  Hubble Space Telescope observations of Uranus- and Neptune-crossing object
(65489) Ceto/Phorcys (provisionally designated 2003 FX128) 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 microns are combined with visible photometry to
constrain the system's effective radius (109 +10/-11 km) and geometric albedo
(0.084 +0.021/-0.014). We estimate the average bulk density to be 1.37
+0.66/-0.32 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 SM165, and (47171) 1999 TC36, 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.
