Icarus, 219, 676-688 (2012).

We report new Hubble Space Telescope and Spitzer Space Telescope results concerning the physical properties of the trans-neptunian object (TNO) binaries (120347) Salacia-Actaea (formerly 2004 SB60), and (42355) Typhon-Echidna (formerly 2002 CR46). The mass of the (120347) Salacia-Actaea system is 4.66 ± 0.22 × 1020 kg. The semi-major axis, period, and eccentricity of the binary orbit are a = 5619 ± 87 km, P = 5.49380 ± 0.00016 days, and e = 0.0084 ± 0.0076, respectively. In terms of the ratio of the semimajor axis to the radius of the Hill sphere, a/rH, (120347) Salacia-Actaea is the tightest TNO binary system with a known orbit. Based on hybrid Standard Thermal Model (hybrid-STM) fits to the data, the effective diameter and V-band geometric albedo of the system are D = 954 ± 109 km (making it one of the largest known TNOs), and pV=3.57-0.72+1.03%. Thermophysical models for (120347) Salacia suggest that it probably has a thermal inertia ⩽5 J m-2 s-1/2 K-1, although we cannot rule out values as high as 30 J m-2 s-1/2 K-1. Based on the magnitude difference between Salacia and Actaea, δ = 2.37 ± 0.06, we estimate their individual diameters to be d1 = 905 ± 103 km and d2 = 303 ± 35 km. The mass density of the components is ρ=1.16-0.36+0.59 g/cm3. Hybrid-STM fits to new Spitzer data for Typhon-Echidna give an effective diameter and V-band geometric albedo for the system of D = 157 ± 34 km, and pV=6.00-2.08+4.10%. Thermophysical models for (42355) Typhon suggest somewhat lower albedos (probably no higher than about 8.2%, as compared to the hybrid-STM upper limit of 10.1%). Taken together with the previously reported mass, this diameter indicates a density of ρ=0.60-0.29+0.72g/cm3, consistent with the very low densities of most other TNOs smaller than 500 km diameter. Both objects must have significant amounts of void space in their interiors, particularly if they contain silicates as well as water-ice (as is expected). The ensemble of binary-TNO densities suggests a trend of increasing density with size, with objects smaller than 400 km diameter all having densities less than 1 g/cm3, and those with diameters greater than 800 km all having densities greater than 1 g/cm3. If the eccentricity of the binary orbit of (42355) Typhon-Echidna is not due to recent perturbations, considerations of tidal evolution suggest that (42355) Typhon-Echidna must have a rigidity close to that of solid water ice, otherwise the orbital eccentricity of the system would have been damped by now.

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