The orbit, mass, size, albedo, and density of (65489) Ceto/Phorcys: A tidally-evolved binary Centaur

• Published in: Icarus (New York, N.Y. 1962) Vol. 191; no. 1; pp. 286 - 297
• Main Authors: Grundy, W.M., Stansberry, J.A., Noll, K.S., Stephens, D.C., Trilling, D.E., Kern, S.D., Spencer, J.R., Cruikshank, D.P., Levison, H.F.
• Format: Journal Article
• Language: English
• Published: San Diego, CA Elsevier Inc 01.11.2007; Elsevier
• Subjects: Astronomy; Centaurs; Earth, ocean, space; Exact sciences and technology; Kuiper belt; Satellites; Solar system; Trans-neptunian objects; Kuiper belt; Satellites; Trans-neptunian objects; Centaurs; Bulk density; Phoebe; Eccentricity; Ice; Orbits; Albedo; Effective radius; Neptune planet; Uranus planet; Close binary stars; Saturn satellite; Solar system; Age; Photometry
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2007.04.004

Hubble Space Telescope observations of Uranus- and Neptune-crossing object (65489) Ceto/Phorcys (provisionally designated 2003 FX 128) reveal it to be a close binary system. The mutual orbit has a period of 9.554 ± 0.011 days and a semimajor axis of 1840 ± 48   km . These values enable computation of a system mass of ( 5.41 ± 0.42 ) × 10 18   kg . Spitzer Space Telescope observations of thermal emission at 24 and 70 μm are combined with visible photometry to constrain the system's effective radius ( 109 −11 +10   km ) and geometric albedo ( 0.084 −0.014 +0.021 ) . We estimate the average bulk density to be 1.37 −0.32 +0.66   g cm −3 , consistent with ice plus rocky and/or carbonaceous materials. This density contrasts with lower densities recently measured with the same technique for three other comparably-sized outer Solar System binaries (617) Patroclus, (26308) 1998 SM 165, and (47171) 1999 TC 36, and is closer to the density of the saturnian irregular satellite Phoebe. The mutual orbit of Ceto and Phorcys is nearly circular, with an eccentricity ⩽0.015. This observation is consistent with calculations suggesting that the system should tidally evolve on a timescale shorter than the age of the Solar System.