The moon has been orbiting Earth for nearly four billion years - but for most of that time, it's had at least one unseen companion.
The 'minimoons' are only a few feet across, and each tends to do a stint of around a year in orbit - before resuming their previous lives as asteroids.
Scientists hope to one day 'catch' one of the objects - which could offer a valuable sample of material largely unchanged since the dawn of the solar system 4.6 billion years ago.
The 'minimoons' are only a few feet across, and each tends to do a stint of around a year in orbit - before resuming their previous lives as asteroids.
Our 2,000-mile-diameter Moon, so beloved by poets, artists and romantics, has been orbiting Earth for over 4 billion years. Its much smaller cousins, dubbed ‘minimoons,’ are thought to be only a few feet across and to usually orbit our planet for less than a year before resuming their previous lives as asteroids orbiting the Sun.
A team including Robert Jedicke of the University of Hawaii at Manoa calculated the probability that at any given time Earth has more than one moon.
They used a supercomputer to simulate the passage of 10 million asteroids past Earth.
They concluded that at any given time there should be at least one asteroid with a diameter of at least one meter orbiting Earth.
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.
This is 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. While the typical minimoon would orbit Earth for about nine months, some of them could orbit our planet for decades.
When a minimoon 'breaks free', the space rock will resume its orbit around the sun.
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
‘This was one of the largest and longest computations I’ve ever done,’ said Vaubaillon. ‘If you were to try to do this on your home computer, it would take about six years.’
In 2006, the University of Arizona’s Catalina Sky Survey discovered a minimoon about the size of a car.
Known by the unimaginative designation 2006 RH120, it orbited Earth for less than a year after its discovery, then resumed orbiting the Sun.
‘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.’
The team’s paper, ‘The population of natural Earth satellites,’ appears in the March issue of the journal Icarus.
