The Kuiper Belt vs the Asteroid Belt: Discoveries about the Kuiper Belt Size Distribution from New Horizons Data of Pluto, Charon, and Arrokoth

New Horizons observed impact craters on Pluto, Charon, and Arrokoth (also known as 2014 MU69) that were produced by other Kuiper belt objects in the size range of $\sim$50 meters to $\sim$50 km in diameter. The majority of Kuiper belt objects observed by Earth- and space-based telescopes are larger than 50 km in diameter, thus the New Horizons data provides new information on much smaller objects. The new information revealed a previously-unknown deficit of small Kuiper belt objects (KBOs) less than $\sim$1-2 km in diameter. This result was seen both at Pluto and Charon (Singer et al., 2019, Science) and at Arrokoth (Spencer et al., 2020, Science; McKinnon et al., 2020, Science). The images returned by New Horizons in early 2019 show Arrokoth is only modestly cratered, and potential craters on the surface show a shallow size-frequency distribution (SFD) similar to that of craters on Pluto and Charon. Both the apparent lack of craters overall, and the shallow SFD slopes, are consistent with a relatively benign collisional environment for Arrokoth. The size-distribution of small body populations are a signature of solar system formation and evolution processes. This deficit of objects smaller than 1 km gives the Kuiper belt population a different shape than the asteroid belt for objects between $\sim$1 km and 200 m in size (the lower end is bound by the smallest impact craters we can see in the New Horizons data). The slope of the Kuiper belt size distribution has a shallow differential power-law slope of approximately -1.8, whereas the Asteroid belt has an average slope closer to -3 in this size range. The shallow slope seen in the Kuiper belt is not representative of a population in traditional collisional equilibrium, and we discuss the implications for formation and evolution of the Kuiper belt and planetesimals in our own solar system, and in other solar systems. Many models assume dust or debris has a collisional size distribution, but the New Horizons data implies there may be more possible outcomes for the size-distribution of evolved planetesimal populations than the traditional collisional equilibrium slope (of approximately -3.5).