When the mass you are trying to move is a billion metric tons, you're gonna need a lot of advance warning. We can land on asteroids, changing their trajectory

When the mass you are trying to move is a billion metric tons, you're gonna need a lot of advance warning. We can land on asteroids, changing their trajectory enough to avoid a planet strike is uncharted and entirely unpracticed territory. Part of me thinks that NASA's recent foray into asteroids is just that, practice.