On February 15, 2020, a pair of astronomers spotted a faint asteroid moving across their telescope's field of view. This is pretty common — they were observing as part of the Catalina Sky Survey, a collection of three telescopes in Arizona that scans the skies looking for near-Earth objects, asteroids that orbit the Sun and get near Earth.
But this one was different. It's not orbiting the Sun, it's orbiting the Earth!
That makes it a rare object indeed: a minimoon