That could work if your villain is on a budget, but a time frame of a few centuries is MUCH too optimistic. It would expect diverting even a small asteroid to t

That could work if your villain is on a budget, but a time frame of a few centuries is MUCH too optimistic. It would expect diverting even a small asteroid to take several million years, let alone one of the really dangerous ones. Making the sail (much) bigger would help, but then you have additional problems because people would see it coming and could take countermeasures.