The high-albedo Kuiper Belt object (55565) 2002 AW197

We detected thermal emission from the Kuiper Belt object 2002 AW(197) in 2003 December and again in 2004 April using the Multiband Imaging Photometer on the Spitzer Space Telescope. In combination with the absolute visual magnitude, the thermal measurements indicate a geometric albedo of 0.17 +/- 0.03 and a diameter of km. The albedo of 2002 AW(197) is significantly higher than the 0.04 value typically assumed for trans-Neptunian objects, and consequently the object is smaller than previously thought based on that assumption. Our thermal measurements at two wavelengths ( 24 and 70 mm) allow us to constrain the surface temperature and thereby place constraints on the thermal inertia. We find that the standard thermal model (STM) is inconsistent with the 24/70 mu m color unless we set the beaming parameter eta > 0.95, indicating that the object has a significant thermal inertia and, therefore, that the STM is inappropriate. The other end-member thermal inertia model is the fast-rotator, or isothermal-latitude, model (ILM). The data are well represented by an ILM with the pole of rotation inclined to the Sun by 45 degrees +/- 10 degrees. The high albedo is consistent with a surface containing significant amounts of weakly absorbing materials, with ices and/or fine-grained silicates as likely candidates.