We actually today have programs to keep an eye out for various rocks in space that may hit Earth.
It's a common sci-fi trope that when one is on course to hit us, we deflect it.
That's pretty much done "using nukes" or in some schemes, ingeniously using some sort of relatively low power rocket-like engine which is bolted on place on the Danger Object and slowly deflects it over a period of time.
All fair enough.
Can we put some numbers on this?
Say there was an incoming Danger Object of mass 1 tonne, 1000 kg.
We have a realistic "1 or a few years" to deflect it.
I'm pretty sure that although it might cost the lives of Bruce and Billy-Bob, we could probably deflect a 1-tonne object using current, real, actual technology. Rocket ships, robots, bombs and engines as we know them.
10 tonne .. I'd guess again yes. (But maybe I'm totally wrong, maybe that's hopelessly outside the scales of energy involved.)
100 tonne .. I have no clue.
1000 tonne? A million? 100 million?
A recent interesting question asked if an incoming planet-sized body could be deflected in any way at all with current actual technology; of course the answer to that is absolutely not.
But it got me to thinking.
In order of magnitude terms, in fact what is the biggest incoming Danger Object we could deflect currently?
Has this in fact already been worked out and is well-known by the various danger-object-tracking programs? Can someone here who drinks and knows stuff figure it out with a calculator?
It would seem to be a critical basic info-fact for worldbuilders!
Tag here is hard-science.