail on the head ... you have to make a huge effort (with rockets, engines, spinning .. whatever) to make that happen, it's a huge challenge: you have to BOTH set it up correctly AND (one way or another) correct it continually, forever. Both are difficult.

That's the answer to your question. A satellite does NOT just sit "correctly facing".

{As a footnote, somewhat confusingly: as aramis explains perfectly there is a very subtle force that DOES "magically" do this - in some situations - "naturally" over long periods of time, in some cases. But as a simple matter in answer to your specific question, yes, when satellites are put in to orbit, there is a tremendous job that must be done to (a) make them "correctly facing" to begin with and (b) making them continue to be "correctly facing".}

Hope it helps!

One small point -- your toy car analogy is really totally unrelated, heh!

And your question here ... "If I launch a rocket which reaches LEO, [...] would that rocket turn around and show the same side towards the Earth as it orbits?"

The answer is extremely simply and absolutely and unequivocally NO.

Satellite launchers have to make a massive, incredibly complicated effort (of one kind or another) to achieve your points 1 and 2 -- to get it "correctly facing" and then keep it "correctly facing" over the years.

It's that simple. Hope it helps!

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I very much appreciate the other lengthy answers above, but you state it most clearly. As I understand it now, curved space does not change the attitude of a satellite traveling through it, not in any practically meaningful way for actual man made satellites anyway. –  LocalFluff May 7 at 9:38
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I realised I know just what you were asking Fluff: "IS THERE A 'LOCAL DOWN' IN ORBIT..." Right? Like any game programmer would assume there is a "local down". Just as in the case of an object being swung by a string. In orbit, the answer is no, the objects quaternion is unaffected, it will just keep pointing "the same" ... towards some distant stars. Cheers!! –  Joe Blow May 7 at 15:59
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There is a local "down" in orbit for real objects with extent, called the gravity gradient. It is actually both up and down, but with a definite orientation toward the center of the planet. The force is small, but very real and very useful. ISS and many other observing satellites use the gravity gradient to help keep their instruments pointed at Earth. The same force keeps the same side of the Moon always pointed at the Earth. It is misleading to say there is no local down with such certitude and emphasis. –  Mark Adler 2 days ago
    
Hey Mark I delete that sentence then. The situation is this: "That's the answer to your question. A satellite does NOT just sit 'correctly facing'." –  Joe Blow yesterday
    
Mark -I would suggest: there's a problem on these sites where an extreme expert (like you!!), sees (what is actually) an incredibly simple question (perhaps phrased confusingly), and, because coincidentally there is a related incredibly technical - literally arcane - issue, the expert mistakenly offers an explanation (in itself, fascinating!) of that arcane issue. TBC here, the only thing the OP is asking is: say a human (in a spacesuit!) is floating in orbit: does she always stand face down? Of course, obviously, you and I know "heh, of course not!!" that's the whole question here! –  Joe Blow yesterday

Is any of the following true about what is needed to achieve this:

  • continuous change in its rotation,
  • once and for all giving it the right spin to begin with,
  • it happens naturally.

The answer is "yes" to all three questions.

If a vehicle is shaped right and is given the right rotation to start with, torques that naturally occur such as gravity gradient torque and torque from atmospheric drag from can help keep the vehicle rotating in the desired orientation. However, this is never perfect and there are always residual undesired torques.

Vehicles need to have some kind of active attitude control system so they can keep themselves properly oriented. If that attitude control relies on fuel, the depletion of the fuel tanks marks the end of the vehicle's useful life.


Update: Approaches to attitude control

Use thrusters.
The vehicle can only do this so often before it runs out of fuel. For most vehicles, that's the end of the mission. Approaches that reduce the need to use thrusters will extend the vehicle's useful life or enable a bigger payload. In some cases thee alternate approaches entirely eliminate the need for thrusters.

Take advantage of torques from the environment.
Vehicles from Landsat to the Space Station take advantage of rather than fight the external torques exerted on the vehicle by the environment. Environmental torques include gravity gradient torque, atmospheric torque, and magnetic torque. (There's also solar radiation pressure torque, but this is a tiny disturbance.) Some small vehicles in low Earth orbit equipped with magnetic torquers don't use any fuel. They remain functional until they reenter the atmosphere.

Take advantage of rotation.
A rotating object has angular momentum, which makes it harder to turn than if the object wasn't rotating. This adds stability to the vehicle (but also instabilities in some cases). Some of the earliest satellites were spin stabilized.

The next step up in complexity is to construct the vehicle so that it has comprises two parts that rotate about a common axis but at different speeds. Most communications satellites are dual spin satellites. The rotor (plastered with solar arrays) rotates rather quickly for stability while the communications platform rotates but once per day.

Another approach is to place the rotating parts inside the vehicle. These internal rotating devices include momentum wheels, reaction wheels, and control moment gyros. A momentum wheel, like the rotor in a communications satellite, is intended to rotate at a constant angular velocity. A motor with a simple controller is needed to bring the wheel up to speed and then keep it at that speed.

Adding the ability to change the commanded rotation rate to that momentum wheel controller turns the momentum wheel into a reaction wheel. With this ability, angular momentum can be transferred between the main body of the spacecraft to the reaction wheel. A vehicle with three reaction wheels, one per rotation axis, provides an active means of controlling vehicle rotation. Reaction wheels have a basic problem in that rotation speed must be between a minimum value (lest the stabilizing influence be lost) and a maximum value (lest the wheel lose structural integrity). A vehicle that uses reaction wheels needs some alternate control mechanism to help keep the vehicle stable while reaction wheels at their limits are brought back to the nominal rotation rate.

