Starship Asterisk*

Chris Peterson wrote: ↑Thu Sep 01, 2022 1:07 pm All of the angular momentum is in the same direction. Doesn't matter whether it's revolution or rotation. You'd need some kind of turbulence to create retrograde rotation, and an accretion disk probably isn't turbulent. So planets with retrograde rotation acquire that from interactions with other bodies once they form (they get their inclination shifted... basically swapping north and south).

MarkBour wrote: ↑Thu Sep 01, 2022 6:53 am

Chris Peterson wrote: ↑Wed Aug 31, 2022 6:58 pm Are you talking about protoplanetary disks? I don't think there's any mechanism by which planets could form with retrograde motion in an accretion disk.

Thanks for asking.

It certainly makes sense that really no planet would get a retrograde orbit, excepting some freakish occurrences. I'm just talking about why planets never seem to even get a retrograde rotation for the day/night.

Victor seemed to be clear on that, but I have yet to understand why it should almost never occur, if that is in fact true, beyond the example of our own solar system.

Chris Peterson wrote: ↑Wed Aug 31, 2022 6:58 pm Are you talking about protoplanetary disks? I don't think there's any mechanism by which planets could form with retrograde motion in an accretion disk.

MarkBour wrote: ↑Wed Aug 31, 2022 6:52 pm

VictorBorun wrote: ↑Thu Jul 28, 2022 6:32 pm the spin conservation law works like this.

When a bunch of massive things spin and travel without any external interaction, the sum of every spin along an axis is constant however you choose an axis to calculate the spins.
If a thing is travelling along a line, it has an orbital spin of mvR, where m is the mass, v is the velocity component perpendicular to your chosen axis and R is the distance between the line and your axis.
If a thing is spinning and the spin is parallel to your chosen axis, the spin along your chosen axis is mvr, where m is the mass, v is an effective linear velocity or just the linear velocity of the spinning in case the thing is a thin ring, and r is an effective radius or just the radius in case the thing is a thin ring. If the spin is at an angle with your chosen axis, only its component along your axis is counted as an investment in the total spin along your axis.

Both kinds of spins invest in the total spin of the bunch.

Example: Moon+Earth bunch is internally tidally exchanging spins, Earth slowing down to smaller mvr and Moon retreating to higher orbits with greater mvR.

A protoplanetary disk collapses to star (or several stars), planets, satellites and secondary satellites. All the collapsing progress only as much as the protoplanetary disk sheds its total spin

Well, this makes some sense. But I'm wondering -- if a planet in the solar disk were to rotate in the retrograde direction, would it not contribute equally to the total angular momentum calculation? Would its L value be negative to the others? That doesn't make sense to me; if I think about an arrangement of gears, it seems it would be a positive contribution.

Today I saw a new paper sponsored by NSF about the behavior of accreting disks.
https://iopscience.iop.org/article/10.3 ... ac62d5/pdf
It claims the results can be applied in a wider context, but the simulation particularly had the features to match planetary or proto-planetary disks around young stars. They did not discuss or mention the question that's troubling me here, but it does seem to give more insight into the mechanisms you've been trying to describe to me.

VictorBorun wrote: ↑Thu Jul 28, 2022 6:32 pm the spin conservation law works like this.

When a bunch of massive things spin and travel without any external interaction, the sum of every spin along an axis is constant however you choose an axis to calculate the spins.
If a thing is travelling along a line, it has an orbital spin of mvR, where m is the mass, v is the velocity component perpendicular to your chosen axis and R is the distance between the line and your axis.
If a thing is spinning and the spin is parallel to your chosen axis, the spin along your chosen axis is mvr, where m is the mass, v is an effective linear velocity or just the linear velocity of the spinning in case the thing is a thin ring, and r is an effective radius or just the radius in case the thing is a thin ring. If the spin is at an angle with your chosen axis, only its component along your axis is counted as an investment in the total spin along your axis.

Both kinds of spins invest in the total spin of the bunch.

