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"Halo" Orbit


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1 hour ago, Cheif Operations Director said:

Is it possible to have a craft in space orbit the pole of a planet instead of go al the way around? I doubt it but I'm curious

No, you need tangential velocity in order to not fall back down. If it's hovering there it would need to produce some kind of force to counter gravity.

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On 7/22/2018 at 6:00 PM, UmbralRaptor said:

As in hover over the pole? It's not an orbit, but cleverly exploiting things that aren't gravity can help: https://en.wikipedia.org/wiki/Statite

Oooh, Rocheworld fan, eh?

Statites are cool and all, but you have to be fairly far out from the parent body for them to actually work.

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On 7/22/2018 at 9:09 PM, Bill Phil said:

A highly eccentric polar orbit where a large portion of the time is above the pole could work.

Basically, A polar Kolniya orbit.   With a few satellites, and a high enough Ap, it would almost appear to a ground station to be a stationary relay sat. 

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5 hours ago, MaverickSawyer said:

Statites are cool and all, but you have to be fairly far out from the parent body for them to actually work.

How is that so? One of the advantages of Statites is that hey work at any altitude...?

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1 hour ago, Xd the great said:

You still need a force, provided by the solar sail.

Basically, light hit the sail, produce force, lifts stuff upward.

I know, and light hits the sail even if you are close to the parent body...

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17 minutes ago, p1t1o said:

I know, and light hits the sail even if you are close to the parent body...

The thrust generated by light sail is very low compared to its mass. At low altitude, Earth's gravity will easily overpower it. You have to be far enough from Earth so that the gravity is weak enough for light sail to keep the craft suspended.

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5 hours ago, Gargamel said:

Basically, A polar Kolniya orbit.   With a few satellites, and a high enough Ap, it would almost appear to a ground station to be a stationary relay sat. 

Yeah.

Kolniya? You mean Molniya? KSP really does get into our heads.

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Sure you can. Just flip the planet's axis 90° so the pole faces one of the Lagrange points and then put a satellite there. It won't last very long since the planet's pole won't face the star all the time but it would stay over the pole for a while.

BTW does anyone know if you can have a tidally locked planet that spins kind of like Uranus but the pole faces the star all the time? I'd imagine it would be very volcanic or at least behave like Triton with all the surface cracks and geysers.

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12 minutes ago, Wjolcz said:

BTW does anyone know if you can have a tidally locked planet that spins kind of like Uranus but the pole faces the star all the time? I'd imagine it would be very volcanic or at least behave like Triton with all the surface cracks and geysers.

It would precess like hell.

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11 minutes ago, Wjolcz said:

Sure you can. Just flip the planet's axis 90° so the pole faces one of the Lagrange points and then put a satellite there. It won't last very long since the planet's pole won't face the star all the time but it would stay over the pole for a while.

BTW does anyone know if you can have a tidally locked planet that spins kind of like Uranus but the pole faces the star all the time? I'd imagine it would be very volcanic or at least behave like Triton with all the surface cracks and geysers.

If the axis of rotation was pointing towards the star, and the pole tidally locked, the axis would have to change orientation as the planet orbited its star.

This requires a truly massive change in angular momentum, so no, this planet would very strongly resist being tidally locked.

If the gravity environment was strong enough, and the planet materially strong enough to withstand it (doubtful if this is physically possible due to extreme energies involved) then energy would very quickly be sapped from the system and the planet would fall into the sun.

Edited by p1t1o
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1 hour ago, p1t1o said:

If the axis of rotation was pointing towards the star, and the pole tidally locked, the axis would have to change orientation as the planet orbited its star.

This requires a truly massive change in angular momentum, so no, this planet would very strongly resist being tidally locked.

If the gravity environment was strong enough, and the planet materially strong enough to withstand it (doubtful if this is physically possible due to extreme energies involved) then energy would very quickly be sapped from the system and the planet would fall into the sun.

