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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1$\begingroup$ Probably adds 'delay' to keep it sun-synchronous. $\endgroup$– BobTCommented Feb 10, 2017 at 20:31
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$\begingroup$ @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! $\endgroup$– uhohCommented Feb 10, 2017 at 20:55
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1$\begingroup$ 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. $\endgroup$– CodyCommented Feb 10, 2017 at 23:48
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$\begingroup$ The eccentricity of an orbit has nothing to do with its sun-synchonicity. $\endgroup$– ErikCommented Feb 11, 2017 at 2:43
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1$\begingroup$ If they want low periapsis, and it being over a certain area consistently, then a semi-synchronous orbit is a must. $\endgroup$– SF.Commented Feb 11, 2017 at 6:59
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1 Answer
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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.
answered Feb 11, 2017 at 5:50
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2$\begingroup$ 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. $\endgroup$– uhohCommented Feb 11, 2017 at 6:11
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1$\begingroup$ That's a good reason, although nowadays for the Keyholes it's the other way around: some have a perigee in the Southern hemisphere $\endgroup$– gosnoldCommented Feb 11, 2017 at 10:03
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$\begingroup$ You forgot Antarctica (also not a threat). $\endgroup$– VikkiCommented Mar 29, 2020 at 16:00
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