Example: Moon+Earth bunch is internally tidally exchanging spins, Earth slowing down to smaller mvr and Moon retreating to higher orbits with greater mvR.

A protoplanetary disk collapses to star (or several stars), planets, satellites and secondary satellites. All the collapsing progress only as much as the protoplanetary disk sheds its total spin

VictorBorun wrote: ↑Thu Jul 21, 2022 7:30 pm But is it not true that the spin of a proto-disk is blocking gravitational collapses, and after some spin is shed to evaporating (i.e. partially to kicked-out clumps in the plane of the disk and partially to a pair of conical jets from every new star or a large planet), what finally forms is spinning at maximum, all in the same direction?

MarkBour wrote: ↑Thu Jul 21, 2022 5:59 pm Search me! I looked around on the Internet again for a while about planetary rotation.

The rationale for planets to generally rotate in the prograde direction (for their orbit) is not a topic I can find much discussed. Perhaps nobody knows why that would be the most likely scenario -- or why it might not be that likely in general, yet would have occurred in our solar system. Of course there is a tendency toward tidal locking, which would be slowly prograde, but for our system of planets, almost all of them are not at all near tidal locking.

I don't know if this pattern (spins are prograde) holds for most of the asteroids for which we can detect spin thus far. There is a reference to a study of this (ref in https://en.wikipedia.org/wiki/Retrograd ... ade_motion -- to Poznan Observatory), but it was hard to get to the data or any conclusions from this study.

I did find one general theory put forth by an Ian Miller at:
https://www.researchgate.net/post/Plane ... exceptions.
I would have to buy his eBook to go further into it. He summarizes a theory that as a planet forms, it accumulates mass from the surrounding gas (of the circumstellar disk) in a way that it picks up most of the mass on its leading side. And then he further theorizes that this will, for some reason, impart a prograde spin on the growing planet. I have no idea why, or whether or not other people are finding this theory reasonable.

So, in summary -- I have not found much in the way of explanation.

VictorBorun wrote: ↑Tue Jul 19, 2022 6:48 pm

MarkBour wrote: ↑Thu Jul 14, 2022 5:36 pm As to Saturn spinning fast, I know that it and Jupiter are similar, with Jupiter spinning even faster. A basic idea would be that they got it from their accretion and formation process. That's consistent with your suggestion, that angular momentum comes from what has fallen inward.

This one effect I've been thinking about in this thread about Phobos, would seem to induce a retrograde spin in the satellite. However, almost all of the planets in our solar system have a prograde spin in their orbits around the sun. I don't know how to make any sense of that fact.

On one hand, Jupiter is spinning faster
Equatorial rotation velocity = 12.6 km/s = Escape velocity 59.5 km/s * 0.212
than Saturn
Equatorial rotation velocity = 9.87 km/s = Escape velocity 35.5 km/s * 0.278

On the other hand a more massive giant like Jupiter would have taken longer to slow down from initial spin when Equatorial rotation velocity was Escape velocity.

On the other hand still Jupiter is more slowed by Sun's tides.

I am lost here. Is Saturn spun up by the ring shower? Was Saturn even more spun up at the event that created the ring?

MarkBour wrote: ↑Thu Jul 14, 2022 5:36 pm As to Saturn spinning fast, I know that it and Jupiter are similar, with Jupiter spinning even faster. A basic idea would be that they got it from their accretion and formation process. That's consistent with your suggestion, that angular momentum comes from what has fallen inward.

This one effect I've been thinking about in this thread about Phobos, would seem to induce a retrograde spin in the satellite. However, almost all of the planets in our solar system have a prograde spin in their orbits around the sun. I don't know how to make any sense of that fact.

VictorBorun wrote: ↑Thu Jul 14, 2022 3:46 am Saturn's moonlets in the rings are small in radius even doubled with their ridges so their spinning up by the rings' differential rotation would be tiny.
They should migrate to Saturn faster than the rings' particles, too.
Not sure they live long enough to spin up before they crash into Saturn.

Saturn itself is spinning fast. Was Saturn spinned up by its rings' and satellites' shower?

