Asteroid (469219) 2016 HO₃, the smallest and closest Earth quasi-satellite

A number of Earth co-orbital asteroids experience repeated transitions between the quasi-satellite and horseshoe dynamical states. Asteroids 2001 GO<inf>2</inf>, 2002 AA<inf>29</inf>, 2003 YN<inf>107</inf> and 2015 SO<inf>2</inf> are well-documented cases of such a dynamical behaviour. These transitions depend on the gravitational influence of other planets, owing to the overlapping of a multiplicity of secular resonances. Here, we show that the recently discovered asteroid (469219) 2016 HO<inf>3</inf> is a quasi-satellite of our planet – the fifth one, joining the ranks of (164207) 2004 GU<inf>9</inf>, (277810) 2006 FV<inf>35</inf>, 2013 LX<inf>28</inf>, and 2014 OL<inf>339</inf>. This new Earth co-orbital also switches repeatedly between the quasi-satellite and horseshoe configurations. Its current quasi-satellite episode started nearly 100 yr ago and it will end in about 300 yr from now. The orbital solution currently available for this object is very robust and our full <it>N</it>-body calculations show that it may be a long-term companion (time-scale of Myr) to our planet. Among the known Earth quasi-satellites, it is the closest to our planet and as such, a potentially accessible target for future <it>in situ</it> study. Due to its presumably lengthy dynamical relationship with the Earth and given the fact that at present and for many decades this transient object remains well positioned with respect to our planet, the results of spectroscopic studies of this small body, 26–115 m, may be particularly useful to improve our understanding of the origins – local or captured – of Earth's co-orbital asteroid population. The non-negligible effect of the uncertainty in the value of the mass of Jupiter on the stability of this type of co-orbitals is also briefly explored.