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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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@JoeBlow the moon is NOT a really unusual and freaky example. See this list of tidelocked bodies: en.wikipedia.org/wiki/… – HopDavid Jul 26 '15 at 3:43
    
HI Hop! To help the OP, simply answer yes/no to the question in the title. When we launch a satellite ... "Do satellites naturally turn in phase with its orbit, always facing Earth?" it's a very simple question with a very simple answer. – Joe Blow Jul 27 '15 at 4:09
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zerognews.com/special/sp8000/archive/00000107/01/sp8071.pdf says "The TRANSIT-5A, which was the first man-made object to achieve GG stabilization…" Table 2 gives a list of satellites that attempted to use Gravity Gradient stabilization, some successfully. This was a 1971 PDF so I expect the list is longer now. And once again, your statement "The moon is a really unusual and freaky example" is absolutely false. Please acknowledge that tidelocked moons are NOT unusual and freaky. – HopDavid Jul 29 '15 at 23:40
    
An overlooked candidate for best answer. – uhoh Jul 14 at 2:35

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 '14 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 '14 at 12:19
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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 '14 at 15:38
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The torque from gravity gradient on ISS works against its most common flight attitudes. Vehicles will tend to orbit with their minimum moment of inertia oriented vertically. This is not how the ISS orbits. selenianboondocks.com/wp-content/uploads/2010/11/… – Erik Jul 25 '15 at 16:21
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Here is a nice gaming-related explanation: youtu.be/-WsuNSuIhG0 – Erik Jul 25 '15 at 16:27

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 '14 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 '14 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 '14 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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A satellite can naturally remain aligned to the local vertical.

In orbit are two forces to consider: force of gravity and centrifugal force. Centrifugal force is actually inertia in a rotating frame. But if you happen to be on the merry-go-round it feels like a force.

Centrifugal force is $\omega^2r$ and gravity is $GM/r^2$

To portray these up and down tugs I'll use balloons and passengers being carried by balloons.

enter image description here

This picture portrays a balance of gravity and centrifugal force. Net force is zero.

What happens if we double r, the distance from body center?

enter image description here

Doubling radius doubles upward tug. Downward tug is cut to 1/4. Net acceleration is up.

And if we cut radius in half…

enter image description here

Upward tug is cut in half while downward tug is quadrupled. Net acceleration is down

Tie these three together and you get a tether that remains aligned to the local vertical:

enter image description here

There are satellites that use gravity gradient stabilization to remain aligned. This also what keeps a lot of moons tidelocked. If we ever have vertical tethers or space elevators, this is what would hold them vertical.

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I like the way you get to the point here also. If I had my way, this answer would float to the top of the stack. – uhoh Jul 14 at 2:34

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 '14 at 18:01
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This is not the answer to your question, LocalFluff. – David Hammen May 5 '14 at 18:11
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"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 '14 at 14:00
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This answer is wholly and totally incorrect. Gravity pulling is, exactly, completely different from a string pulling. – Joe Blow May 3 '15 at 11:38
    
This answer discusses how and why a satellite remains in orbit. It says nothing about its orientation, which is what the question is about. – Keith Thompson Jul 13 at 21:18

A projectile fired horizontally will fall to earth eventually under the influence of gravity and air friction. It will keep the same face towards the earth throughout its flight if it has not been given an initial spin. If the same projectile is fired in space with enough force that it falls to earth at the same rate as the earth is falling away, it will never fall to earth and will be "in orbit". Providing it has been given no initial spin then it will always keep the same face towards the earth throughout its orbit. Tidal and other force may disturb the orientation of the satellite and thrusters are needed to maintain the general orientation but the principle condition is to keep the same face towards the earth throughout the orbit.

The same applies to the moon. It is a projectile that is trying to go in a straight line. The earth's gravity pulls it into orbit and it must therefore keep the same face towards the earth.

There is nothing special about the moon's rotation about the earth and it has no magical spin that keeps the same face towards the earth.

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If a projectile is launched with no initial spin, it will (ignoring other forces acting on it) continue to point in the same direction with respect to the distant stars -- which means it won't keep the same orientation with respect to Earth. For a satellite in a 90-minute orbit to keep the same orientation with respect to Earth, it needs to rotate every 90 minutes in the proper direction. Your second paragraph explains why the Moon remains in orbit around the Earth, but says nothing about its rotation. The Moon keeps the same face toward Earth because of tidal effects, which you ignore. – Keith Thompson Jul 13 at 21:17
    
So you are saying that if the projectile was 100 metres long say, as it went through its orbit round the earth, it would start off parallel to the earth's surface at the equator, then travelling north, it's end would point at the North Pole, then it's other side would face the equator on the other side of the earth and so on. This cannot happen because the force that is pulling it into orbit is acting on the entire length of the projectile, pulling each part into the orbital curve. Therefore, one side would face the earth for its complete orbit without the need to give it a spin. – David Taylor Jul 14 at 13:09
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Suppose we have a satellite in a 90-minute equatorial orbit. Assume it's not spinning, and let's say it's pointed directly at Sirius. As it orbits around the Earth it continues to point at Sirius. Its absolute orientation is constant, but its orientation with respect to the Earth's surface changes. The force of gravity, which is what keeps it in orbit, affects its velocity, not its orientation. There is a tidal effect which probably tends to orient its long axis perpendicular to the Earth's surface, but that's minor. – Keith Thompson Jul 14 at 15:46
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The ISS keeps a "horizontal" orientation, with its long axis parallel to the surface, because (a) it's spinning with the same period as its orbital period, and (b) they carefully adjust its orientation using gyroscopes, and possibly thrusters. – Keith Thompson Jul 14 at 15:47
    
Keith, so you are saying that if someone were to fire a bullet from a gun from a satellite and the bullet took the same orbital path as the satellite, it would keep the same orientation as it completed a full circuit of the earth: ie it's tip would stay pointing at the same star all the way round it's orbit? That's not what projectiles do in their trajectories. The bullet is in free fall and will continue in orbit with the same side facing the earth. Does the ISS really spin? – David Taylor Jul 15 at 9:38

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