How Earth picks up its extra moons

Everyone is familiar with the Moon, the Earth’s oldest and nearest companion in space. Now however, a team of astronomers have suggested that at any given moment, our planet has more than one satellite in orbit around it, though most of the time we are not even aware of it.

An impression of an asteroid orbiting Earth

Our Solar System is littered with millions of asteroids, pieces of debris left over from the formation of the planets, some 4.6 billion years ago. Some asteroids measure several kilometres in diameter, but many more are smaller than a metre across. Thousands of these asteroids orbit the Sun at distances similar to the Earth, and every now and again one will pass relatively close by.

Astronomers at the University of Helsinki, the University of Hawaii, and the Paris Observatory have calculated that there is a high probability that, at any given time, at least one of these small asteroids will have passed close to the Earth and that our planet will have temporarily captured it.

Smaller asteroids (those on the scale of a few metres or less in diameter), moving at a similar velocity as the Earth around the Sun would be the ones most susceptible to having their orbits interfered with by the Earth’s gravitational pull. As they pass close to our planet, they can be wrenched from their original path and snared by the Earth into a chaotic, corkscrewing orbit around our planet.

These new orbits are extremely tenuous and unstable however, and it is not usually long before the gravitational influence of the Sun overpowers that of Earth, or perhaps a close pass near our actual Moon kicks the captured asteroid back out into the wider Solar System. This entire process may happen over the course of just a few months, but it is thought that occasionally an asteroid will remain a mini moon for several decades.

The probability that Earth usually has at least one of these mini moons was calculated using the Jade supercomputer at CINES in Montpelier, France. Ten million asteroid fly-bys of Earth were simulated by the team using the supercomputer. Of these, 18,000 asteroids were temporarily deviated into an Earth orbit, and it was these asteroids that allowed the astronomers to determine their findings.

“This was one of the largest and longest computations I’ve ever done,” says Jeremie Vaubaillon of the Paris Observatory. “If you were to try to do this on your home computer, it would take about six years.”

The path that a mini moon might take as it is temporarily captured by Earth’s gravity. The inset shows an image of the near-Earth asteroid 1999 JM8. Credits: Karen Teramura/NASA

Although most mini moons are difficult to see directly due to their small size, in 2006 a relative large object was observed. 2006 RH120 has a diameter of approximately five metres, and was observed to orbit Earth four times over a period of eleven months, before breaking free in June 2007 to resume its journey around the Sun.

The ability to locate small asteroids within relatively easy reach of our planet also has possible future ramifications for our understanding of the formation of the Solar System. “Mini moons are scientifically extremely interesting,” says Robert Jedicke of the University of Hawaii at Manoa. “A mini moon 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.”

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