[meteorite-list] fireball speed

From: Sterling K. Webb <kelly_at_meteoritecentral.com>
Date: Thu Apr 22 10:31:25 2004
Message-ID: <40394A23.881002CA_at_bhil.com>

         For a meteorite to be "lit," it has to be traveling faster
     than the speed of sound (in air) and probably faster than
     that. So a meteorite can only get to the ground while "lit" if
     hypersonic.
         When an object hits something at hypersonic velocities,
     the shock wave from the impact is traveling backwards through
     the impacting object at the speed of the impactor PLUS the
     speed of the shock wave itself.
         This combined velocity will almost certainly be greater
     than the speed of sound in the interior of the object.
     (Testing of sample meteorites reveal they don't transit sound
     very rapidly.)
         If the shock wave exceeds the speed of sound in the
     object, it will convert the object into something that will
     either transmit sound at the higher speed or allow the energy
     of the shock wave to escape, namely, a ball of superheated
     rock or iron vapor or plasma!
         As far as to whether a give meteoroid will reach the
     ground and get promoted to the status of "meteorite," the
     general rule is this: If the mass behind each unit area of the
     frontal surface of the incoming object is greater than the
     weight of the atmosphere that lies in the path of that unit
     area from the top of the atmosphere to the ground, it has a
     chance of landing on Earth in one piece.
         Of course, if the re-entry forces are strong enough to
     fragment the object, then you have to re-calculate the chances
     for each individual fragment (always less). Fragmentation is
     what prevents most cosmic visitors from checking into the
     terrestrial hotel.
         An object that takes a low entry angle (like a re-entering
     spacecraft) and has a low entry velocity has the best chance.
     It has been calculated that the maximum weight for an iron to
     survive to the reach the ground is no more than 100 tons.
         This is why the rumored Mauritanian meteorite the size of
     a big hill is almost certainly a myth. (Sorry, I can't spell
     its name, Chingui... something.) Unless, of course, it was an
     iron reef from a great impact in the distant past.
         Hoba was probably that size before it started rusting
     away, and the great Greenland irons are less than 100 tons
     also (only 31 tons). Interesting that these two locations are
     far north and south, towards the poles, where captures of
     objects that are moving in the plane of the solar system is
     possible as they just graze the atmosphere.
         Might explain why there are so many nice meteorites in
     Antarctica...
         By the way, the maximum weight for a stone to reach the
     ground is less than for an iron, only 40 tons or so. That's a
     stone roughly spheroidal and about 25 feet in diameter! So, if
     you notice a fusion crusted rock, say, 9 meters across, be
     sure and check it with a magnet.


     Sterling K. Webb
Received on Sun 22 Feb 2004 07:32:36 PM PST