Don't you remember? The ship was incapable of diverting it on its own. Spock crippled the ship trying, and they had to limp back to the planet
That never made ANY sense. With a few months of time until the asteroid was due to hit, they could have nudged it sideways with a tractor beam, rather than trying to destroy it. A horribly written & conceived situation. Seriously, they could have just added a tiny, 1 foot-per-second sideways vector to it, and the asteroid would have completely missed the planet.
Why the hell wasn't the ship able to destroy it, anyway? Was it made of neutronium or something?
That business is baloney, top-to-bottom.