"enough to make a difference" depends a lot on timescale in question Earth travels its radius in 3-4 minutes. Delay asteroid by that in its orbit and you did i

"enough to make a difference" depends a lot on timescale in question Earth travels its radius in 3-4 minutes. Delay asteroid by that in its orbit and you did it If you learn of impact 100 years in advance, your task is only 2 seconds per orbit - less than 1mm/s