New Horizons and the case of the missing Kuiper belt objects

Twenty years ago, only a few thousand objects in the asteroid belt between the orbits of Mars and Jupiter had been observed precisely enough to determine their orbits. Since then, that number has grown to more than half a million. Newer telescopes can observe smaller asteroids, many just hundreds of meters in diameter, and there are enormously more small asteroids than large ones. The size distribution reflects the asteroid belt’s collisional equilibrium. For billions of years, asteroids have been smashing into each other and either sticking together or shattering into smaller fragments, and the balance between those processes has reached a steady state.

Something fundamentally different seems to be going on in the Kuiper belt, the torus-shaped zone of dwarf planets and smaller objects beyond the orbit of Neptune. Kuiper belt objects (KBOs) smaller than tens of kilometers in diameter are difficult to see with Earth-based telescopes. But in 2015, when NASA’s New Horizons probe transmitted the first close-up images of Pluto and its largest moon Charon (shown in the figure), the presence of KBOs of various sizes was manifest in the hundreds of craters created by KBO impactors. Now Kelsi Singer (Southwest Research Institute in Boulder, Colorado) and her colleagues on the New Horizons team have analyzed the crater size distributions. The smallest craters they observed still outnumber the large ones—but by more than an order of magnitude less than collisional equilibrium would predict.

The deficit of small KBOs suggests not only that the Kuiper belt is not in collisional equilibrium, but that its collisional evolution is nearly negligible. If KBOs only rarely collide, then they’re a good approximation of the original planetesimals that populated the infant solar system more than 4 billion years ago. That insight could help researchers seeking to model the still unknown mechanism by which dust grains combined to form pebbles, boulders, and eventually planets (see, for example, Physics Today, November 2015, page 16 ): Somewhere along the way, the model would have to reproduce the KBO size distribution. (K. N. Singer et al., Science 363, 955, 2019 .)