Earth usually has more than one moon, say astronomers. Asteroids orbiting the sun might temporarily become minimoons, following complicated paths around Earth for a time. Eventually, they would break free of Earth’s gravity – only to be immediately recaptured into orbit around the sun. Astronomers simulated the orbits of minimoons with a supercomputer and published their work in the March 2012 issue of the journal Icarus.
Moons are defined as Earth’s natural satellites. They orbit around the Earth. The little moons envisioned by these astronomers might be only a few feet across and might orbit our planet for less than a year before going back to orbit the sun as asteroids.
According to astronomers’ simulation, most asteroids that are captured by Earth’s gravity wouldn’t orbit Earth in neat ellipses, as our 2,000-mile-diameter (3,000-kilometer-diameter) moon does. Instead, these small bodies in space – minimoons sometimes less than a meter across – would follow complicated, twisting paths, as shown in the image above.
The moon’s neat orbit comes from the fact that it’s tightly held by Earth’s gravity. Minimoons would be literally tugged from many sides by the Earth, moon and sun, resulting in their complicated – and temporary – orbits.
According to these astronomers, a minimoon would remain captured by Earth until a particularly strong tug by the sun or moon broke the pull of Earth’s gravity. At that point, the sun would once again take control of the former minimoon, which would then go back to being an asteroid. While the typical minimoon would orbit Earth for about nine months, some of them could orbit our planet for decades, the astronomers say.
Mikael Granvik (formerly at UH Manoa and now at Helsinki), Jeremie Vaubaillon (Paris Observatory) and Robert Jedicke (UH Manoa) used a supercomputer to simulate the passage of 10 million asteroids past Earth. They said the calculations were so complex that – if you tried them on your home computer – it would take you six years to finish them. They concluded that at any given time there should be at least one asteroid with a diameter of at least one meter orbiting Earth. Of course, there may also be many smaller objects orbiting Earth, too.
We have seen a minimoon in the past. The University of Arizona’s Catalina Sky Survey discovered a minimoon in 2006. Known to astronomers as 2006 RH120, it was about the size of a car. It orbited Earth for less than a year after its discovery, then resumed its orbit around the sun.
Bottom line: Astronomers Mikael Granvik, Jeremie Vaubaillon and Robert Jedicke used a supercomputer to simulate the passage of 10 million asteroids past Earth. They determined that Earth often captures an asteroid into a temporary orbit, resulting in a new natural satellite for our planet – a second moon.
Deborah Byrd created the EarthSky radio series in 1991 and EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.