You're right about what you say Chuck, and I think that we're beginning to get serious about cataloguing all the rocks and bits of junk sailing around up there. I'm absolutely in favor of that, mostly because of the science we get out of it and partly because it's a good idea to know the dangers we face. But as I said in an earlier version of this thread, it's been 65 million years since the last mass extinction that could have been caused by an impacter. All of the other mass extinctions in the geological record have evidence they may have been due to other causes. And there's some increasing evidence that the Chixulub impact may not have been the only cause of the KT extinctions. So while I wouldn't welcome a direct hit by an asteroid, it's way way down my list of worries. We are currently undergoing a mass extinction more rapid than any we've encountered in geology, with nothing to blame but our little old selves.
Michael, my understanding is that the numbers don't add up for this strategy to work unless implemented decades to scores of years in advance- same for "low relative thrust/long term" rocket motors such as ion engines anchored to the surface. Any impactor discovered months to a few years beforehand defaults to the Atari strategy. Certainly it is the most expensive option, and it just costs too much to build the levees to withstand a category 5, right? :(
There's the justification for funding inventory searches more aggressively.