  The most pristine remnants of the Solar system's planet formation epoch orbit
the Sun beyond Neptune, the small bodies of the trans-Neptunian object
populations. The bulk of the mass is in ~100 km objects, but objects at smaller
sizes have undergone minimal collisional processing, with New Horizons recently
revealing that ~20 km effective diameter body (486958) Arrokoth appears to be a
primordial body, not a collisional fragment. This indicates bodies at these
sizes (and perhaps smaller) retain a record of how they were formed, and are
the most numerous record of that epoch. However, such bodies are impractical to
find by optical surveys due to their very low brightnesses. Their presence can
be inferred from the observed cratering record of Pluto and Charon, and
directly measured by serendipitous stellar occultations. These two methods
produce conflicting results, with occultations measuring roughly ten times the
number of ~km bodies inferred from the cratering record. We use numerical
models to explore how these observations can be reconciled with evolutionary
models of the outer Solar system. We find that models where the initial size of
bodies decreases with increasing semimajor axis of formation, and models where
the surface density of bodies increases beyond the 2:1 mean-motion resonance
with Neptune can produce both sets of observations, though comparison to
various observational tests favours the former mechanism. We discuss how to
evaluate the astrophysical plausibility of these solutions, and conclude
extended serendipitous occultation surveys with broad sky coverage are the most
practical approach.
