Is it possible to have a Lunar "dusk-dawn" Sun-synchronous orbit, where a spacecraft can always be exposed to sunlight 24x7 over the terminator line?

  • $\begingroup$ See the unanswered question Is there a Lunar Sun-synchronous orbit over the terminator?. The comment there points back to Is a lunar Sun-synchronous orbit possible at the “frozen” inclination of 86°? and to Are sun-synchronous orbits possible around any body?. tl;dr: No. The moon rotates very slowly and so has a very small oblateness and therefore J₂. See en.wikipedia.org/wiki/Sun-synchronous_orbit#Technical_details for more about the use of J₂. $\endgroup$ – uhoh Jan 28 '18 at 13:12
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    $\begingroup$ As already noted, the Moon's $J_2$ is too small to support sun synchronous orbits. However, one of the advantages of the Near Rectilinear Halo Orbit (see this question) is that the Sun is hardly ever eclipsed by either the Moon or the Earth. $\endgroup$ – David Hammen Jan 28 '18 at 14:11
  • $\begingroup$ @uhoh Thanks. I had a look at the question that you've linked, but I found those answers a bit difficult to understand, and were quite ambiguous to an inexperienced person. I get it, so the answer is 'no'. $\endgroup$ – Aditya Radhakrishnan Jan 28 '18 at 16:48
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    $\begingroup$ @AdityaRadhakrishnan I understand what you mean! Take a look at my recent comment where I've tried to expand on the answer there - several other people were not sure either. Yes, the answer is indeed no. I'll try to post an additional, longer, simpler, step-by-step answer there tomorrow. Briefly, the reason Sun-synchronous orbits are close to polar is because precession around the oblate Earth is just too darn fast; about 38 days near the equator and 73 days for the ISS. Need to go close to polar to slow it down to 365 days. $\endgroup$ – uhoh Jan 28 '18 at 17:56
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    $\begingroup$ @AdityaRadhakrishnan but the moon is much closer to spherical, and so precession is about 10x slower. So only near-equatorial orbits can precess fast enough to complete one rotation in a year and still have a safe distance from the Moon's surface, and those are eclipsed by the moon for almost an hour every 2 hour period. $\endgroup$ – uhoh Jan 28 '18 at 18:01