 The densities and rock–ice fractions are known for only a small number of objects for which the diameters and the masses have been determined.The diameter can be determined by imaging with a high-resolution telescope such as the Hubble Space Telescope, by the timing of an occultation when an object passes in front of a star or, most commonly, by using the albedo of an object calculated from its infrared emissions.The masses are determined using the semi-major axes and periods of satellites, which are therefore known only for a few binary objects.