the extinction of mankind
"Now since what we're talking about here is preventing the extinction of mankind, do you really want a second best system to be used?"
Fact: Mankind did not become extinct in the past 1.5 Million years, despite having no technology to address the "rocks-from-space" issue.
Deduction: we have *at least* 1% of this time before it will happen in the future.
Conclusion: we have *at least* 15,000 years to worry about space objects.
Don't panic, the next Ice Age will arrive before then, and exterminate 2/3 of the global population, unless we get REALLY good with global warming and exterminate 50% of the population by other means...