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Re: An uncomplicated question

We know from the study of meteorites that there were many separate
asteroids in the early history of the solar system that never were
once part of larger bodies.  The chemical, isotopic, physical,
and petrologic properties of the chondrites are consistent with each 
chondrite group having arisen on a separate body.  While know that CV 
chondrites came from different parent asteroids than, say, H chondrites, 
we don't know how many CV-like asteroids or H-like asteroids there 
might have once been. (Cosmic-ray exposure ages do give us clues about 
how many of each type of asteroid is currently supplying meteorites 
to the Earth.)  Anyway, we can say with a fair degree of confidence 
that there was at least one unique, undifferentiated asteroid out there 
for each chondrite group (H, L, LL, EH, EL, CI, CM, CO, CV, CR, CH, CK, 
R), plus more for the ungrouped chondrites (like Kakangari, and a 
bunch of others).

We can also say that there were more separate asteroids early in the
history of the solar system to account for the differentiated 
meteorites (coming from asteroids that melted).  The chemical and
isotopic compositions of igneous-group iron meteorites tell us that 
they represent many (perhaps >50) separate melted asteroids.  Of course, 
these asteroids have now been mostly destroyed, which is why pieces of
their cores can land on Earth.  The achondrites (HED-group, aubrites,
ureilites, etc.) probably sample a bunch more different asteroids.  In 
fact, there are only a few associations of different types of meteorites 
for which a good case can be made to support them coming from a single 
asteroid (like the howardites, eucrites, and diogenites all coming from 
Vesta, the IIE irons coming from an H-chondrite-like asteroid, or the 
main-group pallasites coming from the IIIAB iron parent body).

When you sum all of this up, we know with some certainty that there
were at least ~100 different bodies in existence at the end of 
the very early "accretional" epoch of the asteroid belt.  And, 
these are just the ones that happen to be supplying the Earth with 
meteorites at this moment, and doesn't take into account the possibility 
that there might have been more than one asteroid of each "type".

Jeff
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