Chris,
> The meteor crater in Arizona was formed about 45,000 years ago. I'm not > much on statistics, but an interesting guess might be that such a strike > has a probability of 1/45,000 per year. This strike was a small one. True, but would you want it to happen in your city? > Zodiacal dust has a life on the order of 20,000 years, so why is there > any? It's because it is formed continuously by collisions in the > asteroid belt. I assume that means there are lots of collisions and > therefore lots of asteroids. This seems a little unclear (with respect to dust lifetime). Across my desk (source unknown) there was a document this morning regarding the complete "decoding" of a speck of space dust -- included in its contents were several red giants and probably a couple of supernova. So I question the dust "life-time" and source figures (though I do not dispute that much of the dust may be produced via the mechanism described so we may be dealing with "distribution" abundances). > I previously had the impression that there were maybe hundreds of > asteroids, but there must be hundreds of thousands, maybe hundreds of > millions. Number of NEO's is here: http://neo.jpl.nasa.gov/neo/number.html or here http://spaceguard.ias.rm.cnr.it/NScience/neo/neo-when/number.htm (thank google) so I suspect the number of asteroids (in total) is a few orders of magnitude larger (than the number of NEOs -- numbered at a few thousand) and the number of bodies that represent potential risk (mostly Kuiper Belt objects) is *very* much larger. Hundreds of millions does not seem like an extreme estimate to me. > What me worry? But you should be. As I've discussed offlist with several people the pace of molecular biology and nanotechnology *will* defeat aging within this century -- in that situation the "average" human lifespan transitions to 2000-8000 years -- entirely limited by the "accident" rate. At that point one becomes seriously interested in "accidents" like asteroid impacts. If you view your probable longevity as less than 100 years than sending a mission to Pluto at a cost of $200+ million dollars or Europa at something probably several times that figure makes sense. *BUT* if you view your potential longevity as a few thousand years then spending less than 1% of those amounts (which I believe are the current NASA funding levels for NEO searches) seems like a potentially large mis-allocation of funding priorities. (And we will not even go into the Space Shuttle or the ISS -- Bruce manages to do that very well -- *but* if redirected to "catastrophe avoidance" instead of "contrived scientific research" they might really be useful to humanity...) Best, Robert == You are subscribed to the Europa Icepick mailing list: [EMAIL PROTECTED] Project information and list (un)subscribe info: http://klx.com/europa/
