Zardnaar
Legend
I know this - reaction drives and guidance. It's pretty obvious when you're doing this, as an asteroid big enough to be a dinosaur killer needs a large drive and a naughty word-load of reaction mass to move it in any reasonable length of time. Unless you can accelerate it to velocities in the 1,000's of km/sec it will still take months or years to get to an inner planet from the outer system.
If you've ever done an orbital rendezvous in Kerbal Space Program, hitting the sphere of influence of a planet, let alone the planet itself is not a trivial undertaking from (say) Jool to Kerbin. The asteroid will still need terminal guidance or a very precise initial shove indeed to hit the target. Caught far enough out, it doesn't need a big push to make it miss - a difference of a few metres per second will do it from far enough out. You have to defend the asteroids or the target will pick up on it and nudge it away.
You need a lot more delta-V to orchestrate the attacks then you need to defend against them. Unless the target has no spacegoing capability or lacks the technology to do the intercept then it's easier to defend this than attack with it.
Clever asteroid trick used in the Thrawn Trilogy. Use the as siege weapons.
You load a few onto star destroyers and put cloaking devices on them. They made you blind as well but doesn't matter on asteroids.
Drop 20 of them around a planet and fake fire your tractor beams a few hundred times.
Don't even need asteroids you could use large tungsten rods or go the mass effect route and fire things very fast.
If you've got ftl drives and interstellar flight one would think you can deal with errant asteroids.
Even in sci fi if you can weapon asteroids large enough to wipe out the dinosaurs you're getting into mega engineering levels of sci fi. The dinosaur killer on the smaller scale estimates masses more than a Super Star Destroyer and they're 19 kilometres long.
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