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Formation of asteroid pairs by rotational fission


Pairs of asteroids sharing similar heliocentric orbits, but not bound together, were found recently1,2,3. Backward integrations of their orbits indicated that they separated gently with low relative velocities, but did not provide additional insight into their formation mechanism. A previously hypothesized rotational fission process4 may explain their formation—critical predictions are that the mass ratios are less than about 0.2 and, as the mass ratio approaches this upper limit, the spin period of the larger body becomes long. Here we report photometric observations of a sample of asteroid pairs, revealing that the primaries of pairs with mass ratios much less than 0.2 rotate rapidly, near their critical fission frequency. As the mass ratio approaches 0.2, the primary period grows long. This occurs as the total energy of the system approaches zero, requiring the asteroid pair to extract an increasing fraction of energy from the primary's spin in order to escape. We do not find asteroid pairs with mass ratios larger than 0.2. Rotationally fissioned systems beyond this limit have insufficient energy to disrupt. We conclude that asteroid pairs are formed by the rotational fission of a parent asteroid into a proto-binary system, which subsequently disrupts under its own internal system dynamics soon after formation.

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Figure 1: Primary rotation periods P 1 versus mass ratios q of asteroid pairs.
Figure 2: A disparity between the lightcurve amplitudes of the primary components in asteroid pairs and binary systems.


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Research at Ondřejov was supported by the Grant Agency of the Czech Republic. D.V. was supported by the Czech Ministry of Education. D.P. was supported by an Ilan Ramon grant from the Israeli Ministry of Science, and is grateful for the guidance of N. Brosch and D. Prialnik. D.J.S. and S.A.J. acknowledge support by NASA's PG&G and OPR research programs. A.W.H. was supported by NASA and NSF. Work at Modra Observatory was supported by the Slovak Grant Agency for Science. The observations at Cerro Tololo were performed with the support of CTIO and J. Vasquez, using telescopes operated by the SMARTS Consortium. Work at Pic du Midi Observatory was supported by CNRS, Programme de Planétologie. Operations at Carbuncle Hill Observatory were supported by the Planetary Society’s Gene Shoemaker NEO Grant. Support for PROMPT has been provided by the NSF. F.M. and B.M. were supported by the NSF. We thank O. Bautista, T. Moulinier and P. Eclancher for assistance with observations with the T60 on Pic du Midi.

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Authors and Affiliations



This work was a team effort; here we specify only the most important contributions by individual authors. P.P. led the project and worked on most of its parts, except Supplementary Information sections 1 and 4. D.V. performed the backward orbit integrations and contributed to interpretations of the results. D.P. ran the observations and data reduction, and contributed to interpretations. D.J.S. developed the fission theory and worked out its implications for the observational data. A.W.H. contributed to interpretations and implications of the data. A.G., O.V., F.P., A.B., P.L., F.V., F.C., D.P.P., J.P., D.R., K.I., J.H., A.L., P.K., T.H., F.M., B.M., Yu.N.K., A.V.S. and A.L. carried out the observations, data reduction and analyses. S.A.J. ran simulations of the satellite ejection process in a proto-binary after fission.

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Correspondence to P. Pravec.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Figures 1- 45 with Legends, Supplementary Methods, Supplementary Data, Supplementary Discussions 1-2 with additional References and Supplementary Tables 1-2. (PDF 10292 kb)

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Pravec, P., Vokrouhlický, D., Polishook, D. et al. Formation of asteroid pairs by rotational fission. Nature 466, 1085–1088 (2010).

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