Curious about gravitational sluing detail

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Discussion Overview

The discussion revolves around the dynamics of a rock approaching a moon or planet, particularly focusing on the effects of gravitational slingshot maneuvers on the rock's rotation and angular momentum. Participants explore the implications of the rock's initial state of non-rotation and how its trajectory and interaction with a larger body might influence its motion and orientation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants question whether a gravitational encounter would impart any spinning or tumbling motion to the rock, given its initial state of non-rotation.
  • Others argue that the rock's perspective during the maneuver would not reveal any rotation, as it maintains alignment with its orbital path.
  • A participant suggests that for the rock to rotate synchronously while on a hyperbolic orbit, its angular momentum would need to be altered continuously during the approach.
  • Another viewpoint emphasizes that tidal forces could theoretically affect the rock's rotation, but the effects would likely be negligible unless extreme conditions are present.
  • Some participants note that the shape and density of the rock are critical factors, with non-uniform shapes potentially experiencing torque and changing rotation, while a perfectly spherical rock would not.
  • There is a discussion about the implications of maintaining axial alignment versus keeping the same face toward the planet during the slingshot maneuver.

Areas of Agreement / Disagreement

Participants express differing views on whether the rock would experience any change in rotation due to the gravitational encounter. While some believe that no additional angular momentum results from the slingshot, others propose that factors such as shape and tidal forces could influence the outcome. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants acknowledge that the initial conditions, such as the rock's shape and the gravitational characteristics of the bodies involved, play a significant role in determining the outcome of the encounter. There are also discussions about the definitions of rotation and the frames of reference used in the analysis.

  • #31
mfb said:
It can change if the rock does not have a spherical symmetry

Yes, I know, if the rock is not perfectly spherically symmetric. Thats why I went back and edited the post and added "negligible tidal forces".

Besides, the amount of change in rotation of a non-symmetric rock would be extremely small. The poster in post #2 thought that the object would align with the path of deflection. That means in fig. 1 the object (whether symmetric or not) would have rotated several degrees. I was trying to correct that. Now I guess the point is hopelessly lost, but thanks anyway.

mfb said:
as has been mentioned before.

Honestly. Did you really think that was necessary? For once I wish we could just stick to physics.
Why do you do that?
 
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  • #32
MikeGomez said:
but they did not rotate to align themselves to the path.
I think we talk about different orientations here. Baluncore was asking about the "mutual orientation" of the two test masses. I assume he means the orientation of the line connecting the two test masses. You are talking about the individual orientations of each test mass.

If the distance changes then the orientation of the connecting line can change too.
 
  • #33
D H said:
There's no need to invoke general relativity here.
...
For now, assume the planet is a rogue planet, i.e., it's not orbiting a star. After the encounter is over, the object's linear velocity will have changed but it's speed will be the same as it was before the encounter began. That's because the approach and departure are symmetrical with respect to translation.

There is no such symmetry for rotation. The object's orientation and angular velocity prior to and after the encounter are not conserved quantities.

That the OP specifically asked about a planet makes this even more complicated. The encounter can now transfer linear and angular momentum to the object.

You think a symmetrical object would pick up any spin if slingshot dramatically around the periphery of a black hole?

And... you lost me at "but it's speed will be the same as it was before". It's well known that NASA employs the maneuver to boost a craft's speed.
 
  • #34
bowlegged said:
And... you lost me at "but it's speed will be the same as it was before". It's well known that NASA employs the maneuver to boost a craft's speed.
In the reference frame of the planet, the speed is unchanged. In another reference frame (like the sun's), where the planet is moving, the speed can change.

MikeGomez said:
Besides, the amount of change in rotation of a non-symmetric rock would be extremely small.
I'm not so sure about that, it would be interesting to find an upper limit.

Did you really think that was necessary?
If the same wrong things get repeated? I think it is useful.
 
  • #35
MikeGomez said:
Besides, the amount of change in rotation of a non-symmetric rock would be extremely small.
Not true. It's not all that hard to get an angular acceleration on the order of 10-6 radians/second2 at closest approach in the scenario described in the OP. That doesn't sound like much, but keep in mind that the Earth's rotation rate is on the order of 10-4 radians/second, and that the Moon's is on the order of 10-6 radians/second. An angular acceleration of 10-6 radians/second2 is pretty large given the slow rotation rate of a typical space rock.
 

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