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

Published in 2008:
*Icarus* **197,** 260-268.

**W.M. Grundy ^{1},
K.S. Noll^{2}, J. Virtanen^{3}, K. Muinonen^{4},
S.D. Kern^{2}, D.C. Stephens^{5}, J.A. Stansberry^{6},
H.F. Levison^{7}, and J.R. Spencer^{7}
**

(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.

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 ± 0.006
days and a semimajor axis of 1628 ± 29 km, implying a system mass of
(9.49 ± 0.52) × 10^{17} 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.