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Are sodium lasers useful for Earth observing space telescopes/spy satellites?

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    $\begingroup$ I'm voting to close this question as off-topic because it is about satellites and related technologies, not astronomy. $\endgroup$
    – HDE 226868
    Aug 2, 2015 at 16:43
  • $\begingroup$ @HDE226868 Well, it is about adaptive optics used by astronomers. If astronomy isn't the right place to ask if an Earth observing space telescope could use that technology, then where? $\endgroup$
    – LocalFluff
    Aug 2, 2015 at 16:57
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    $\begingroup$ Space Exploration, perhaps? $\endgroup$
    – HDE 226868
    Aug 2, 2015 at 16:58
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    $\begingroup$ It could be argued both ways. I'm interested in optical hardware for astronomy, so I thought the question makes sense here. But I'm not happy with it for a different reason - it's quite speculative, exploratory and open-ended, and I don't think anybody knows the real answer. I think @dotancohen has given the best possible answer already, within reason. A speculative answer for a speculative question. $\endgroup$ Aug 2, 2015 at 22:05
  • $\begingroup$ I suggest leaving the question open. Not because it is a great question per se, but rather because it addresses some incorrect notions about the technologies used in astronomy: telescopes, satellites, and guide star lasers. Also, the answer addresses these misconceptions in a way that is relevant both to the question and also in general for those who may not understand how, and why, the guide star laser is used. That is very relevant to this site. $\endgroup$
    – dotancohen
    Aug 3, 2015 at 10:49

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The guide star laser is used to adapt the optics at a specific time and location, for specific atmospheric conditions. It is not, as your question implies, used to calibrate the optics for long-term use.

Therefor, if order to calibrate the satellite with a laser fired from Earth, the reconnaissance target would have to provide the laser. This is useless for non-cooperative targets, which I would imagine make up the bulk of the reconnaissance targets.

I suppose that one could fire a laser from the satellite itself, but this would have the obvious disadvantage of making it even more obvious when a spy satellite was overhead.

In any case, the guide star image does not penetrate the entire atmosphere, I believe that it produces an image just a few tens of kilometers up. This is useful because it is the thicker, denser and more turbulent lower air of which is the most concern to optics. Firing the laser from above would thus see the laser energy absorbed in the thinner, stiller, more predictable upper atmosphere, thus it would have must less advantage.

One final reason not to use a guide star laser on a reconnaissance satellite would be power budget. These satellites are as small as possible, with as small a heat and reflectivity index as possible. The added power source, be it RTG, conventional batteries, or solar, would likely increase the reflectivity of the satellite.

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