A more interesting question is : what kind of delay is worth worrying about?<p>I mean, for instance it is known that life will remain possible on Earth for about 500 million years, after which the Sun will be too bright.  We don&#x27;t seriously worry about it, since so many things can happen in half a billion years that it does not make much sense to try to plan or predict anything in such a timeframe.<p>Similarly, I personally don&#x27;t worry about an impactor of 10 km diameter or so.  Such an event is so rare that it&#x27;s not likely to happen in several million years.  To me that&#x27;s still the kind of timeframe it does not make sense to worry about.  I&#x27;m sure some people will disagree though.<p>What is the limit?  10,000 years? 1,000?  100?