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Those artificial satellites placed in the Lagrange points, like SOHO, are they orbiting the Sun or the Earth?

Or is it a satellite of both?

Joe Jobs
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Those orbits would not exist if the Earth did not exist. Those orbits also would not exist if the Sun did not exist. They are satellites of both bodies, as both bodies must exist for those Lagrange point pseudo-orbits to exist.

As an aside, SOHO et al. are not at the Lagrange points. The L1 and L2 Lagrange points are highly unstable. They are instead in pseudo-orbits about the L1 or L2 Lagrange points. These pseudo-orbits are also unstable, but require much less propellant for orbit maintenance than would be needed for staying exactly at one of those two Lagrange points.

In addition, there is an extreme benefit from being somewhat removed from those Lagrange points. An object at the Sun-Earth L1 point would require pointing an Earth-bound antenna straight toward the Sun, while an object at the Sun-Earth L2 point would require pointing a spacecraft antenna straight toward the Sun. Neither is a good idea.

David Hammen
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  • It is circling around the Sun but it is not circling around the Earth, is that right? – Joe Jobs Dec 08 '20 at 22:35
  • At L1, the antenna points straight away from the sun, no? – Camille Goudeseune Dec 09 '20 at 03:08
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    @JoeJobs, like everything else, it's circling around the sun. Technically, it also circles around the earth, once every 365 days. But there's no practical reason to call that orbiting the earth. – Camille Goudeseune Dec 09 '20 at 03:12
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    @CamilleGoudeseune -- I intentionally wrote "an Earth-bound antenna" regarding a satellite at Sun-Earth L1. While a satellite at L1 would indeed point straight away from the Sun to send a signal to the Earth, an antenna on the Earth intended to receive that signal would have to point straight toward the Sun. – David Hammen Dec 09 '20 at 08:11
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    @CamilleGoudeseune It goes around the Sun once per year. It goes around the Earth once per year. It is thus reasonably considered to be in orbit around both. It is not, however, in a Keplerian orbit. – John Doty Dec 09 '20 at 14:56
  • "Earth-bound" is unfortunately ambiguous. For example, are the signals it produces bound for earth or is the antenna physically at or near the earth's surface? I know which is the correct answer ; but a layperson might find it confusing. – zaTricky Dec 09 '20 at 16:32
  • Not sure I agree with these criteria “this orbit wouldn’t exist if Earth didn’t exist.” You could say the same thing about the L3, L4 and L5 points, but it seems slightly ridiculous to claim that objects there are in orbit around the earth. Likewise solar orbits that are in resonance with a planet. Are they in orbit around that planet also? Is this a standard/official astronomical definition? – RBarryYoung Dec 09 '20 at 17:17