From: g_d_pusch_remove_underscores@xnet.com   
      
   cray74@hotmail.com (Mike Miller) writes:   
      
   > g_d_pusch_remove_underscores@xnet.com (Gordon D. Pusch) wrote in message   
   news:...   
   >   
   >> For a glancing impact, on the order of 90 minutes --- the time it takes   
   >> for the debris to orbit the Earth once.   
   >   
   > Just to be clear: are you saying that the oceans would boil off in 90   
   > minutes,   
      
   No, I am saying that for a glancing impact, most of the energy of the   
   incoming body is still stored in the spray of debris, which could in   
   principle make nearly a complete orbit around the Earth before re-impacting   
   the Earth.  To an _ORDER OF MAGNITUDE APPROXIMATION_ (i.e., power of ten)   
   the timescale to orbit the Earth is on the order of 90 minutes, so I would   
   expect that most of the debris would re-impact on a time scale that is long   
   compared to 9 minutes, but short compared to 900 minutes; the later would   
   require the chunk of debris to be on a fairly eccentric elliptical fractional   
   orbit.   
      
   The energy involved is so enormous that the oceans would be boiled off   
   essentially as fast as the debris re-impacted the Earth. (The debris makes   
   its fractional orbit around the Earth at many times the speed of sound,   
   even in rock.)   
      
      
   > or is that also a matter of hours as the seismic waves   
   > propogate around the globe?   
      
   I was saying that it takes on the order of several hours for the seismic   
   waves produces by the impact to propagate from one side of the Earth to the   
   other --- where again, by "order of magnitude," I meant "more than several   
   tenths of an hour, and less than several tens of hours." However, unfortunately   
   I mis-remembered how long it takes for seismic waves to propagate through   
   the Earth: Characteristic seismic wave velocities are actually in the range   
   of 4--8 kps, so it will actually take less than an hour for P-waves (which   
   are the fastest seismic waves) to propagate completely through the planet.   
      
   Again, the amount of energy involved in such a large impact is so enormous   
   that the seismic shock waves it produces will completely shatter the crust   
   as they propagate through it. In the wake of the shock wave from the main   
   impact, plus the shock waves produced by the re-impacting debris, nothing   
   will be left but an ocean of magma, perhaps dotted with rapidly disintegrating   
   rafts of melting crust.   
      
      
   -- Gordon D. Pusch   
      
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