Tidal locking can only happen in the first place by slowing the rotation of an orbiting body until its rotational period matches its orbital period. A tidally-locked body, by definition, has a rotational axis (and thus a pole) perpendicular to its orbital plane. Tidal locking should be thought of not as a continual force applied to the satellite by its parent body; it's the end result of tidal forces gradually bleeding off angular momentum until the rotational period and the orbital period match. It's the lowest-energy state for the system.

If you smoothed out the Moon's internal structure so there were no bulges whatsoever for Earth's tidal gradient to "lock" onto, it would still retain its current one-month rotational period and would still appear tidally locked.

Similarly, if the Earth suddenly disappeared, our moon would continue to orbit the sun in much the same trajectory, with the exact same one-month rotational period.

A world which somehow was initially rotating around its star with a rotational axis parallel to its orbital plane (by the way, this sort of thing would be EXCELLENT evidence against a deterministic/naturalistic formation of a solar system) would be subject to the same tidal forces as a star with a rotational axis perpendicular to its orbital plane. However, those forces would produce a torque perpendicular to the rotational axis, rather than producing a "dragging" effect as with a perpendicularly-rotating satellite. This torque would be greatest at periapse and lowest at apoapse, and would induce ever-increasing precession of the axis at each periapse. This precession would tend to pull the rotational axis up out of the orbital plane until it stabilized in a perpendicular state, at which point tidal forces would complete the process of slowing down the rotation to tidally lock.

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38 minutes ago, Vanamonde said:

To get an intuitive understanding of why this can't work, fire up KSP and try to put a satellite there. 

Should be go-to advice for most astronomical questions ;)

 

1 hour ago, sevenperforce said:

<snikt>

Question - if you had a hypothetically perfect sphere of uniform mass distribution, rotating on a vertical axis, orbiting a body.

This body could *not* become tidally locked?

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17 hours ago, Bill Phil said:

Kolniya? You mean Molniya? KSP really does get into our heads.

Dang it.. Yeah... I first heard it as Kolniya, and that stuck.   Explains why I was having trouble googling that for the spell check....

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23 hours ago, Vanamonde said:

To get an intuitive understanding of why this can't work, fire up KSP and try to put a satellite there. 

Can you even do a halo orbit in KSP?  I suspect an iterative process where if you find yourself beating against the limits of KSP you fire up RSS, and if that doesn't work include principia (there are reasons you might not want principia in your RO/RSS stack).

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23 hours ago, p1t1o said:

Question - if you had a hypothetically perfect sphere of uniform mass distribution, rotating on a vertical axis, orbiting a body.

This body could *not* become tidally locked?

Tidal gradients pull harder on the near side of the satellite than on the far side of the satellite, causing the satellite to stretch along the vector pointing toward the primary. If the rotation period is greater than the orbital period, then the rotation will rotate the ellipse out of alignment with that vector, and so those same tidal forces will exert a torque on the ellipse which pushes it back toward alignment.

So a perfect, rigid sphere of uniform mass distribution would not become tidally locked, as long as it remains a perfect sphere. If the body is not rigid, however, it will become tidally locked.

A spherically asymmetric body like our Moon already has bulges, which both increase the amount of torque (hastening the locking process) and provide a lowest-energy state that dictates the final orientation of the body. A nonrigid perfectly spherical, uniform satellite would eventually become tidally locked but there would be no particular preferential orientation (e.g., no particular region facing toward the primary). The Moon's orientation toward Earth is fixed by its collection of bulges.

1 hour ago, wumpus said:

Can you even do a halo orbit in KSP?  I suspect an iterative process where if you find yourself beating against the limits of KSP you fire up RSS, and if that doesn't work include principia (there are reasons you might not want principia in your RO/RSS stack).

You cannot do a halo orbit but you can do an effective halo orbit by placing a spacecraft just outside a moon's ROI but at an identical orbital period along its orbital path. You can even throw in some inclination if you want the "halo" oscillation effect. 

Granted, you're next to the moon rather than behind it, but the result is the same.

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