An alternative approach is a control moment gyro (CMG). These are essentially momentum wheels with another motor that pushes against the rotating wheel. (Think of the apocryphal stories of physicists who put airplane gyros in suitcases and then spun them up as a practical joke.) The amount of torque generated by CMGs per unit of power applied can be quite impressive. Just as reaction wheels have operational issues, so do CMGs. In the case of CMGs the problem is gimbal lock. Rotations about one or more axes eventually become uncontrollable. A vehicle that uses CMGs needs some alternate control mechanism to help keep the vehicle stable while CMGs are restored to their nominal rotation axes.

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Because the OP was asking how to keep the dish pointed to the Earth while orbiting the Earth, it could be good to mention, that spin stabilization is not useful to achieve that. Spin stabilization keeps the satellite pointed to a distant target, not towards nadir in the Earth orbit. –  mpv May 5 at 7:51
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@mpv - The first communications satellites were spin stabilized, and in a sense, many still are. You apparently are thinking that the antenna has to point along the rotation axis. That's not the case for comsats. Their rotation axis points to Polaris, not the Earth. –  David Hammen May 5 at 12:19
    
How does a spin stabilized satellite keep its dish pointed towards the Earth? Is it rotating at the same rate as the orbital period? So the dish rotates always towards the Earth? –  mpv May 5 at 15:19
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That's exactly how they work. The same goes for nadir viewing satellites. They don't "look" along the rotation axis. They look normal to it. –  David Hammen May 5 at 15:38
    
Spin-stabilized satellites are spun at a relatively high rate. I don't have numbers, but watching images from early shuttle deployments, it looks to be in the neighborhood of 30 rpm. These satellites contain a section that is "de-spun", which allows precise pointing of the antenna, regardless of the spin of the rest of the vehicle. –  Tristan May 5 at 15:54

The best way to keep an antenna always pointed at Earth, if you can manage it, is to stick a large weight at the tip of your antenna. The weight will receive more pull, and naturally keep the antenna pointed at that direction.

Short of having something like that to help passively, the next best solution is to spin stabilize. By spinning around an axis, you can guarantee that the axis always maintains it's direction, like spinning a top. Of course, there can be some wobble, which might become an issue, but this can be managed if worked carefully enough.

If you can't do one of those two, then you will most likely have an unstable system. Density fluctuations, turning to maintain solar power, solar wind and light pressure, thermal gradients, all can cause a very small perturbation. These will be magnified with time.

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For a graphical example of gyroscopes in orbit, see this footage by Don Pettit onboard the ISS: youtube.com/watch?v=gdAmEEAiJWo –  Davidmh May 4 at 17:06
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Unless I'm missing something, giving the satellite spin ought to make it point in the same direction throughout its orbit. That's good if you want it to point at, say, Alpha Centauri, but not much use if you want it to point at the ground. –  Harry Johnston May 4 at 21:29
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You are correct about spin stabilized, it's mostly used for spacecraft destined to the outer solar system, where the stability required is mostly to point toward Earth. –  PearsonArtPhoto May 5 at 0:16

Keeping the Same Face "Down"

There's a term for this when it naturally occurs: Tidelocking.

Natural orientation

One can make use of tidal stress to keep an orientation naturally.

When an object of significant length is placed into orbit, the side closer to the center of gravity receives somewhat more "pull" than the far end, and it rotates around its own center of mass. This eventually damps rotation to match the orbital duration. This can, however, take years to accomplish.

It also can take a lot of material, and has other effects. It's a tiny force, but it's constant and profound. It's fractions of a centimeter per second per second at geosynchronous orbit. Just enough to have a stable effect.

Short bodies, and especially ones that are round, blob-like, or blocky, will eventually tide-lock as well,but much more slowly.

Further, even large objects have orbital decay issues. Orbital decay comes from several sources: atmospheric drag, solar wind drag, solar wind force, and tidal stresses. Atmospheric drag at most low-earth orbits results in falling before tidal force matters much. Solar wind drag is similar, but several orders less. Solar wind acceleration is always "attempting" to force the periapsis to be on the sunward side, but is a tiny force. Tidal stresses attempt to drag the orbit to the same duration as the rotation of the body orbited.

Most objects people are considering are too small to self-orient naturally before decay.

Unnatural orientation

If one places an object in orbit, and sets its rotation length to the same as its orbit length, then one has essentially replicated the effects of tidelocking... as long as the long axis is also down.

Keep in mind that the object rotates on its center of mass. The center of gravitic force, however, may not be on the center of mass, and so tidal stress will slowly alter the orientation of the object. In earth orbit, this is complicated by the tidal stress of the moon, as well. Mind you, the moon's tidal stress is very tiny - nanometers per second per second - dwarfed by the millimeters per second per second of the earth, but sufficient to induce orbital deformations.

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Picture this: Take a toy airplane and tie a string to one wing. Now spin in place and let the plane fly at the end of the string. You are the Earth and the plane is a satellite. Does the plane really "rotate"? Or is it flying straight all the time but its course is being changed because of the string?

Its the exact same thing with a satellite only the string is gravity. In reality the satellite is flying straight because it was launched forward, and it is constantly falling towards the Earth, but its forward speed exactly offsets the pull of gravity.

So don't think of it so much as turning as it flying forward with continuous automatic course changes into a circular path.

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If I understand you (and others) correctly, pushing a circular LEO satellite towards the Sun, just turns the pushing point in its orbit into perigee without disturbing its orientation. If it was facing the Sun to begin with, it will keep doing so after the push too, as it makes its new eccentrical orbit. –  LocalFluff May 5 at 18:01
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This is not the answer to your question, LocalFluff. –  David Hammen May 5 at 18:11
    
"So don't think of it so much as turning as it flying forward with continuous automatic course changes into a circular path." That's pretty much a simplified explanation of an orbit, however it says nothing about spacecraft attitude in that orbit. –  Michael Kjörling May 6 at 14:00

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