  A considerable amount of information regarding the processes that occurred
during the accretion of the early planetesimals is still present among the
small bodies of our solar system. A review of our current knowledge of the
density of small bodies is presented here. Intrinsic physical properties of
small bodies are sought by searching for relationships between the dynamical
and taxonomic classes, size, and density. Mass and volume estimates for 287
small bodies are collected from the literature. The accuracy and biases
affecting the methods used to estimate these quantities are discussed and
best-estimates are strictly selected. Bulk densities are subsequently computed
and compared with meteorite density, allowing to estimate the macroporosity
within these bodies. Dwarf-planets apparently have no macroporosity, while
smaller bodies can have large voids. This trend is apparently correlated with
size: C and S-complex asteroids tends to have larger density with increasing
diameter. The average density of each Bus-DeMeo taxonomic classes is computed.
S-complex asteroids are more dense on average than those in the C-complex that
in turn have a larger macroporosity, although both complexes partly overlap.
Within the C-complex, B-types stand out in albedo, reflectance spectra, and
density, indicating a unique composition. Asteroids in the X-complex span a
wide range of densities, suggesting that many compositions are included in the
complex. Comets and TNOs have high macroporosity and low density, supporting
the current models of internal structures made of icy aggregates. The number of
density estimates sky-rocketed during last decade from a handful to 287, but
only a third of the estimates are more precise than 20%. Several lines of
investigation to refine this are contemplated, including observations of
multiple systems, 3-D shape modeling, and orbital analysis from Gaia
astrometry.
