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Name: Lewis S.
Status: other
Grade: other
Location: FL
Country: N/A
Date: 4/2/2005


Question:
Can a rotating object in earth orbit maintain axial alignment with the sun? For example: a rotating parabolic photonic collector, a space station, a solar observing satellite.


Replies:
Sort of, and for a while. The forces on an object in orbit are pretty weak, normally, but there are forces. Any drag created by whatever atmosphere is left at the altitude of the object could cause the object to rotate if its cross section is nonuniform. If the object is electrically conducting, there may be some current induced by motion through the Earth's magnetic field, and this current will produce a weak magnetic field that will tend to reorient the object.

But the biggest force is probably tidal. Imagine that the object is made of sand grains glued together, and then imagine that the glue disappears. Each grain of sand is now actually in its own orbit: those closer to Earth are in slightly tighter, faster orbits than the orbit of the object's center of mass, so they will not stay in place forever. The force that would have caused this motion is still in play, even though the object is *not* made of unglued sand grains.

The fact that the object is rotating makes the situation more complicated, but does not remove the forces tending to reorient the object.

Tim Mooney


Yes, there is no first-order effect trying to move the axis of rotation, so if it is pointed at the sun it will not take much maintenance energy to keep it pointed at the sun. But there are always drifts and weak higher-order effects, and the Earth's annual orbit around the sun changes the subjective direction of the sun at about 1 degree per day. So it will take some feedback mechanism determine the right direction and occasionally provide modest amounts of torque in that direction.

Of course, this means that the satellite does not get to keep any one face pointed steadily at earth below. This can give rise to satellites that are part-rotating, part-stationary.

Some of our tiny pico-satellites try to stay in "sun-synchronous orbit", always over the day/night line on the Earth below them. That way they are always in steady sunlight, and have an easier time maintaining a steady temperature. But I think orbit-corrections would be required to stay that way for very many days. The higher the orbit, the more margin you have for staying in sunlight.

Jim Swenson



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