The five known satellites of Uranus are too small and distant to permit direct measurement of their diameters. Consequently, products of the masses of the satellites, and estimates of their bulk densities and surface albedos have been used in discussions of their probable dimensions in the absence of measurements. The presence of water ice on the surfaces of Ariel, Umbriel, Titania and Oberon has been established spectrophotometrically1-4 and the brightnesses of the satellites have been measured photoelectrically 5. Determination of the masses of the uranian satellites depends on observations of perturbations of the satellite orbits and results chiefly in upper limits to the products of the masses of various pairs of satellites 6,7. The new diameter measurements reported here were made using the photometric/radiometric technique of diameter determination. This widely used technique has enabled the measurement of the diameters and albedos of approximately 250 asteroids and many planetary satellites. A recent recalibration of the technique using independent measurements of the diameters of three Solar System objects results in absolute accuracies of better than 5%. Our new albedo measurements show that Umbriel, Titania and Oberon are similar in albedo to J4 Callisto while Ariel is very similar in albedo to S7 Hyperion; the diameters of all four satellites are similar to those of the large, icy saturnian satellites S5 Rhea and S8 Iapetus.
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