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?
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.
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.
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.
There's a term for this when it naturally occurs: Tidelocking.
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.
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.
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.
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?
Doubling radius doubles upward tug. Downward tug is cut to 1/4. Net acceleration is up.
And if we cut radius in half…
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:
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.
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.
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.
