> Asteroids with a diameter more than 30–50 metres (100–150ft) are large enough to make it through our atmosphere intact, however, and the chance of this happening is estimated to be around once in every 100 years.
> The damage from an impact of this size would be wide-ranging, and could wipe out an entire city if they were to impact a heavily populated area.
We lack the technology to detect this size of asteroid with enough time to do anything about it.
Luckily, the majority of the earth's surface is unpopulated. Most of these rocks hit the ocean or Siberia and cause few or no casualties. The odds of it hitting a major city are quite low.
That "if it hits a populated area" reduces the probability by a factor 100 (because about 1% of the world's surface is urbanized [1]).
So the actual frequency of "city wiped" is more like once every 10000 years. Completely negligible compared to other city-wiping events (earthquakes or floods for example).
City killers? That size hits more often
> Asteroids with a diameter more than 30–50 metres (100–150ft) are large enough to make it through our atmosphere intact, however, and the chance of this happening is estimated to be around once in every 100 years.
> The damage from an impact of this size would be wide-ranging, and could wipe out an entire city if they were to impact a heavily populated area.
https://www.skyatnightmagazine.com/space-science/chances-ast...