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I'm examining entering lunar orbit through the Earth-Moon L1 point with low-thrust propulsion. As part of this, I'm interested in all examples of spacecraft (both actually flown and ones that didn't make it past planning phases) that have entered lunar orbit by passing through L1. And by "passing through", I mean that the unique dynamics of that region played a significant factor in the trajectory and subsequent lunar capture, not spacecraft that have engaged in Hohmann-like transfers and happen to have passed near L1 as a matter of course.

While I am specifically interested in low-thrust propulsion, I think it would be instructive to see any high-thrust spacecraft that also used a low-energy transfer through L1, "threading the L1 needle" I believe I've seen it called, if any exist.

I've found several examples of spacecraft that entered highly elliptical orbits and used three-body dynamics to perform a low-energy transfer to lunar orbit, presumably "from the back" through L2. Examples include the GRAIL spacecraft and the upcoming Lunar IceCube CubeSat that will piggyback off of the Artemis I mission. These are not what I'm looking for, I'm specifically looking for trajectories through L1.

If I understand correctly, SMART-1 performed such a transfer, and it's low-thrust to boot. So far, that's been the only one I could find. Any others?

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  • $\begingroup$ The Earth-Moon L1 point is not a keyhole in the sense that that is the only way to go to the Moon. With a high thrust vehicle, it is possible to simply go toward the Moon. Applying enough delta-V will place a vehicle on a translunar trajectory, and then later applying more delta-V will place the vehicle on a lunar orbit (or lunar entry trajectory). The optimal burn-coast-burn trajectory will inevitable take the vehicle far from the line between the Earth and the Moon, and hence far from the Earth-Moon L1 or Earth-Moon L2 points. $\endgroup$ Feb 25, 2022 at 7:44
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    $\begingroup$ @uhoh "Going from LEO to lunar..." is definitely related, that's what I'm hoping to do. I was going to ask a version of that question until I saw it had already been asked, hence my current scope of asking which spacecraft have done it already. The answer there also mentions SMART-1, but doesn't indicate whether this is the only spacecraft to have ever done this and was answered nearly four years ago, so I wanted to see whether the list either was or had grown longer. $\endgroup$ Feb 25, 2022 at 16:51
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    $\begingroup$ @uhoh, for the CAPSTONE posts, CAPSTONE will initially be launched on a low-energy ballistic lunar transfer, and thus will not be performing a multi-spiral, extended low-thrust maneuver like I was trying to get at with my question. Although it does look like its thrusters that it will use for final orbit insertion would count as low-thrust. $\endgroup$ Feb 25, 2022 at 16:53
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    $\begingroup$ @DavidHammen I hadn't realized that keyhole has a specific definition already in astrodynamics, I'll use that term more correctly from now on. $\endgroup$ Feb 25, 2022 at 16:54

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