Nasa to crash $330m spacecraft into asteroid to see if impact can alter course

[Slammer]

4. question

Of course a massive impact can alter the course of the an object. That's basic physics and can be proved mathematically without an experiment.

The question is whether they can deliver the craft onto a target.

There's also a question of whether the object being hit is solid enough to survive the hit intact (rather than splintering into pieces flying off into random directions).

Asteroids, likely solid enough.

Comets? Certainly massive enough to really mess up the Earth if one ever impacted here. But probably not solid enough for a solid deflect (like hitting one pool ball with another). A comet would more likely fracture and the pieces might or might not continue on to devastate the planet.