How likely is Earth to be hit by a killer asteroid?

Our planet is hit by meteorites and asteroids all the time.  We never hear about these strikes because they are insignificant, small and don’t really do any damage.  Most of these small objects either fall into the oceans or in very isolated areas.  Earth is hit so much by tiny objects that it works out to be over 100 tonnes of material per year.  The only time we hear about meteorite strikes is when they are big enough to cause a bit of damage.  

Tiny size strikes

These are generally no bigger than a small ball-bearing and are usually just dust.  These strike Earth in the thousands per day with the larger versions commonly seen as “shooting stars“.  The chances of being harmed, in any way, by one of these is practically non-existent.  

Small size strikes

These are generally the size of a small car up to the size of a bus.  Asteroids of this size generally hit Earth once every thousand years and will cause serious damage if they were to hit a populated area.  Should one hit in the centre of a major metropolis it would seriously damage an area approximately the size of 4 city blocks.  

Medium size strikes

These asteroids would be from 250 metres (820 feet) to 1,000 metres (3,280 feet) wide and strike maybe once every 50,000 years.  Should one of these asteroids hit the centre of Manhattan it would cause massive damage and would destroy most, if not all, of Manhattan Island.  Further devastation would be global with the onset of a nuclear winter causing mass famines.  

Large size strikes (extinction level events)

For an asteroid to be big enough to destroy all human life it would need to be, at least, 20km (12.5 miles) wide.  It doesn’t matter where on Earth this asteroid hits, we would all be toast.  Such asteroids generally strike Earth once every 1 to 2 billion years. It doesn't matter where on Earth this asteroid hits, we would all be toast. Click To Tweet  

The Spaceguard Survey

The Spaceguard Survey is a NASA run program designed to track every asteroid in our Solar System and calculate their trajectories to determine if they will hit us.  Unfortunately, space is so large that it is practically impossible for such a program to track everything.  We usually only hear about large asteroids on a potential collision course after they have already passed us. The chance of being hit by a large enough asteroid to kill a large amount of people is high but we will not get any warning until it’s too late.