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Discussion below this answer involves my feeling that calling halo orbits in very asymmetric systems (e.g. Sun-Earth or Earth-Moon) as orbits around the primary, in 1:1 resonance with the secondary.

Thinking about this has led me to ask something specific:

Question: Is the Gateway's halo orbit bound to Earth? Would the Gateway remain in Earth orbit if the Moon disappeared but drift away from the Moon if the Earth disappeared?

For more on the Gateway's orbit, see

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  • $\begingroup$ This is not primarily opinion-based. All it requires is a straightforward calculation. Just math, no opinions. $\endgroup$
    – uhoh
    Jul 10 '19 at 13:02
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    $\begingroup$ I guess the close votes are because asking what would happen if the Earth or Moon just disappeared is completely hypothetical and counterfactual and "primarily opinion-based" is being used as a proxy for that. $\endgroup$ Jul 10 '19 at 15:08
  • $\begingroup$ @DavidRicherby that could be; that darn Add Comment button is getting so hard to push these days, perhaps it could use a little oil ;-) I'm using the hypothetical as a gedankenexperiment; as way to define "bound to Earth". I don't think the Moon will actually disappear of course; it's just a mathematical technique. $\endgroup$
    – uhoh
    Jul 10 '19 at 15:11
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No. The gateway is not bound to Earth, if we interpret "bound to Earth if the Moon disappears" as not drifting into interplanetary space regardless of what instance the Moon's gravitational influence is disregarded.

There's a neat property of zero-gain flybys: At the point of closest approach, the velocity vector of the spacecraft is parallel to the velocity vector of the secondary body.

A near-rectilinear halo orbit depends on performing zero-gain flybys regularly. From their flyby schedule, we can also see that the gateway must be orbiting around the Earth in the same direction as the Moon, which means that the velocity vectors are not merely parallel at closest approach, but also in the same direction.

They also state the close approaches can happen with an altitude of as little as 3000km, which means that the gateway is travelling faster than the lunar escape velocity at that altitude, 1439m/s.

Transforming the vector into geocentric coordinates is now trivial, as everything is travelling in the same direction. Addition!
With the velocity of the Moon added, the gateway is travelling ~2.5km/s relative to the Earth.

That's much higher than the escape velocity of Earth at a lunar distance, so if the Moon suddenly disappeared at that point in time, the gateway would fly off into interplanetary space.

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  • $\begingroup$ Is this true only at a limited number of positions along the NHRO? Or is it true at any point during the orbit? $\endgroup$ Aug 17 at 23:30
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    $\begingroup$ @AntonHengst This is only a part of the orbit. $\endgroup$ Aug 18 at 0:57

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