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I'm writing an SF book and want to know if a 400 by 1000 meter tubular habitat at Lagrange point 4 (or L5) would be easily detected from earth?

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    $\begingroup$ You need to define what measures (if any) have been taken to make it hard, or easy to detect! Is it painted white, or black, or is it fairly specular (shiny metal)? Is it at some equilibrium temperature, or is the side facing the Earth insulated or even actively cooled to very low temperature in order to be less visible in thermal IR imaging? $\endgroup$ – uhoh Jul 23 '17 at 3:39
  • $\begingroup$ Hm, I'll have to work on those, my only assumptions were that one end would be facing the sun with gold/silver/dichroic amber to blue layers/filters sort of how digital projector and LCD work. the whole thing would be layered graphene plus other things. Maybe white for the cylinder, dark or black would absorb too much heat... it's maker is a super advanced billion year old AI so... maybe the cylinder would have micro shutters as well and could vary... $\endgroup$ – Craig Jul 23 '17 at 4:23
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    $\begingroup$ Then your task is simpler. The super advanced AI would have thought about the possibility of being seen from a civilization that might appear on the well-placed goldilocks planet, and simply decided if it wanted to be hard to spot or not and included features accordingly. Since this is drifting away from on-topic here, I'd recommend you adjust this question to specify a habitat that we might build this century and ask how visible it would be for various types of observations. $\endgroup$ – uhoh Jul 23 '17 at 4:35
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    $\begingroup$ After seeing those answers and understanding some of the science behind the visibility issues, you can consider a new question with the AI-built version of your habitat. That gives you more time to consider if the AI wanted its habitat to be visible or not, and also gives you time to decide if it's better to ask your follow-up AI-built question here or in worldbuilding SE. Remember, you can ask as many questions as you want (as long as they are proper SE questions) in as many different SE sites as you like! $\endgroup$ – uhoh Jul 23 '17 at 4:40
  • $\begingroup$ There is How large does a spacecraft need to be to be visible from the surface of the Earth at 400 km altitude? on Worldbuilding, which I believe you will find helpful. Just plug in your own numbers instead of those in the question and answers. $\endgroup$ – a CVn Jul 26 '17 at 11:17

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