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He has theorized in the past that the chance of a disaster on Earth adds up over time, so that it's a "near certainty" in the next 1,000 or 10,000 years, but the human race will survive if it expands into outer space.
http://www.time.com/4767595/stephen-hawking-100-years-new-planet/

Professor Stephen Hawking thinks the human species will have to populate a new planet within 100 years if it is to survive. With climate change, overdue asteroid strikes, epidemics and population growth, our own planet is increasingly precarious.
http://www.bbc.co.uk/mediacentre/mediapacks/tomorrows-world/television

Maybe he's a little bit overdramatizing... I don't know.

Aha, ok, so in general he thinks that we are about to be hit by a giant rock because we weren't hit recently.
How optimistic. LOL

Stochastics... :D

Oh and I'm a little bit curious... what led you to this 2 month old post of mine?

I guess the mention of the human extinction and Mars.