Why does it need to be aim along the planet?
No particular reason. It's just that the arbitrary task of planetary self destruction that Multipartite specified happens to be that of destroying the planet with a bomb on the surface. If you were just trying to destroy the planet then doing so from the surface seems like a terrible idea.
(For thoroughness, noting that the other approach was also wondered about a little earlier. Surface action is an alternative to look at if projectile-launching would definitely be ineffective, but if the projectile approach would in fact be better then there'd no reason not to focus on it instead.)
http://neo.jpl.nasa.gov/risk/2012da14.html
http://rt.com/news/paint-asteroid-earth-nasa-767/
Seems like a good opportunity to bring up existential risks. And A friendly reminder that NASA is in fact pretty damned important.
Thoughts?