A trio of horseshoes: past, present and future dynamical evolution of Earth co-orbital asteroids 2015 XX169, 2015 YA and 2015 YQ1

It is widely accepted that a quasi-steady-state flux of minor bodies moving in and out of the co-orbital state with the Earth may exist. Some of these objects are very good candidates for future in situ study due to their favourable dynamical properties. In this paper, we show that the recently discovered near-Earth asteroids 2015 XX_{169}, 2015 YA and 2015 YQ₁ are small transient Earth co-orbitals. These new findings increase the tally of known Earth co-orbitals to 17. The three of them currently exhibit asymmetric horseshoe behaviour subjected to a Kozai resonance and their short-term orbital evolution is rather unstable. Both 2015 YA and 2015 YQ₁ may leave Earth's co-orbital zone in the near future as they experience close encounters with Venus, the Earth-Moon system and Mars. Asteroid 2015 XX_{169} may have remained in the vicinity of, or trapped inside, the 1:1 mean motion resonance with our planet for many thousands of years and may continue in that region for a significant amount of time into the future.