Infrequent visitors of the Kozai kind: the dynamical lives of 2012 FC71, 2014 EK24, 2014 QD364, and 2014 UR

Abstract

Context. Asteroids with semi-major axes very close to that of a host planet can avoid node crossings when their nodal points are at perihelion and at aphelion. This layout protects the asteroids from close encounters, and eventual collisions, with the host planet. Aims. Here, we study the short-term dynamical evolution of four recently discovered near-Earth asteroids (NEAs) --2012 FC71, 2014 EK24, 2014 QD364, and 2014 UR-- that follow very Earth-like orbits. Methods. Our analysis is based on results of direct N-body calculations that use the most updated ephemerides and include perturbations from the eight major planets, the Moon, the barycentre of the Pluto-Charon system, and the three largest asteroids. Results. These four NEAs exhibit an orbital evolution unlike any other known near-Earth object (NEO). Beyond horseshoe, tadpole, or quasi-satellite trajectories, they follow co-orbital passing orbits relative to the Earth within the Kozai domain. Our calculations show that secular interactions induce librations of their relative argument of perihelion with respect to our planet but also to Venus, Mars, and Jupiter. Secular chaos is also present. The size of this transient population is probably large. Conclusions. Although some of these NEAs can remain orbitally stable for many thousands of years, their secular dynamics are substantially more complicated than commonly thought and cannot be properly described within the framework of the three-body problem alone owing to the overlapping of multiple secular resonances. Objects in this group are amongst the most atypical NEOs regarding favourable visibility windows because these are separated in time by many decades or even several centuries.