I was at a performance of William Missouri Burroughs' play Mad Gravity last night. It turns out the play involves a bit of audience participation, so at one point a character asks the audience if anyone knows the difference between an asteroid and a comet. My date pointed to me, so I offered that asteroids were rocky denizens of the inner Solar System while comets were icy wanderers of its distant outskirts.
It seems that the plot involved a close approach and possible impact by a comet.
I didn't get around to mentioning it, but overall comets are a heck of a lot more menacing than asteroids. Orbiting mostly in our near neighborhood, asteroids are pretty familiar, and the big ones on nearby trajectories are pretty well cataloged. If a really big one (major extinction class event) has our number, modern astronomy should see it coming for a few hundred years before impact, probably giving us enough time to persuade even the dumbest
Republican climate denialist that we ought to do something about it. Also, asteroid orbits are mostly fairly similar to ours, so impact velocities would be "only" a few tens of thousands of kilometers per hour.
Comets, though, are invisible till they get pretty darn close, and falling through that big potential well of hundreds of astronomical units are moving very fast. Because they are spewing gas, dust and rock by the time they get this close to the Sun, even a near miss might be pretty calamitous. Warning times in weeks or less would imply that the most effective action we could take would be to bend over, place our heads between our legs, and kiss our asses goodbye.
Of course in 50 years, we might have other options.