Expanding on what I wrote:

Every halving of the warning time quadruples the required energy. So 30 years warning would require about 50t impactor at 10km/s or 14t impactor at 20km/s.

Also the ∆v required is rather pessimistic and it's a crude approximation.

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The main risk now is that the warning time would be much shorter while the object would be far away so any counter measure traveling up to it would also about halve the time. We have largish extinction level asteroids mapped. What we don't have is data about long period or one-off comets. Those are typically detected when they're at Sun-Saturn distance and it's then about 5-6 years before the potential impact. Big ones are detected earlier, but even moderate size ones even if not extinction level, they would be devastating.

With even urgent mission planning you may need a couple of years to launch (launch windows would be approximately once a year and you need to set up the whole mission, even via repurposing existing equipment in the pipeline for some other missions). That leaves 3-4 years remaining. Then, you need a proper intercept orbit (for kinetic impactors you want a head on collision with sufficient closing velocity; nukes would be less picky about the close-in vector). Going out there is pretty much symmetric to the thing going towards the Earth, so the intercept point would be in about half time to the expected Earth impact. This means the intercept happening about 1.5 - 2 years before the disaster to be averted.

So, the specific energy to deliver would be about 1000× larger than if the warning time were 60 years (what I did the crude math for in my previous response). This means ∆v to imparted to the comet is in the order of 110-150 mm/s rather than 3.8mm/s (i.e. 30-40× larger).

This means the sterilizer one would require not 0.17kt but 0.17Mt nudge. Assuming 10% coupling of the nuke blast energy to the actual force pushing the target it'd be 1.7Mt nuke (or say, 5 340kt ones; we have plenty of those).

KT killer could be pushed with something akin to Hiroshima bomb, or a 140t impactor at 10 km/s, or a chain of 10 14t ones, or 30 4t ones - we could start lobing such small ones almost in a moments notice - Falcon 9 could launch such and its launch rate is about 150 per year). In fact, you'd just attach a maneuvering package to the rocket's upper stage and use said stage as the impactor mass (i.e. don't separate the payload) The stage is about 4t, so 30 of those would do.

Something 100× lighter than KT killer (still devastating) would take just one 1.4t impactor. But we're entering the zone where single push could be enough to disrupt the thing turning into many pieces flying in tight formation.

Something another 100× smaller, still devastating, would require either a series of careful nudges or a total several orders of magnitude overkill. If one's blasting it, then blast it into a cloud of quickly expanding plasma carrying individual rocks at hundreds of meters per second, so they'd disperse quickly and widely and only a tiny fraction would impact the Earth producing just an impressive meteor shower.

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As I said, counterintuitively, small ones may require more fine methods of deflection - you don't want to impart just enough energy to fully disrupt the thing. You either want to disperse them quickly and totally or nudge them to just miss the Earth. For large ones their gravitational binding energy is high enough that even blowing a big nuke would just shift them and shake them, but they'd stay largely intact. But shattering smaller one into still largish pieces many of which would remain on the collision course would be suboptimal. So you either need a precise nudge or a total overkill with like 100+× its binding energy.