To estimate the density and, hence, bulk composition of a planetary body requires separate measurements of its mass and size. Of all trans-neptunian objects, this has been accomplished only for Triton, where Doppler tracking of the gravitational deflection of Voyager 2 yielded its mass and Voyager 2 images gave accurate measurements of size. The lack of precise radii and masses for Pluto and Charon makes it difficult to reach a robust conclusion on their densities. Improved values of the masses of Pluto and Charon will probably be provided by more precise measurements of the barycentric wobble. To achieve the necessary accuracy in measurements of their sizes will require a spacecraft flyby with a solar occultation. To determine the degree of differentiation of these bodies it will be necessary to measure high-order moments of their gravitational fields via Doppler tracking of a close flyby.
While the current location and orbital parameters are useful information for dynamical studies of the Kuiper Belt, better estimates of the sizes of KBOs are vital for estimating the total mass of the Kuiper Belt. The size distribution of the population (i.e., the number of objects as a function of their size) is a key indicator of collisional and accretional processes. Currently, the sizes of KBOs are inferred from their brightness by assuming a value for their albedo. Another way to measure size is to compare an object's thermal output with the amount of sunlight that it receives. At trans-neptunian distances from the Sun, equilibrium temperatures are 30 to 50 Kelvin so that thermal emissions from KBOs peak, according to Wein's law, in the infrared region at wavelengths between 10 and 100 microns. Accurate size measurement therefore requires a radiometric observation of a KBO's thermal flux at wavelengths of ~10 to 100 microns.
The smaller KBOs are at current lower limits for detectability with moderate-aperture telescopes. Deep searches have, in the past, been confined to the ecliptic plane and have covered only a small region of the sky. Statistical results imply that many bodies lie just at the edge of detectability by HST. For larger KBOs, search efforts on 2-meter-class telescopes must be continued to answer the question of the existence of objects with diameters between 200 and 2,400 km, objects apparently missing from the current population-number distribution.