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D. T. Britt (University of Tennessee), G. J. Consolmagno (Vatican Observatory)
Measurements of asteroid bulk density, along with data on the grain densities of analogue meteorites, can provide insight on asteroid porosity and internal structure. A number of asteroid bulk density measurements have been made in recent years either by spacecraft tracking, observations of satellites, or observations of mutual gravitational events. These data have some intriguing implications. The densities of many (but not all) asteroids are substantially below their most likely meteorite analogues indicating large porosities. These data appear to divide into two rough groups. The first group are objects that, given the most likely meteorite analogues, appear to have bulk porosities between 0% and 35%. These probably represent asteroids that are coherent, although the objects at the high end of the range are probably heavily fractured. The second group, with estimated bulk porosities greater than 35%, are probably gravitationally-bound rubble piles, some of which have more empty space than solid material. Group 1 asteroids include the S type asteroids 243 Ida and 433 Eros that show bulk densities of approximately 2.6 g/cc and the C type asteroid 121 Hermione with a density of approximately 1.8 g/cc. The most likely meteorite analogues for these asteroids indicate bulk porosities probably ranging between 25-30%. Also in Group 1 are the large asteroids 1 Ceres, 2 Pallas, and 4 Vesta whose bulk densities and likely analogues indicate very low porosities. Group 2 asteroids are objects whose bulk densities are substantially below the grain density of the least dense meteorites. These include Phobos and Deimos, the C-type asteroids 253 Mathilde and 45 Eugenia, and the M-type 16 Psyche. In the case of Psyche, if its bulk composition is analogous to metallic meteorites, this object may be extremely porous with substantially more internal empty space than solid material.
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