simple asteroid defense

put fission nuclear reactor on asteroid, 

put robots / machinery that collect material from asteroid, 

use energy from nuclear reactor to energize material from asteroid, 

until gas or plasma is created, 

expel gas or plasma through rocket engines, 

use rocket engines to accelerate asteroid toward target asteroid that is deemed a threat 

using an asteroid as projectile instead of a rocket makes it possible to accelerate a larger amount of mass toward target asteroid, 

which increases probability that travel path of target asteroid is affected or that target asteroid is partially or completely destroyed 

less politically sensitive if project is done by a private company and fusion reactor is used