VictorBorun wrote: ↑Tue Jul 05, 2022 5:06 am

MarkBour wrote: ↑Mon Jul 04, 2022 7:58 pm

Capture1.png

I'm just looking at this image from the Wikipedia "Roche Limit" page and thinking about it intuitively (https://en.wikipedia.org/wiki/Roche_limit)
As the object approaches the Roche limit, its inner chunks are wanting to orbit angularly more rapidly than the rest, and its outer chunks are wanting to orbit angularly more slowly than the rest. If there is a period of time in which this is appreciable, but during which the body can still "hold it together", then I think it would induce a rotational motion. You might even have some chunks breaking loose, but then rolling along the surface in that direction as well.

Well, if there is a ring, with some differential rotation, and a large globe orbiting along the median circle of the ring, the globe does get some spinning up, though far from infinitely large. The friction will work just until the equatorial velocity of the globe catches up with the headwind/tailwind of the matter of the ring at orbits higher/lower by the radius of the globe.

Chris Peterson wrote: ↑Tue Jul 05, 2022 3:08 pm

johnnydeep wrote: ↑Tue Jul 05, 2022 3:00 pm

Chris Peterson wrote: ↑Tue Jul 05, 2022 2:17 pm
The Roche limit only applies to objects that are held together by their own gravity. It isn't some clearly defined distance from the parent. It's related to the structure of the satellite, to its shape, to its material strength, to its rotation. The nature of the tidal forces on an object don't change with distance.

So I guess there's no "tendency" for the lower orbit parts of bodies - whatever their structure - to "want" go faster than the higher orbit parts? (The rationale being that bodies in lower orbit travel faster.)

There is. But all that does is slightly alter the fixed orientation of a tidally locked body very slightly. It can't create any spin (except in a limited sense before the body becomes tidally locked).

johnnydeep wrote: ↑Tue Jul 05, 2022 3:00 pm

Chris Peterson wrote: ↑Tue Jul 05, 2022 2:17 pm

johnnydeep wrote: ↑Tue Jul 05, 2022 2:09 pm

So the idea that "As the object approaches the Roche limit, its inner chunks are wanting to orbit angularly more rapidly than the rest, and its outer chunks are wanting to orbit angularly more slowly than the rest." is irrelevant in that case? I would have thought that any such "twisting" forces would still be acting on any orbiting object regardless of its composition.

The Roche limit only applies to objects that are held together by their own gravity. It isn't some clearly defined distance from the parent. It's related to the structure of the satellite, to its shape, to its material strength, to its rotation. The nature of the tidal forces on an object don't change with distance.

So I guess there's no "tendency" for the lower orbit parts of bodies - whatever their structure - to "want" go faster than the higher orbit parts? (The rationale being that bodies in lower orbit travel faster.)

Chris Peterson wrote: ↑Tue Jul 05, 2022 2:17 pm

johnnydeep wrote: ↑Tue Jul 05, 2022 2:09 pm

Chris Peterson wrote: ↑Mon Jul 04, 2022 10:20 pm
A rigid body that is held together by forces much stronger than gravity doesn't have a Roche limit. I'd expect your rod to orbit in the perpendicular orientation forever, that being where it is tidally locked.

So the idea that "As the object approaches the Roche limit, its inner chunks are wanting to orbit angularly more rapidly than the rest, and its outer chunks are wanting to orbit angularly more slowly than the rest." is irrelevant in that case? I would have thought that any such "twisting" forces would still be acting on any orbiting object regardless of its composition.

The Roche limit only applies to objects that are held together by their own gravity. It isn't some clearly defined distance from the parent. It's related to the structure of the satellite, to its shape, to its material strength, to its rotation. The nature of the tidal forces on an object don't change with distance.

johnnydeep wrote: ↑Tue Jul 05, 2022 2:09 pm

Chris Peterson wrote: ↑Mon Jul 04, 2022 10:20 pm

johnnydeep wrote: ↑Mon Jul 04, 2022 8:08 pm

Hmm. What would happen to a 50 km carbon nanotube rod (assumed to be strong enough to resist being torn apart) oriented perpendicularly pointing to Mars and starting out in orbit at the Roche limit? Would it just rotate to be tangential to the orbit, overshoot a bit, then rotate back, etc., eventually settling in to a stable tangential orientation?

