An asteroid whose orbit is no where near intersecting earth’s had another baby asteroid orbiting it. The impactor hit the little guy and made it slow down a skosh. It showed that an asteroid redirect is possible using an impactor.

So let’s say we found a planet killer out there. And you calculated an intercept 20 years from now. So you send up an impactor at the 10 year mark. It offsets the velocity of the asteroid by 1 km/hr. After another 10 years it’s 87,600 km off from where it would have been.