In 1908, an asteroid about 100m across exploded 6km above the Earth, releasing energy of a thousand Hiroshimas over Tunguska in Siberia. An object coming in at 20km per second hitting the atmosphere goes kerblam. It needs to be bigger than 100 metres in size to reach the ground intact but that does not mean we are safe. The British government had taken no action on a report from one of its own committees on asteroid hazard. It would be delicate work.
Asteroids are piles of rubble held together by their own gravity.
There would be no blast wave in space, but one side of the asteroid would be heated, and start to vaporise. This would have a jetting effect, and could slow the monster by half an inch a second. It might not sound much, but half an inch over 20 years adds up to greater than the radius of the Earth. It would miss us. But no matter how good the surveillance, no one will get a chance to sound the alarm or deflect a chance vacuum fluctuation before it happens.