Collisions in the Kuiper belt

The Kuiper belt is a torus shaped reservoir of small icy bodies, located beyond the orbit of Neptune. The billions of comets, constituting the belt, are believed to have never undergone thermal evolution, and hence consist of the primordial material out of which the Solar System formed. Furthermore, the Kuiper belt is well recognized as the source of Short-Period Comets (SPCs. Several of these comets have been studied in-situ by spacecrafts. What is remarkable about these comets is their irregular shapes – as if two separate bodies have been “glued” together.

In 2015 the NASA New Horizons spacecraft made a close fly by the dwarf planet Pluto and then in 2019 it flew past the Cold Classical Kuiper Belt Object (CCKBO) - “Arrokoth”. The New Horizon’s mission was the first ever spacecraft to make a close approach and provide detailed images of the surface of a Kuiper belt object. Some of the most important features that New Horizons spacecraft observed, while flying only 3500 km above Arrokoth’s surface, were its bi-lobate shape¹ (similar to SPCs) and the deficit of small craters on its surface². The latter of these features might very well be indicative of a lack of catastrophic collisions in the trans-Neptunian region³, where most of the collisions between Kuiper belt objects are in a cratering regime rather than disruptive. In other words, the size-frequency distribution that we observe today, may be the same primordial distribution the objects have had since the formation of the Solar System, which may put constraints on the planet formation models – “planetesimal accretion” versus the “Streaming Instability” model^4,5. Lastly, the bi-lobate shape of Arrokoth and some SPCs may be indicative of formation of TNOs in a low collision speed environment.

Bibliography:

1. Spencer, J. et al. (2021) - The geology and geophysics of Kuiper Belt object (486958) Arrokoth; Science, Vol: 367

2. Singer, K. et al. (2019) - Impact craters on Pluto and Charon indicate a deficit of small Kuiper belt objects; Science, Vol: 363

3. Abedin, A. et al. (2022) - OSSOS XXVI: On the lack of Catastrophic Collisions in the Kuiper Belt; The Astronomical Journal, Vol:164;

4. Nesvorny, D. et al (2019) - Trans-Neptunian binaries as evidence for planetesimal formation by the streaming instability; Nature Astronomy; Vol: 3

5. Simon, J. et al. (2016) - Astrophysical Journal, Vol: 822