Gaia Space Telescope and Lagrangian Point 2

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Discussion Overview

The discussion centers around the behavior of the Gaia Space Telescope at the Lagrangian Point 2 (L2), particularly addressing misconceptions about its motion and the effects of various gravitational influences. Participants explore theoretical aspects of L2, including stability, potential wells, and the dynamics involved in maintaining a satellite's position in this region.

Discussion Character

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

Main Points Raised

  • One participant expresses confusion about the animation of Gaia orbiting L2, believing it should remain stationary at that point.
  • Another participant notes that several satellites, including Gaia, are already in orbit around L2, indicating that there is space for more satellites like the Webb telescope.
  • Discussion includes the idea that if a satellite is placed exactly at L2 without disturbances, it could theoretically remain there, but practical factors like Coriolis acceleration and the need for thruster corrections complicate this.
  • One participant questions the effect of the Moon and other celestial bodies on the stability of satellites at L2, acknowledging these as disturbances.
  • There is a debate about the representation of effective potential in diagrams, with one participant asserting that the diagram does not account for the Coriolis effect and does not represent spacetime accurately.
  • A participant seeks clarification on the existence of a net potential well at L2 and whether it resembles the force field surrounding a satellite, questioning the size of such a satellite.
  • Another participant challenges the notion of a local minimum in potential at L2, stating that the Coriolis force is velocity-dependent and cannot be expressed as a potential.

Areas of Agreement / Disagreement

Participants express varying degrees of understanding and interpretation regarding the dynamics at L2, with no consensus on the nature of the potential well or the effects of external bodies like the Moon. The discussion remains unresolved on several technical points.

Contextual Notes

Limitations include the lack of consensus on the stability of L2, the influence of external gravitational forces, and the interpretation of diagrams representing potential and forces at L2.

Johnnyallen
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I'm confused (what else is new) about L2.
While watching a video from PBS Digital Spacetime about the latest data drop from Gaia Space Telescope, Matt O'Dowd showed a CGI animation of the telescope leaving Earth then circling/orbiting L2 perpendicular to the Earth/sun plane.
I thought that the craft would remain stationary at L2. That didn't seem to be the case in the animation.
Why is this?
BTW I'm just an old retired guy who is fascinated by this stuff and never studied Physics in college. This PBS Spacetime series is awesome and very accessible (usually)
 
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There are already several satellites in the L2 orbit, including the Wilkinson Microwave Anisotropy Probe,the Gaia Space Telescope and the Herschel and Planck space observatories. But there's plenty of room for another neighbor, and the Webb telescope will be heading out to L2 in the near future (2021).
 
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Johnnyallen said:
I thought that the craft would remain stationary at L2.
If you placed it exactly there, and there were no disturbances:
https://en.wikipedia.org/wiki/Lagrangian_point#Stability

Below is the effective potential in the rotating rest frame of Sun and Earth. If you place something approximately at a Lagrangian point it can go "downhill" in various directions. But the Coriolis acceleration tries to make it go in circles, so it stays in the vicinity of the Lagrangian point. And some probes can make small corrections with their thrusters.

330px-Lagrangian_points_equipotential.jpg
 

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A.T. said:
If you placed it exactly there, and there were no disturbances:
https://en.wikipedia.org/wiki/Lagrangian_point#Stability

Below is the effective potential in the rotating rest frame of Sun and Earth. If you place something approximately at a Lagrangian point it can go "downhill" in various directions. But the Coriolis acceleration tries to make it go in circles, so it stays in the vicinity of the Lagrangian point. And some probes can make small corrections with their thrusters.

View attachment 236765
This is very helpful. Thanks.
Next question: Does the Moon have any effect?
 
Johnnyallen said:
Does the Moon have any effect?
That's one of the disturbances. There are also the other planets, the eccentricity of the Earth's orbit, etc.
 
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A.T. said:
If you placed it exactly there, and there were no disturbances:
https://en.wikipedia.org/wiki/Lagrangian_point#Stability

Below is the effective potential in the rotating rest frame of Sun and Earth. If you place something approximately at a Lagrangian point it can go "downhill" in various directions. But the Coriolis acceleration tries to make it go in circles, so it stays in the vicinity of the Lagrangian point. And some probes can make small corrections with their thrusters.

View attachment 236765
This attachment was very helpful. It's a good representation of Spacetime in 3 dimensions. Given this, this would explain why the CGI of the Gaia orbit (in the PBS series) is or can be perpendicular to the Earth/sun plane.
Am I correct in this assessment?
 
Johnnyallen said:
This attachment was very helpful. It's a good representation of Spacetime in 3 dimensions.
That isn't spacetime. It is the effective potential in 2 space dimensions.

Johnnyallen said:
Given this, this would explain why the CGI of the Gaia orbit (in the PBS series) is or can be perpendicular to the Earth/sun plane.
The diagram doesn't show the spatial dimension perpendicular to the Earth's orbital plane. And it shows only the potential, not the Coriolis effect.
 
I find this discussion very helpful. But, still, in the neighborhood of L2, there must be a net potential well to hold an object. Is there a similar illustration of just L2 and its force field? It seems to me that it must be similar to the force field that would surround a real satellite at that position? How big would such a satellite be?
 
Bill McKeeman said:
But, still, in the neighborhood of L2, there must be a net potential well to hold an object.
The net potential doesn't have a local minimum there, and the Coriolis force cannot be expressed as a potential, because it is velocity dependent, not position dependent.
 

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