There would be tremendous earthquakes and tsunamis, followed by massive volcanism around the impact zone. The ozone layer would be destroyed. The oceans would turn acidic. The Sun would be blotted out, probably for decades.
Answer: Well it depends on the size. If the asteroid was the size of a basketball, it would most probably turn to dust by the time it reaches the surface. But, if it was the size of Australia, then most probably everyone would die.
