In a series of comments below For a given 48 hour observing period, what fraction of the celestial sphere is available to CHEOPS? there's been a discussion of that spacecraft's dawn-dusk Sun-synchronous orbit and it's orientation with respect to the Sun.

Trying to visualize it I first came up with four of them. The ascending node could be on the dawn side, or the dusk side ("RAAN" of 6 AM or 6 PM mean solar time, roughly speaking) and for each of those it could go on either side of the North pole.

For example if it's December, the North pole is in darkness, away from the Sun. starting from the ascending node it could go to the "back" (night) side of the pole or the "front" (day) side of it.

That makes four combinations, but then I realized that two of them would precess in the wrong direction and couldn't be Sun-synchronous.

Question: So are there still two different flavors of dawn-dusk Sun-synchronous orbits, or only one?


As you state, you can have a 6 AM descending node time or a 6 PM descending node time. Both will be 97ish inclinations, and both will be sun-synchronous. The other two quasi-polar orbits you mention (front vs back of pole) would be at 83ish inclination, and therefore not sun-sync.

And, of course, you can have an infinite number of altitudes for the two descending node times.

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