What plan does the government have if the earth is on collision course with an asteroid?
There are no current public plans for what happens if an asteroid is on a confirmed collusion course with the earth by any government. Although many governments most notably the UK have commissioned companies to submit plans for the inevitable event. On the other hand it is widely believed that most governments have secret plans on what their government will do in the event.
Plans have circulated the scientific community about what to do. These plans include breaking the asteroid up with nuclear devices, despite the general consensus that asteroids are too dense for a nuclear device to do any damage and even if it does it will simply break the asteroid into several objects still on a collusion course. Other plans include pushing/diverting the asteroid with a variety objects including a laser beam to slowly push the asteroid.
Currently NASA tracks any object that is 700m or greater in diameter although plans are in the works to make that 70m. Another possibility that has been documented is that the Russians have 3 50 kiloton nuclear missles in silos strategically placed across siberia. although this approach would work it would send a debris cloud hurtling towards earth causing wide spread but minor damage.