Earth has a collection of minimoons, which are small temporary moons that get captured for varying amounts of time. There was a NASA Future In-Space Operations (FISO) Working Group Presentation on minimoons.
Mikael Granvik (formerly at UH Manoa and now at Helsinki), Jeremie Vaubaillon (Paris Observatory) and Robert Jedicke (UH 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 then 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. Of course, there may also be many smaller objects orbiting Earth, too.
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. 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. While the typical minimoon would orbit Earth for about nine months, some of them could orbit our planet for decades.
There should be on average 2 one meter diameter minimoons and 12 half meter diameter minimoons. Once every 50 years there will be one about 5 meters in diameter. A 3 meter diameter object is one in ten year event.
The average is they are captured for 2.9 orbits and 9.5 months. Some can be captured for decades.
Minimoons are already temporarily captured which can make them easier targets for exploration or mining.
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