A rigid body that is held together by forces much stronger than gravity doesn't have a Roche limit. I'd expect your rod to orbit in the perpendicular orientation forever, that being where it is tidally locked.

So the idea that "As the object approaches the Roche limit, its inner chunks are wanting to orbit angularly more rapidly than the rest, and its outer chunks are wanting to orbit angularly more slowly than the rest." is irrelevant in that case? I would have thought that any such "twisting" forces would still be acting on any orbiting object regardless of its composition.

Chris Peterson wrote: ↑Mon Jul 04, 2022 10:24 pm

johnnydeep wrote: ↑Mon Jul 04, 2022 3:55 pm

Chris Peterson wrote: ↑Mon Jul 04, 2022 3:25 pm

Here's a different way of looking at it, less rigorous but maybe more intuitive. Escape velocity from the surface of Mars is about 5 km/s. That's the lowest speed a body that wasn't gravitationally bound to Mars could strike the surface (but most things would hit much faster). Escape velocity (with respect to Mars) at the height of Phobos is about 3 km/s. Basically, material from Phobos striking the surface will be impacting at a much lower speed. The speed of something already gravitationally bound to Mars. So it makes sense that there wouldn't be enough energy to eject material fast enough to exceed the much greater martian surface escape velocity.

Thanks. That helps some, but the orbital speed of Phobos averages 2.14 km/s (per Wikipedia). Wouldn't that increase the impact speed (and energy) over the 3 km/s escape velocity?

It's in free fall. Its orbital speed isn't a factor. Consider as well that this isn't a rigid body with high material strength. When it breaks up, it's going to be boulders and gravel. Which will mostly burn up without even reaching the ground, and all of which will be substantially reduced in speed by the atmosphere, even if the ground is reached.

Chris Peterson wrote: ↑Mon Jul 04, 2022 10:20 pm

johnnydeep wrote: ↑Mon Jul 04, 2022 8:08 pm

MarkBour wrote: ↑Mon Jul 04, 2022 7:58 pm

Capture1.png

I'm just looking at this image from the Wikipedia "Roche Limit" page and thinking about it intuitively (https://en.wikipedia.org/wiki/Roche_limit)

As the object approaches the Roche limit, its inner chunks are wanting to orbit angularly more rapidly than the rest, and its outer chunks are wanting to orbit angularly more slowly than the rest. If there is a period of time in which this is appreciable, but during which the body can still "hold it together", then I think it would induce a rotational motion. You might even have some chunks breaking loose, but then rolling along the surface in that direction as well.

My intuition, which is certainly a poor guide here, is that for a fluid satellite, it would not happen. The fluid would just stretch into an ellipsoid and slightly rotate forward, but only less than a single visible rotation of the "body". and then it would begin to disperse (exactly like this image).

But for a rubble pile, I have even more trouble guessing.

I wonder if someone has recorded an experiment that has found a way to show the details at the end before disintegration, or if some numerical simulation has worked it out with satisfactory conclusiveness.

The only example observation I know of is Comet Shoemaker–Levy 9, also discussed in that same Wikipedia article, but not much could be concluded from that example in regard to the fate of Phobos. If someone had a lot of money, they could easily enough set this up orbiting a few small satellites around the Moon. Of course, I doubt anyone is going to do that as a purposeful experiment. But maybe, with lots of varied activity up there, some related observations will be part of life in cislunar space.

Hmm. What would happen to a 50 km carbon nanotube rod (assumed to be strong enough to resist being torn apart) oriented perpendicularly pointing to Mars and starting out in orbit at the Roche limit? Would it just rotate to be tangential to the orbit, overshoot a bit, then rotate back, etc., eventually settling in to a stable tangential orientation?

