The figure above shows the mass density of the solar system as a function of distance from the Sun. The primordial density level is derived from the mass distribution of the planets. The decrease in density from 30 to 50 AU may be due to the scattering of material by the giant planets. Beyond 50 AU there are no observations to constrain the level and the density may increase. Interstellar Probe will observe the spatial density, composition, and size distribution of dust that results from collisional processes in the Kuiper Belt. Also under consideration is the possibility of surveying the population of Kuiper Belt objects >1 km in size with a small telescope.

Interstellar Probe can address the fundamental question of the radial extent of the primordial solar nebula, or, more precisely, the extent of the primordial planetesimal disk. This can be accomplished most directly by measuring the variation with heliocentric radius of the population of small bodies in the Kuiper Belt, or, less directly, by measuring the radial variation of the spatial density of dust grains derived from Kuiper Belt objects. Moreover, the Kuiper Belt is an analog for circumstellar disks around other stars. Improved understanding of its properties will aid in the interpretation of astronomical observations of planet-forming or planet-harboring disks around other stars.