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Size and albedo of Kuiper belt object 55636 from a stellar occultation


The Kuiper belt is a collection of small bodies (Kuiper belt objects, KBOs) that lie beyond the orbit of Neptune and which are believed to have formed contemporaneously with the planets. Their small size and great distance make them difficult to study. KBO 55636 (2002 TX300) is a member of the water-ice-rich Haumea KBO collisional family1. The Haumea family are among the most highly reflective objects in the Solar System. Dynamical calculations indicate that the collision that created KBO 55636 occurred at least 1 Gyr ago2,3. Here we report observations of a multi-chord stellar occultation by KBO 55636, which occurred on 9 October 2009 ut. We find that it has a mean radius of 143 ± 5 km (assuming a circular solution). Allowing for possible elliptical shapes, we find a geometric albedo of in the V photometric band, which establishes that KBO 55636 is smaller than previously thought and that, like its parent body, it is highly reflective. The dynamical age implies either that KBO 55636 has an active resurfacing mechanism, or that fresh water-ice in the outer Solar System can persist for gigayear timescales.

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Figure 1: Light curves for KBO 55636.
Figure 2: Occultation chords and the radius of KBO 55636.

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We are grateful to W. M. Grundy for supplying HV magnitudes from his database and for discussions of water-ice surfaces in the outer Solar System; to W. B. McKinnon for discussions of the physical state of water-ice at low temperatures; to E. D. Schmidt for use of the telescope and participating in the observations at Behlen Observatory; to B. Carter for help in obtaining telescope time at Mt Kent, and to L. A. Young for assisting with the McDonald observations. We thank D. Byrne of the Visitor Information Station at the Onizuka Center for International Astronomy on Mauna Kea for use of their equipment and facilitating the observations from their site. J.W. thanks E. Gates of Lick Observatory, and E. Becklin, E. Pfueller, M. Wiedemann and M. Burgdort of SOFIA, for support of his observations. B. Sicardy provided several comments that improved the paper. Occultation research at MIT and Williams College is supported by NASA and NSF.

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Authors and Affiliations



J.L.E. helped plan the observations, consulted on the occultation prediction, analysed the data, and wrote the paper. M.J.P. organized the observers, performed observations from Brownsville, Texas, and consulted on the prediction, data reduction, text and figures. C.A.Z. analysed the data for the stellar occultation prediction and constructed the light curves. A.S.B. directed the data analysis for the occultation prediction. E.R.A. wrote the light-curve generation software. S.E.L. made astrometric observations and performed observations of the occultation from the USNO in Flagstaff. M.L. designed and built 12 PICO camera systems and attempted observations from Cairns. J.M.P. arranged for observations at several sites and helped to plan the observations. S.P.S. consulted on the design of the PICO. L.B., E.W.D., S.S.S. and T.T. supplied astrometric data for the occultation prediction. D.R. provided information used to derive the geometric albedo of KBO 55636. Authors identified in Supplementary Table 2 were responsible for the observations. All authors were given the opportunity to review the results and comment on the manuscript.

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Correspondence to J. L. Elliot.

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The authors declare no competing financial interests.

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This file contains Supplementary Information and Data, Supplementary Tables 1-5, Supplementary Figures 1-3 with legends and References. (PDF 1244 kb)

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Elliot, J., Person, M., Zuluaga, C. et al. Size and albedo of Kuiper belt object 55636 from a stellar occultation. Nature 465, 897–900 (2010).

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