How big and how fast does an asteroid have to be to destroy the earth?

Question: How big and how fast does an asteroid have to be to destroy the earth?
- Keywords:
  asteroid,
  destroy,
  earth
Asked by ZEBEAST to Anne, Beth, COLFlight, Jon, Tom on 14 Oct 2015.
- Columbus Flight Directors answered on 14 Oct 2015:
  
  Sergio:
  Hi Zebeast!
  It will have to be pretty big to survive the atmosphere friction without burning up completely.
  And then it will not only depend on the speed, but also on the angle of impact.
  
  So I don’t have a direct answer but… why don’t you check it out yourself!
  There are a couple of websites I know of where you can simulate an impact Have fun with them! 🙂
  
  1) https://www.purdue.edu/impactearth/
  
  2) http://simulator.down2earth.eu/input.html?lang=English

Question: How big and how fast does an asteroid have to be to destroy the earth?