>The only problem I can understand is that the sky is a pretty big place, but surely we also have a lot of time to detect such objects.
I'm no expert, but as I understand it, the kind of objects we would be worried about are extremely hard to detect as they are dark, cold and relatively tiny.
At the same time, our "eyes" can only look at tiny patches of sky, generally through the distortion of our atmosphere for only a portion of each day with parts of the sky only visible part of the year. Furthermore, we generally have those eyes trained to other, known targets - stars, galaxies, nebulae and all that.
Consider that we didn't spot Apophis [1], which is set to come extremely close, until 2004 even though it's on our lawn [2] in terms of distance.
There is an effort [3] or two [4] afoot related to tracking asteroids and a new DARPA [5] program for tracking our own local junk.
I'm no expert, but as I understand it, the kind of objects we would be worried about are extremely hard to detect as they are dark, cold and relatively tiny.
At the same time, our "eyes" can only look at tiny patches of sky, generally through the distortion of our atmosphere for only a portion of each day with parts of the sky only visible part of the year. Furthermore, we generally have those eyes trained to other, known targets - stars, galaxies, nebulae and all that.
Consider that we didn't spot Apophis [1], which is set to come extremely close, until 2004 even though it's on our lawn [2] in terms of distance.
There is an effort [3] or two [4] afoot related to tracking asteroids and a new DARPA [5] program for tracking our own local junk.
1: http://en.wikipedia.org/wiki/99942_Apophis
2: http://www.howcloseisapophis.com/
3: http://www.skyandtelescope.com/news/160839035.html
4: http://www.planetaryresources.com/mission/
5: http://www.spaceviewnetwork.com/