What is considered a good starting prior in Bayes theorem for an event that hasn't happened yet?

Question

There are certainly things that to our knowledge have not yet occurred yet do stand a significant chance of occurring (Probability human lives to year $x$? for $x>120$ for example or Probability of asteroid hitting earth) for such events and even less plausible sounding events how does one assign a prior? We can't assign the value of zero due to zero being a fixed point under Bayes theorem, but there doesn't seem to be a good way to pull out priors from infinite hypothesis space. A really good example is how do we assign a prior that the current prisoners dilemma known solution is the best solution there is certainly evidence but there are infinite such strategy that we have not tested.