Long rod in orbit

[GTOM]

Lets suppose we could place a long rod in orbit, at the distance of Moon.
Which would be the stabile position of the rod, perdendicular to the orbital path, or parallel to it?
I think parallel, to minimize tidal forces between the two opposite parts of the rod.

 

[FactChecker]

As a casual amateur, here are my two cents. Consider some extreme cases:
1) If the rod has the exact curvature of a circular orbit, parallel would be stable because every particle of the rod would have the same orbital speed, even if it was separated.
2) If the "rod" was separate pieces and vertical, the pieces would separate due to their different orbital velocities.
3) If the rod is solid, rather than separate pieces, and vertical, the ends at different orbital speeds start to rotate to a parallel position. Whether they would overshoot and oscillate or become a stable parallel bar is a harder question.

 

[Hornbein]

As a casual amateur, here are my two cents. Consider some extreme cases:
1) If the rod has the exact curvature of a circular orbit, parallel would be stable because every particle of the rod would have the same orbital speed, even if it was separated.
2) If the "rod" was separate pieces and vertical, the pieces would separate due to their different orbital velocities.
3) If the rod is solid, rather than separate pieces, and vertical, the ends at different orbital speeds start to rotate to a parallel position. Whether they would overshoot and oscillate or become a stable parallel bar is a harder question.

My rule of thumb is, if it can oscillate then it will.

 

[FactChecker]

My rule of thumb is, if it can oscillate then it will.

Good point! I guess there is no damping force, so that seems right.

 

[Bandersnatch]

A rod would try to orient itself radially, due to tidal forces, in the same way as tidally-deformed ellipsoids of planets and moons orient themselves with their long axes radially, towards the centre of the gravitational field. If there's oscillation, it's around that position.
Orbit-parallel/tangent is unstable to perturbations.

 

[FactChecker]

A rod would try to orient itself radially, due to tidal forces, in the same way as tidally-deformed ellipsoids of planets and moons orient themselves with their long axes radially, towards the centre of the gravitational field. If there's oscillation, it's around that position.
Orbit-parallel/tangent is unstable to perturbations.

I think I see why. (Please stop me if my amateur ideas are off the track.)
Suppose it starts parallel with orbital speed $V_o$, and there is a small perturbation which raises one end up. That end is at a higher altitude but still has speed $V_o$. Orbital speed decreases with altitude, so that end is going faster than its new orbital speed. Therefore, that end tends to "escape" orbit and go up more. That is unstable.
My original idea did not consider a perturbation from parallel.

 

[GTOM]

A rod would try to orient itself radially, due to tidal forces, in the same way as tidally-deformed ellipsoids of planets and moons orient themselves with their long axes radially, towards the centre of the gravitational field. If there's oscillation, it's around that position.
Orbit-parallel/tangent is unstable to perturbations.

And what makes that stabile? At the end farer from planet, bigger speed than lower end, but weaker gravity field. Wouldnt that rotate further until it oscillate around parallel position?

 

[Baluncore]

Lets suppose we could place a long rod in orbit, at the distance of Moon.

How long is the rod?
Is the rod straight or curved?
When it was placed in orbit, was it rotating once every month, so it always remains parallel with the Earth's surface, or was it aligned with a fixed star?

 

[GTOM]

A rod would try to orient itself radially, due to tidal forces, in the same way as tidally-deformed ellipsoids of planets and moons orient themselves with their long axes radially, towards the centre of the gravitational field. If there's oscillation, it's around that position.
Orbit-parallel/tangent is unstable to perturbations.

And what makes that stabile? At the end farer from planet, bigger speed than lower end, but weaker gravity field. Wouldnt that rotate further until it oscillate around parallel position?

How long is the rod?
Is the rod straight or curved?
When it was placed in orbit, was it rotating once every month, so it always remains parallel with the Earth's surface, or was it aligned with a fixed star?

Lets suppose km long rod, straight, aligned with Sun.