Groundhog Day in the Kuiper Belt? How strength can trap KBOs in a collisional loop

The Pluto-Charon binary is generally assumed to have formed via a giant impact, though the impact conditions, including the impact angle, impact velocity, and pre-impact spin, remain poorly understood. Previous impact simulations, which approximated the progenitors as fluids, confirmed that collisional capture can produce a Pluto-Charon-like system (Canup 2005, 2011, Arakawa et al., 2019). However, ongoing advances in smoothed-particle hydrodynamics (SPH) codes have found that material strength is critical to impact outcomes, even at larger sizes (Emsenhuber et al., 2024). Implementation of strength for the Pluto-Charon-forming collision shifts the range of impact angles and velocities at which collisional capture is possible, and alters the process by which collisional capture is achieved. The presence of strength creates a "kiss-and-capture," in which Charon briefly merges with a pre-rotating Pluto before separating to become an intact satellite. Here, we present the results of impacts at higher impact velocities and more grazing impact angles, in which the impacting body becomes gravitationally bound, as before, but is locked into a more eccentric orbit that results in repetitive collisions. We find that this regime of "sisyphean collisions" is characterized by a proto-Charon that appears gravitationally bound, but is locked into a highly eccentric orbit and repetitive collisions. This semi-stable state can last for days to weeks, and occurs in an identifiable range of impact parameters, suggesting that it is another variation on collisional capture driven by the inclusion of material strength. We propose that such "sisyphean collisions," which could occur for other large KBO binaries, could drastically alter both bodies' thermal-orbital evolution.