A rigid body that is held together by forces much stronger than gravity doesn't have a Roche limit. I'd expect your rod to orbit in the perpendicular orientation forever, that being where it is tidally locked.

MarkBour wrote: ↑Mon Jul 04, 2022 7:58 pm

Capture1.png

I'm just looking at this image from the Wikipedia "Roche Limit" page and thinking about it intuitively (https://en.wikipedia.org/wiki/Roche_limit)
As the object approaches the Roche limit, its inner chunks are wanting to orbit angularly more rapidly than the rest, and its outer chunks are wanting to orbit angularly more slowly than the rest. If there is a period of time in which this is appreciable, but during which the body can still "hold it together", then I think it would induce a rotational motion. You might even have some chunks breaking loose, but then rolling along the surface in that direction as well.

johnnydeep wrote: ↑Mon Jul 04, 2022 3:55 pm

Chris Peterson wrote: ↑Mon Jul 04, 2022 3:25 pm

johnnydeep wrote: ↑Mon Jul 04, 2022 2:43 pm

Thanks for the formatting effort it took to explain that, but I'm afraid I'll need to read it about a dozen more times while simultaneously referencing Wikipedia. It's been over 40 years since I took high school physics. :-(

Here's a different way of looking at it, less rigorous but maybe more intuitive. Escape velocity from the surface of Mars is about 5 km/s. That's the lowest speed a body that wasn't gravitationally bound to Mars could strike the surface (but most things would hit much faster). Escape velocity (with respect to Mars) at the height of Phobos is about 3 km/s. Basically, material from Phobos striking the surface will be impacting at a much lower speed. The speed of something already gravitationally bound to Mars. So it makes sense that there wouldn't be enough energy to eject material fast enough to exceed the much greater martian surface escape velocity.

Thanks. That helps some, but the orbital speed of Phobos averages 2.14 km/s (per Wikipedia). Wouldn't that increase the impact speed (and energy) over the 3 km/s escape velocity?

MarkBour wrote: ↑Mon Jul 04, 2022 7:58 pm

Chris Peterson wrote: ↑Mon Jul 04, 2022 1:09 pm

MarkBour wrote: ↑Mon Jul 04, 2022 3:03 am I wonder, if, as it approaches its Roche limit, will it perhaps begin to rotate?

Why would it? Once it starts to break up the individual pieces will assume their own rotations, with the total angular momentum conserved, of course.

Capture1.png

I'm just looking at this image from the Wikipedia "Roche Limit" page and thinking about it intuitively (https://en.wikipedia.org/wiki/Roche_limit)

As the object approaches the Roche limit, its inner chunks are wanting to orbit angularly more rapidly than the rest, and its outer chunks are wanting to orbit angularly more slowly than the rest. If there is a period of time in which this is appreciable, but during which the body can still "hold it together", then I think it would induce a rotational motion. You might even have some chunks breaking loose, but then rolling along the surface in that direction as well.

My intuition, which is certainly a poor guide here, is that for a fluid satellite, it would not happen. The fluid would just stretch into an ellipsoid and slightly rotate forward, but only less than a single visible rotation of the "body". and then it would begin to disperse (exactly like this image).

But for a rubble pile, I have even more trouble guessing.

I wonder if someone has recorded an experiment that has found a way to show the details at the end before disintegration, or if some numerical simulation has worked it out with satisfactory conclusiveness.

The only example observation I know of is Comet Shoemaker–Levy 9, also discussed in that same Wikipedia article, but not much could be concluded from that example in regard to the fate of Phobos. If someone had a lot of money, they could easily enough set this up orbiting a few small satellites around the Moon. Of course, I doubt anyone is going to do that as a purposeful experiment. But maybe, with lots of varied activity up there, some related observations will be part of life in cislunar space.

johnnydeep wrote: ↑Mon Jul 04, 2022 8:08 pm

MarkBour wrote: ↑Mon Jul 04, 2022 7:58 pm

Chris Peterson wrote: ↑Mon Jul 04, 2022 1:09 pm

Why would it? Once it starts to break up the individual pieces will assume their own rotations, with the total angular momentum conserved, of course.

