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I'm wondering how close a lunar orbit can get at periapsis to the south pole. I want an orbit that is sustainable for at least ~20 orbits. I'm working on a small sat concept for studying the lunar south pole. We want to get as close as possible to perform remote spectroscopy on the surface of the craters. Ideally, we'd like to get be in an eccentric orbit that doesn't require crazy amounts of delta v to maintain. We are planning on going to the moon via Rocketlab's photon stage, just to give you a sense of the propulsion capabilities we'll have. I set 20 orbits at the minimum but we'd ideally like to have 100 or more. I know the Lunar reconnaissance orbiter got within 20km but I'm wondering if closer approaches can be achieved. What are the main factors that limit how close a spacecraft can get? Is it the terrain variations, gravitational fluctuations, drag from the thin lunar "atmosphere", or something else?

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    $\begingroup$ Do you allow course corrections at apolune? A somewhat eccentric orbit would allow plenty of time to refine tracking between south pole passes, and very small delta-v corrections could be made. With a good enough gravitational model I think terrain would be the only limit. $\endgroup$ – Russell Borogove Oct 7 '20 at 2:41
  • $\begingroup$ as the previous comment points out limitations like terrain variations, gravitational fluctuations can be directly directly addressed by corrections. ~20 orbits is like two days roughly, if that's all you need then a practical mission could have plenty of propellant to use for that short of a time to do crazy things, then crash as soon as it runs out. In two days it would only sample two 26 degree wide wedges, so choosing those might be beneficial. I think the answer could be really really close if it can crash afterwards! $\endgroup$ – uhoh Oct 7 '20 at 4:22
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    $\begingroup$ Certainly 1 km, and I'd guess probably 100 meters above topography would be possible. It depends on how many "practice orbits" propellant allows before you start counting to 20. It would be great if you explained more about what you had in mind though. Thanks! $\endgroup$ – uhoh Oct 7 '20 at 4:22
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    $\begingroup$ @uhoh I'm working on a small sat concept for studying the lunar south pole. We want to get as close as possible to perform remote spectroscopy on the surface of the craters. Ideally, we'd like to get be in an eccentric orbit that doesn't require crazy amounts of delta v to maintain. We are planning on going to the moon via Rocketlab's photon stage, just to give you a sense of the propulsion capabilities we'll have. I set 20 orbits at the minimum but we'd ideally like to have 100 or more. $\endgroup$ – sgp45 Oct 7 '20 at 15:35
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    $\begingroup$ Wonderful, excellent, fun! I strongly recommend you click "edit" under your post and move all of those juicy details back into the question itself! 1) many people simply don't read comments, 2) editing your post bumps it back into the active queue increasing visibility. $\endgroup$ – uhoh Oct 7 '20 at 15:38

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