- Nuke it -- not ideal, as we'll probably lose a few satellites, however the smaller pieces won't cause nearly so much damage. Most of the fragments will burn up in the atmosphere. Many will miss the earth entirely.
- Send a probe out to it when it's still far away. Position the probe close to the asteroid for a long time (months) -- the pull of gravity exerted by the probe will nudge the asteroid off its collision course.
- Wrap it in a giant piece of plastic, and mount some engines on it, so we can capture it into a nearby orbit. That much mass *in orbit* would be immensely valuable, even if it's just rock. (this one is probably unworkable due to the sheer quantity of fuel that would be required)
A manned mission is laughable, and only fit for the movies.