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G. J. Consolmagno (Vatican Observatory), D. T. Britt (University of Tennessee)
An asteroid's bulk density, its analogous meteorite type (determined from its reflectance spectrum), and the grain density and porosity of the analog meteorite, can be used to estimate its bulk porosity. Subtracting the average meteorite analog microporosity from the asteroid bulk porosity gives an estimate of its large-scale macroporosity.
The estimated macroporosities for asteroids appear to divide into three groups. The large asteroids 1 Ceres, 2 Pallas, and 4 Vesta, as well as the somewhat smaller 20 Massalia, have bulk densities very close to the grain densities of their analog meteorites, indicating essentially zero macroporosity. These asteroids are probably strong, coherent objects that have not been disrupted through solar system history, suggesting a strong size selection for survival within the asteroid belt. The second group, including the S asteroids 433 Eros, 243 Ida, and others, have between 15 and 25% macroporosity indicating that they have been extensively fractured. Spacecraft images of both Eros and Ida show pervasive fracturing, but probably not extensive enough to disrupt the object. Such asteroids have coherent strength; on Earth, well-sorted sedimentary rocks can have up to 30% porosity and still be coherent. The third group, including 16 Psyche, 253 Mathilde, and others, have greater than 30% macroporosity. These objects may have been completely disrupted by collision, then reaccreted into an body with about as much empty space as solid material.
In general, the dark, primitive asteroids seem to be more prevalent in the "rubble-pile" group while the higher albedo, higher-temperature S-type asteroids seem to be more common in the "fractured" group. However, there are notable exceptions to this trend in each group, like the high porosity S-type 15 Eunomia and the low-porosity P-type 87 Sylvia. The collisional disruption history of the asteroid belt is complex and stochastic; mineralogy affects, but does not control, macroporosity.