In April 2016, astronomers discovered an asteroid, duly called 2016 HO3, that orbits the sun in such a way that it never strays too far from the Earth, making it a 'quasi-satellite' or 'quasi-moon' of our planet. The size of this object has not yet been firmly established, but it is likely between 40 and 100 meters in diameter.
This newly discovered satellite is firmly locked onto the Earth. Calculations indicate that 2016 HO3 has been a stable quasi-satellite of Earth for almost a century, and it will continue to follow this pattern as Earth's companion for centuries to come.
The path of 2016 HO3 tends to twist and drift over time, but Earth's gravitational pull keeps the asteroid contained: It never comes closer than 14.5 million kilometers to our planet, and it never gets more than 38.6 million kilometers away, researchers said.
In effect, this small mini-moon is caught in a little dance with Earth.
 NASA: Small Asteroid Is Earth's Constant Companion – June 15, 2016. See here.