We're not talking about the asteroid stopping with a screech of tires and then taking a hard left turn to crash into earth.
It's just that anything traveling through the earth/moon gravitational sphere of influence will have it's trajectory tweaked just a bit. How close to the center of gravity the pass is determines exactly how much of a tweak. There is a small section of space, we'll call it the keyhole, where if the asteroid happens to pass exactly through that area the tweak will result in a collision next time the asteroid comes around. That could be decades hence.
There could even be a case where an unlucky keyhole pass this time lines up another unlucky keyhole pass the next time to an eventual collision in the distance future.
The technology to nudge the asteroid just far enough to miss a critical keyhole pass is within the realm of possibility with today's knowhow. We just need to have these missions ready to go on short (order of a few months to a year) notice.
We see big ones with a few days to hours of notice, sometimes we see them when they hit.
Most likely: this will never come up.
Less likely: if it does we're fucked.
Even less likely: if it does and surprisingly we see it in time we will act for the good of all and not bicker about who pays and we'll make things better rather than worse. If not, see above.
It's just that anything traveling through the earth/moon gravitational sphere of influence will have it's trajectory tweaked just a bit. How close to the center of gravity the pass is determines exactly how much of a tweak. There is a small section of space, we'll call it the keyhole, where if the asteroid happens to pass exactly through that area the tweak will result in a collision next time the asteroid comes around. That could be decades hence.
There could even be a case where an unlucky keyhole pass this time lines up another unlucky keyhole pass the next time to an eventual collision in the distance future.
The technology to nudge the asteroid just far enough to miss a critical keyhole pass is within the realm of possibility with today's knowhow. We just need to have these missions ready to go on short (order of a few months to a year) notice.