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The Kuiper belt contains a vast number of objects in a flattened, ring-like volume beyond the orbit of Neptune. These objects are collisionally processed relics from the accretion disk of the Sun and, as such, they can reveal much about early conditions in the Solar system. At the cryogenic temperatures prevailing beyond Neptune, volatile ices have been able to survive since the formation epoch 4.5 Gyr ago. The Kuiper belt is the source of the Centaurs and the Jupiter-family comets. It is also a local analogue of the dust disks present around some nearby main-sequence stars. While most Kuiper belt objects are small, roughly a dozen known examples have diameters of order 1000 km or more, including Pluto and the recently discovered (and possibly larger) giant Kuiper belt objects 2003 UB, 2003 EL (a binary and a triple system, resp.) and 2005 FY.

Our knowledge of comets has advanced hugely since the 1986 apparition of Comet Halley provided the spur to spacecraft investigation. But our understanding is still very much ‘skin deep’. The mass of the nucleus, the density of the nucleus and its internal structure and strength are poorly understood. Whether the physical and chemical properties vary from place to place in a specific cometary nucleus, or from comet to comet is still a mater of conjecture. Recent low-resolution images of nuclei surfaces are improving our understanding, but the real breakthrough will probably have to wait until a probe lands on a cometary surface in 2014, and until future probes return material to Earth some decades later. In this chapter we summarise our knowledge of the physical properties and chemical composition of the cometary nucleus paying special attention to the four comets that have had their nuclei imaged by spacecraft, these being comets Halley, Borrelly, Wild-2 and Tempel-1. Special attention is paid to the rate at which the cometary nucleus decays and the effect this has on cometary activity and the thickness of the allencompassing surface dust layer. The results of recent cometary space mission are summarised, special attention being paid to the Deep Impact mission to comet Tempel-1.