This is the thread where we discuss some real and some obviously fictional scenarios (hostile spacecraft, missiles, or asteroids in either near-earth, cislunar or interplanetary space) with real world physics only. Specifically, things are detected by optics, radar-system and so on, things that may or may not be designed to be less visible. In the first scenario, it is a perfectly normal rock. -- Scenario #1. The trajectory of an asteroid intersects Earth. How likely are we to detect it before it hits? What kind of detection systems can be used? What kind would ideally be built for the task? -- I'm thinking our current satellite systems aren't optimized for "sweeping" the entire sky, in the case where they are turned towards space by design they're viewing far-away objects. A telescope-equivalent designed for looking at an entire hemisphere of space would either be moving or be built like the eye of an insect, with ,multiple telescopes directed against each portion of sky. Also, am I right in thinking that an active LIDAR-system in the form of a pulsed laser directed against a portion of the sky would greatly improve our chance of detecting an inert object there? Do note that this thread is a bit less specific than this thead, I'm thinking the multi-discipline a third engineering, a third conjecture approach often used when trying to analyze fictional scenarios with real world science might scare knowledgable people off. Here a clearly-defined scenario is given and then it is analyzed. I'll continue the thread with "scenario #2, #3, et cetera if the discussion gets going).