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A collisional family of icy objects in the Kuiper belt


The small bodies in the Solar System are thought to have been highly affected by collisions and erosion. In the asteroid belt, direct evidence of the effects of large collisions can be seen in the existence of separate families of asteroids—a family consists of many asteroids with similar orbits and, frequently, similar surface properties, with each family being the remnant of a single catastrophic impact1. In the region beyond Neptune, in contrast, no collisionally created families have hitherto been found2. The third largest known Kuiper belt object, 2003 EL61, however, is thought to have experienced a giant impact that created its multiple satellite system, stripped away much of an overlying ice mantle, and left it with a rapid rotation3,4,5. Here we report the discovery of a family of Kuiper belt objects with surface properties and orbits that are nearly identical to those of 2003 EL61. This family appears to be fragments of the ejected ice mantle of 2003 EL61.

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Figure 1: The strength of the infrared water-ice absorption feature versus optical colour for all KBOs in our survey and those with published spectra.
Figure 2: The proper eccentricity and inclination of KBOs in our sample as a function of semimajor axis.
Figure 3: Simulations of the dispersion in orbital elements expected after the ejection of fragments in a giant impact.


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We thank R. Sari, B. McKinnon, K. Noll, T. Ahrens and A. Morbidelli for their suggestions and comments on this work. This research is supported by a grant to M.E.B. from NASA Planetary Astronomy. Author Contributions M.E.B. was lead author of this Letter, and K.M.B., D.R. and E.L.S. contributed equally.

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Correspondence to Michael E. Brown.

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Supplementary information

Supplementary Information

This file contains Supplementary Discussion, Supplementary Figures 1-2 with Legends, Supplementary Table 1 and additional references. The parts of Supplementary Information discuss all of the infrared spectra and colours the orbital dynamics of the family. (PDF 1211 kb)

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Brown, M., Barkume, K., Ragozzine, D. et al. A collisional family of icy objects in the Kuiper belt. Nature 446, 294–296 (2007).

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