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The common origin of family and non-family asteroids


All asteroids are currently classified as either family, originating from the disruption of known bodies1, or non-family. An outstanding question is the origin of these non-family asteroids. Were they formed individually, or as members of known families but with chaotically evolving orbits, or are they members of old ghost families, that is, asteroids with a common parent body but with orbits that no longer cluster in orbital element space? Here, we show that the sizes of the non-family asteroids in the inner belt are correlated with their orbital eccentricities and anticorrelated with their inclinations, suggesting that both non-family and family asteroids originate from a small number of large primordial planetesimals. We estimate that ~85% of the asteroids in the inner main belt originate from the Flora, Vesta, Nysa, Polana and Eulalia families, with the remaining ~15% originating from either the same families or, more likely, a few ghost families. These new results imply that we must seek explanations for the differing characteristics of the various meteorite groups in the evolutionary histories of a few, large, precursor bodies2. Our findings also support the model that asteroids formed big through the gravitational collapse of material in a protoplanetary disk3.

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Fig. 1: IMB asteroid families.
Fig. 2: Dependence of the mean eccentricity and the mean inclination on asteroid size.
Fig. 3: Asteroid SFDs.
Fig. 4: Chaotic orbital evolution of resonant and non-resonant asteroids.


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A.A.C. thanks Z. Knežević for his advice on using ORBIT9 and on generating proper elements, and acknowledges the SFI/HEA Irish Centre for High-End Computing (ICHEC) for the provision of computational facilities and support. Astronomical research at the Armagh Observatory and Planetarium is funded by the Northern Ireland Department of Communities (DfC).

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S.F.D. initiated and directed the research and wrote the paper. A.A.C. performed the numerical investigations of chaotic orbital evolution and wrote the corresponding part of the Methods. D.L., T.J.J.K. and J.M.R. contributed to the data analysis.

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Correspondence to Stanley F. Dermott.

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Supplementary Figures 1–5, Supplementary Tables 1–2

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Dermott, S.F., Christou, A.A., Li, D. et al. The common origin of family and non-family asteroids. Nat Astron 2, 549–554 (2018).

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