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?