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Originally Posted by astrophotographer
I am not so certain about this. Many amateurs are recording the ISS with little difficulty and in enough detail to identify it. Yes, I have plenty of satellite streaks in my photographs and all are very small but I use a digital SLR with a low focal length. Many amateurs using CCDs are imaging galaxies in very long focal lengths and with fine resolution. My guess is they could probably identify large satellites (about the size of a bus) if they passed the field of view. Of course, they would just be wide streaks and not show detail.
Then we have the survey's done by NEAT, LONEOS, LINEAR, etc. They are covering larger areas of the sky looking for fast moving asteroids (and sweeping up just about everything else in near earth orbit). I am sure any foreign object invading the earth environment would be imaged and identified. I believe there was one such object some time ago that was thought to be an old Saturn rocket stage that may have returned into earth orbit (although this was never confirmed and could be just a small asteroid).
The amount of sky monitored by these surveys have pretty much eliminated the comet discoveries by amateurs (not completely yet). It would seem these instruments would be able to detect any large objects in earth orbit or approaching earth orbit.
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I too have tracked and imaged ISS, but ISS is unique; it's very large and very low (relatively speaking). In order to see any kind of significant detail you have to really crank up the magnification. Put ISS at a significantly higher orbit, like geosynch orbit, and it'd turn into a dot in most amateur scopes, even at high magnification. And yes, some amateurs still do image small galaxies and such, but I think the trend is towards wider and wider fields. It used to be that with amateur CCDs all you could really do was narrowfield galaxy work, now it's like a competition to see who can soak up the widest field of view in one shot. You're right about LINEAR and so forth, but even that only rules out a "presistant visitation" or a visitation with large numbers. What are the odds a single craft (say no larger than ISS) could go undetected without even trying to evade detection, provided it didn't do anything grandiose and stayed to a relatively high orbit most of the time? NEAT, LINEAR, et al are very good at detecting things that stick around in our neighborhood, but could very easily miss a transient event like someone just "passing through" our neighborhood might be.
One study I found particularly fascinating was an attempt in the 70s to search for ancient von neumann type probes the size of at least skylab hanging out at the lagrange(sp?) points. The theory was that if anything from a distant civilization were to visit us for a significant period of time in order to monitor us it might hang out at these handy gravitational zones to minimize the effort needed to remain in one place for long periods of time. Of course they didn't find anything there, though I believe the search only could have found objects with roughly the same size and albedo of skylab or larger. But what if we were to be visited by someone who didn't stick around? It could have very easily happened before a point history where we had the knowledge and technology to monitor our sky, or even if it happened today or anytime in the near future I doubt we'd catch it, or if some non-professional saw it they'd either disregard it or be disregarded themselves. Just my two cents though.