(42355) Typhon-Echidna: Scheduling observations for binary orbit determination

Published in 2008: Icarus 197, 260-268.

W.M. Grundy1, K.S. Noll2, J. Virtanen3, K. Muinonen4, S.D. Kern2, D.C. Stephens5, J.A. Stansberry6, H.F. Levison7, and J.R. Spencer7

(1) Lowell Observatory, Flagstaff AZ.

(2) Space Telescope Science Institute, Baltimore MD.

(3) Finnish Geodetic Institute, Masala, Finland.

(4) Dept. of Astronomy, University of Helsinki, Helsinki, Finland.

(5) Brigham Young University, Provo UT.

(6) Steward Observatory, University of Arizona, Tucson AZ.

(7) Southwest Research Institute, Boulder CO.

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We describe a strategy for scheduling astrometric observations to minimize the number required to determine the mutual orbits of binary transneptunian systems. The method is illustrated by application to Hubble Space Telescope observations of (42355) Typhon-Echidna, revealing that Typhon and Echidna orbit one another with a period of 18.971 0.006 days and a semimajor axis of 1628 29 km, implying a system mass of (9.49 0.52) 1017 kg. The eccentricity of the orbit is 0.526 0.015. Combined with a radiometric size determined from Spitzer Space Telescope data and the assumption that Typhon and Echidna both have the same albedo, we estimate that their radii are 76+14-16 and 42+8-9 km, respectively. These numbers give an average bulk density of only 0.44+0.44-0.17 g cm-3, consistent with very low bulk densities recently reported for two other small transneptunian binaries.