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

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.04.004 Byline: W.M. Grundy (a), K.S. Noll (b), J. Virtanen (c), K. Muinonen (d), S.D. Kern (b), D.C. Stephens (e), J.A. Stansberry (f), H.F. Levison (g), J.R. Spencer (g) Keywords: Kuiper belt; Trans-Neptunian objects; Centaurs; Satellites; general Abstract: 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[+ or -]0.006 days and a semimajor axis of 1628[+ or -]29 km, implying a system mass of (9.49[+ or -]0.52)x10.sup.17 kg. The eccentricity of the orbit is 0.526[+ or -]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.sub.-16.sup.+14 and 42.sub.-9.sup.+8 km, respectively. These numbers give an average bulk density of only 0.44.sub.-0.17.sup.+0.44 gcm.sup.-3, consistent with very low bulk densities recently reported for two other small transneptunian binaries. Author Affiliation: (a) Lowell Observatory, 1400 W. Mars Hill Rd., Flagstaff, AZ 86001, USA (b) Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA (c) Finnish Geodetic Institute, Geodeetinrinne 2, P.O. Box 15, FI-02431 Masala, Finland (d) Department of Astronomy, Kopernikuksentie 1, P.O. Box 14, FI-00014 University of Helsinki, Finland (e) Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA (f) Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA (g) Southwest Research Institute, 1050 Walnut St. #300, Boulder, CO 80302, USA Article History: Received 23 July 2007; Revised 10 March 2008