What is an acceptable risk to destroy the earth ?

[vanesch]

The distinction was drawn earlier in the thread between an acceptable risk - one that we voluntarily assume, like getting in a car- and those circumstances over which we have no control.

This is true, but baywax also has a point, in that relatively high risk levels associated with choices of action are usually a comparison between risk and benefit, but if the involved risk is much lower than "natural and unavoidable" risks, then it would be a bit strange to prohibit it, as it would in any way just be "noise". If we can obtain the slightest benefit out of an action which poses a risk which is orders of smaller than the natural risk (such as asteroid impact) to get us all done, then I think the discussion is moot.