The Earth's celestial dance partner, the Moon, isn't the only object naturally orbiting our planet. In addition to man-made satellites and the International Space Station, the Moon is often joined by one more or "mini-moons"--asteroids that typically follow orbits around the sun but may occasionally be pulled into Earth orbit for a short period. The dance floor of space can be, in fact, a bit crowded.
Scientists recently used a supercomputer to calculate the trajectories of 10 million asteroids in the solar system that pass within the gravitational field of Earth. In the model, 18,000 of these objects were pulled into orbit around the Earth, in most cases for under a year, but in some instances potentially sticking with us for decades before being attracted back to an orbit around the sun. Based on the calculations, the study concluded that at least one asteroid is statistically likely orbiting the planet at any given time. But these mini-moons aren't visible to the naked eye, given that they can be as small as one meter in diameter. It's because of this small size that these mini-moons have erratic orbits, being affected by the gravity wells of the Earth, Sun, and even the Moon.
Claims of additional moons have been made by astronomers since the 1800s, though temporary mini-moons weren't spotted until 2006. Fortunately, these moons aren't any threat to Earth, either. Even the ones the size of a car would burn up in our atmosphere if they ever converted from mini-moon to killer asteroid.