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This answer mentions that (optical) spy satellites are often in elliptical orbits, and when I look here sure enough the orbits are elliptical, with periapsis altitudes around 250 to 400 km, and apoapsis altitudes around 400 to 1000 km. I'm guessing it is some kind of compromise involving resolution limit by distance and orbit decay, maybe even atomic oxygen, but why would a satellite with a minimum altitude of 400km still go up to 1000 km every orbit instead of being circular?

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uhoh
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    Probably adds 'delay' to keep it sun-synchronous. – BobT Feb 10 '17 at 20:31
  • @BobT sounds very plausible. I'll look at some TLEs next time I get a chance, unless someone else does (and leaves an answer). Thanks for the clue! – uhoh Feb 10 '17 at 20:55
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    I remember reading somewhere that in some cases, it is so that the satellite passes over a certain region consistently, rather than a circular polar orbit, which will eventually pass over everywhere but take a long time to do so. I don't have a source for this, but I think I read it somewhere on this SE. – Cody Feb 10 '17 at 23:48
  • The eccentricity of an orbit has nothing to do with its sun-synchonicity. – Erik Feb 11 '17 at 02:43
  • @Erik It does if you want to also specify the periapsis. Spy satellites want to be close to the ground when they snap their photos. So if you demand a periapsis that gives you an altitude of 250 km, and you also demand synchrony, you arrive at a few choices for eccentricity. – uhoh Feb 11 '17 at 03:21
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    If they want low periapsis, and it being over a certain area consistently, then a semi-synchronous orbit is a must. – SF. Feb 11 '17 at 06:59
  • A sun-synchonous orbit precesses by using the non-spherical geometry of the earth. It is entirely independent of eccentricity and almost entirely dependent on inclination. Now, if you want the orbit to be over a certain area at a certain time, that is an entirely new constraint. – Erik Feb 11 '17 at 07:27
  • @Erik your sentence is correct. But a sun-synchronous orbit that also has to go down to 250 km part of the time but also remain in space for a number of years without running out of station-keeping fuel due to drag might choose to spend as much of it's orbit at a higher altitude. In order to accommodate all of those requirements at the same time, the sun-synchronous orbit would be elliptical. The eccentricity is related to the additional constraints beyond synchrony. Nobody said that a sun-synchronous orbit would necessarily always be elliptical. – uhoh Feb 11 '17 at 07:39
  • @Erik if the spies don't care what part of Earth they look at and how they are looking at it, there is much flexibility in the choice of apoapsis altitude (and therefore resulting eccentricity). However if they want to closely monitor the anti-spy-satellite secret launch pad's progress carefully, then they may want to keep a history of photos with very similar viewing angles. In this case they must choose the apoapsis such that the period is a rational fraction of a solar day. For exampe 1/15, 1/16, 2/31... – uhoh Feb 11 '17 at 08:22

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This is just an educated guess; I'm not connected with the National Geospatial-Intelligence Agency, or any related agency.


Europe is in the northern hemisphere, as is the Middle East and all of mainland Asia. Those are the primary targets of the United State's satellite spying efforts. What about the Southern Hemisphere? It's mostly water (not a threat), a all of Australia and New Zealand (both allies; neither is a threat), parts of South America (some are allies, none are technical threats), parts of Africa (no technical threats there, either), and a bunch of islands (not threats).

Since the Southern Hemisphere is "mostly harmless", it makes sense to have an elliptical orbit with perigee occurring over the Northern Hemisphere and apogee over the Southern Hemisphere. A 250 km circular orbit would either require a lot of fuel for altitude maintenance or would require frequent launches. An elliptical orbit with the perigee small over potential threats but apogee a bit larger over the "mostly harmless" areas reduces overall drag and extends the life of the satellite.

David Hammen
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    This makes a lot of sense, and can be expanded to include spy satellites of the other major players besides the US, since they tend to be northern hemisphere focused as well. For spy satellites that have elliptical orbits with periapsis altitude in the 400 km neighborhood and higher, the synchrony might apply instead, but at 250 km it's gotta be about minimizing total time spent at low altitude. – uhoh Feb 11 '17 at 06:11
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    That's a good reason, although nowadays for the Keyholes it's the other way around: some have a perigee in the Southern hemisphere – gosnold Feb 11 '17 at 10:03
  • You forgot Antarctica (also not a threat). – Vikki Mar 29 '20 at 16:00
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    @Vikki that we know of... – Hobbamok Jan 18 '22 at 08:20