The outer Solar Sytem offers an interesting example of the potential of the Atacama telescope for Solar System studies.
Over the last decade, several hundred Solar System objects beyond Neptune have been discovered. Known as Kuiper Belt Objects (KBOs), they are believed to have formed very early on in the outer reaches of the protoplanetary disk around the Sun, and to have undergone very little evolution since then. The primitive nature of the material in this region holds important clues towards our understanding of the formation and evolution of the Solar System. These objects are small and very distant. The largest among them are Pluto and its moon Charon, respectively 2400 km and 1200 km in diameter; the third largest among KBOs is Varuna, with an estimated diameter of about 900 km. The measurement of the size of KBOs is an important goal, not only because we wish to know the population properties but also because a knowledge of the size allows estimates of the albedo and permits us to draw inferences on the physical conditions of their surfaces. With angular diameters of – at best – a few tens of milliarcseconds, they will appear unresolved in imaging surveys. To the first order, size can be estimated from optical observations, the orbital parameters and some value for the unknown albedo. This, however, teaches us nothing on the physical properties of their surfaces, for the value for the albedo needs to be assumed. A direct measurement of the size is possible from combined optical and IR measurements. At heliocentric distances on the order of 40 AU, KBOs have temperatures near 45 K, and their emissivity peaks near 70 µm. SIRTF and the JW(NG)ST (if it will retain mid IR instrumentation) will be able to detect KBOs in the mid IR. However, the Rayleigh--Jeans part of the KBOs' SED falls in the FIR/submm, and it is possible to measure KBO sizes from the ground. Using that technique, the SCUBA bolometer was recently used at 850 micron to measure the diameter of Varuna and Quaor. Because of the faintness of KBOs FIR emission and because of confusion with background sources, the FIR/submm technique is applicable to only the very brightest KBOs with existing telescopes. The AT would change that, allowing the detection of KBOs to sizes of order 100 km, making possibles a statistical investigation of both the size distribution and surface properties of those objects.