Capture1.png

I'm just looking at this image from the Wikipedia "Roche Limit" page and thinking about it intuitively (https://en.wikipedia.org/wiki/Roche_limit)

As the object approaches the Roche limit, its inner chunks are wanting to orbit angularly more rapidly than the rest, and its outer chunks are wanting to orbit angularly more slowly than the rest. If there is a period of time in which this is appreciable, but during which the body can still "hold it together", then I think it would induce a rotational motion. You might even have some chunks breaking loose, but then rolling along the surface in that direction as well.

My intuition, which is certainly a poor guide here, is that for a fluid satellite, it would not happen. The fluid would just stretch into an ellipsoid and slightly rotate forward, but only less than a single visible rotation of the "body". and then it would begin to disperse (exactly like this image).

But for a rubble pile, I have even more trouble guessing.

I wonder if someone has recorded an experiment that has found a way to show the details at the end before disintegration, or if some numerical simulation has worked it out with satisfactory conclusiveness.

The only example observation I know of is Comet Shoemaker–Levy 9, also discussed in that same Wikipedia article, but not much could be concluded from that example in regard to the fate of Phobos. If someone had a lot of money, they could easily enough set this up orbiting a few small satellites around the Moon. Of course, I doubt anyone is going to do that as a purposeful experiment. But maybe, with lots of varied activity up there, some related observations will be part of life in cislunar space.

Hmm. What would happen to a 50 km carbon nanotube rod (assumed to be strong enough to resist being torn apart) oriented perpendicularly pointing to Mars and starting out in orbit at the Roche limit? Would it just rotate to be tangential to the orbit, overshoot a bit, then rotate back, etc., eventually settling in to a stable tangential orientation?

MarkBour wrote: ↑Mon Jul 04, 2022 7:58 pm

Chris Peterson wrote: ↑Mon Jul 04, 2022 1:09 pm

MarkBour wrote: ↑Mon Jul 04, 2022 3:03 am I wonder, if, as it approaches its Roche limit, will it perhaps begin to rotate?

Why would it? Once it starts to break up the individual pieces will assume their own rotations, with the total angular momentum conserved, of course.

Capture1.png

I'm just looking at this image from the Wikipedia "Roche Limit" page and thinking about it intuitively (https://en.wikipedia.org/wiki/Roche_limit)

As the object approaches the Roche limit, its inner chunks are wanting to orbit angularly more rapidly than the rest, and its outer chunks are wanting to orbit angularly more slowly than the rest. If there is a period of time in which this is appreciable, but during which the body can still "hold it together", then I think it would induce a rotational motion. You might even have some chunks breaking loose, but then rolling along the surface in that direction as well.

My intuition, which is certainly a poor guide here, is that for a fluid satellite, it would not happen. The fluid would just stretch into an ellipsoid and slightly rotate forward, but only less than a single visible rotation of the "body". and then it would begin to disperse (exactly like this image).

But for a rubble pile, I have even more trouble guessing.

I wonder if someone has recorded an experiment that has found a way to show the details at the end before disintegration, or if some numerical simulation has worked it out with satisfactory conclusiveness.

The only example observation I know of is Comet Shoemaker–Levy 9, also discussed in that same Wikipedia article, but not much could be concluded from that example in regard to the fate of Phobos. If someone had a lot of money, they could easily enough set this up orbiting a few small satellites around the Moon. Of course, I doubt anyone is going to do that as a purposeful experiment. But maybe, with lots of varied activity up there, some related observations will be part of life in cislunar space.

Chris Peterson wrote: ↑Mon Jul 04, 2022 1:09 pm

MarkBour wrote: ↑Mon Jul 04, 2022 3:03 am I wonder, if, as it approaches its Roche limit, will it perhaps begin to rotate?

Why would it? Once it starts to break up the individual pieces will assume their own rotations, with the total angular momentum conserved, of course.