Impact of non-gravitational effects on chaotic properties of retrograde orbits

Dynamical studies of asteroid populations in retrograde orbits, that is with orbital inclinations greater than 90 degrees, are interesting because the origin of such orbits is still unexplained. Generally, the population of retrograde asteroids includes mostly Centaurs and transneptunian objects (TNOs). A special case is the near-Earth object (343158) 2009 HC82 from the Apollo group. Another interesting object is the comet 333P/LINEAR, which for several years was considered the second retrograde object approaching Earth. Another comet in retrograde orbit, 161P Hartley/IRAS appears to be an object of similar type. Thanks to the large amount of observational data for these two comets, we tested various models of cometary non-gravitational forces applied to their dynamics. The goal was to estimate which of non-gravitational perturbations could affect the stability of retrograde bodies. In principle, we study the local stability by measuring the divergence of nearby orbits. We numerically determined Lyapunov characteristic indicators (LCI) and the associated Lyapunov times (LT). This time, our calculations were extended by more advanced models of non-gravitational perturbations (i.e. Yarkovsky drift and in selected cases cometary forces). This allowed us to estimate chaos in the Lyapunov sense. We found that the Yarkovsky effect for obliquities of $\gamma=0^{\circ}$ and $\gamma=180^{\circ}$ can change the LT substantially. In most cases, for the prograde rotation, we received more stable solutions. Moreover, we confirmed the role of retrograde resonances in this process. Additionally, the studied cometary effects also significantly influence the long-term behaviour of the selected comets. The LT can reach values from 100 to over 1000 years. Conclusions. All of our results indicate that the use of models with non-gravitational effects for retrograde bodies is clearly justified.