I’m guessing we have a fairly robust detection system for aircraft, between the aviation industry and the military. How powerful are these systems? How far up can we detect objects? How large and fast? Can we detect satellites in orbit? Space rocks falling to Earth?
We currently track little bits of space junk, mostly so we can avoid having a big chunk of something come crashing through the window of the space shuttle and kill everyone and stuff like that (since these are generally considered to be bad things). Some of the bits of junk we track are quite small.
From wikipedia: Space debris - Wikipedia
I don’t know exactly how far out we can detect things and what size is required, or how well we are looking for random stuff as opposed to bits of junk that we already know about. We seem to detect large hunks of rock at a pretty good distance, but once in a while pretty big chunks get pretty close before being detected.
What do you mean by ‘detect’, and what particular objects? Spy planes and satellites? I’ve seen airplanes in the sky flying several miles overhead, both day and night, and on dark cloudless nights I can usually spot a satellite or two in the sky if I wait long enough. I suspect this is pretty universal, at least for civilian aircraft. Are you talking about simply keeping tabs on the aircraft in our airspace or looking for UFOs?
You will not receive robust, factual answers to your questions beyond what may be commercially available. The military doesn’t share their knowledge.
Under a clear, dark sky you can easily pick out many satellites that vary in size from a compact car to a semi trailer or 747 that are anywhere from 100-400 miles up with your naked eye. You can pick out more (from more light-polluted skies) with a cheap pair of binoculars.
Using radar or lidar they can detect much smaller objects. As engineer comp geek pointed out accurately tracking these objects can be difficult, but it’s entirely possible to detect fruit-sized objects in orbit several hundred miles up. Not sure what sort of technologies might be classified, but I trust that the US can detect any object in orbit that’s large enough to either take pictures of us or survive reentry to fall on our heads.
If the OP’s concern is asteroids like Apophis, there is fairly large group of amateurs and scientists who devote themselves to hunting asteroids of all sizes. Here is an article from the Smithsonian that seems to talk about it.
I was thinking more along the lines of radar or similar than visual.
Does composition matter?
Does NASA register incoming space material? Would an incoming rock set off a ping anywhere? Would they notice an UFO in orbit? Further out?
The search for objects from afar that might potentially hit the Earth is woefully underfunded and sparse. There have been near-misses that weren’t even discovered until the object had already passed us and was moving away again.
I’m still curious about how far out our radar works. But since the topic of detecting far away collision potentials has come I have another question.
I know that the hardest object to visually detect as a danger is one on a more or less direct approach. I suppose we could solve this by using parallax, but how far apart would two telescopes or other devices have to be for this to be pragmatic?