Re: We need sentinels out past the asteroid belt looking for incoming.
@Ugotta B. Kiddingme "However, even if we do put the requisite number of sentinels in the described area, that alone is insufficient. We must also develop the technology to eliminate or at least mitigate an incoming threat. "
I'd guess that the engineering difficulty and expense of filling interplanetary space with enough incoming rock sensors to give good coverage of the asteroid belt and beyond would be similar or greater than the difficulty and cost of developing the means to deal with a big incoming rock.
I've long thought that all we really need is detection systems which can give adequate warning time of a big incoming rock - "adequate" in this case meaning "long enough for those holding the purse strings to get the message and throw enough money at the problem soon enough to develop and send a mission to deal with it".
I mean, there are lots of ideas about how to deal with big incoming rocks. There is also, ooh, roughly 70 years of space engineering development under humanity's collective belt by now (a V2/A4 passed the Kármán line in 1944; the USA far exceeded that altitude in 1948) including space-going proper nuclear power reactors (the Soviets flew some), long-duration crewed missions, on-orbit repair and assembly experience, a large global aerospace production capacity, and reliable heavy-lift launchers.
The Yanks even worked on a nuclear reactor powered rocket (NERVA). The designs could be dusted off and developed into modern flight hardware - I'm thinking that getting to and shifting a big rock quickly would take nuclear propulsion.
Yes, normally, you'd not want to run the risk of blowing up a nuclear reactor when attempting to put it into space, but if the alternative is species extinction, minds might well change.
The job of dealing with a seriously threatening incoming rock doesn't seem impossible - just difficult and expensive and I reckon getting the funding to develop the required technology would need a clear and present danger.
The bigger the incoming rock, the longer warning we'd need - but then again, bigger rocks are easier to spot.
Whether or not we'd currently get long enough warning and quick enough funding is anyone's guess.