🚀 go-pugleaf

 > There are sparse data, indeed, on the heterogeneity of the surfaces of
 > single asteroids. As for the innards, I expect they are as diverse as the
 > meteorites, which are pretty diverse.

As to small-scale surface heterogeneity, that is correct.  However one can 
look for changes in color or polarization as an object rotates or as we see
different aspects from one apparition to another, and generally there are no
detectable changes.  Probably many of them are rather compositionally 
homogeneous clear through.  But also I think each asteroid, under multiple 
impacts, tends to blanket itself in a well-mixed layer of its own dust.
Certainly most of the smaller asteroids must be like that.

Exceptions include Vesta, which has long been known to show large-scale
variegation (apparently different lava flows on it surface, and some exposed
pieces of its olivine mantle), and too a lesser degree some of the larger
S-types.  

Years ago I noted that Ceres is remarkably uniform polarimetrically, and I 
predicted that it would not show any large-scale albedo features.  That was 
confirmed recently by HST images by Rudi Albrecht et al.

As to internal homogeneity, we do have the evidence of the Hirayama families.
The large Eos, Koronis, and Themis families each stand out against the 
background spectroscopically; clearly they are fragments of homogeneous
bodies.  Lately Rick Binzel has looked at quite a few small members of the
Vesta family, and they all show the charactistic basaltic composition with
the individual variations that you would expect.  However Vesta is essentially
intact, and we don't have any direct information on its deep interior.

About 15 years ago I drew a fanciful reconstruction of the Nysa-Herta family: 
135 Hertha made the metallic core and 44 Nysa the enstatite mantle, both
differentiated from an original enstatite-chondritic or similar composition.
That picture fell into disfavor when it was discovered that the smaller 
fragments of the family are all of the dark P type, but recently Mike Gaffey
has resurrected my picture with a geochemical explanation for the P's.

Then there are some noteworthy families that don't make a bit of geochemical
sense. 

Ben