minimoon illustration
The path of a simulated minimoon that is temporarily captured by Earth (illustration by K. Teramura, UH Mānoa Institute for Astronomy)

Earth usually has more than one moon, according to the University of Hawaiʻi at Mānoa Institute for Astronomy Specialist Robert Jedicke, Mikael Granvik (formerly of Mānoa and now at the University of Helsinki) and Jeremie Vaubaillon (Paris Observatory). The 2,000-mile-diameter Moon’s much smaller cousins, dubbed minimoons, are thought to be only a few feet across and usually orbit our planet for less than a year before resuming their previous lives as asteroids orbiting the Sun. Their research was published in the March issue of the journal Icarus.

The researchers calculated the probability that at any given time Earth has more than one moon. They used a supercomputer to simulates the passage of 10 million asteroids past Earth and tracked the trajectories of the 18,000 objects that were captured by Earth’s gravity. They concluded that at any given time there should be at least one asteroid with a diameter of at least one meter orbiting Earth.

Minimoons follow crazy path

According to the simulation, most asteroids that are captured by Earth’s gravity would not orbit Earth in neat circles. Instead, they would follow complicated, twisting paths because a minimoon would not be tightly held by Earth’s gravity so it would be tugged into a crazy path by the combined gravity of Earth, the Moon and the Sun.

A minimoon would remain captured by Earth until one of those tugs breaks the pull of Earth’s gravity, and the Sun once again takes control of the object’s trajectory. A typical minimoon orbits Earth for about nine months but some could orbit our planet for decades.

“Minimoons are scientifically extremely interesting,” said Jedicke. “A minimoon could someday be brought back to Earth, giving us a low cost way to examine a sample of material that has not changed much since the beginning of our solar system over 4.6 billion years ago.”

Read the news release for more.