Earth's mini Moon has been circling us for nearly a century, Nasa finds

The little satellite is too far away to be considered a true moon, but has been designated a 'near-Earth companion'

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The Independent Online

Earth has been followed by a mini Moon for nearly a century – and scientists have only just spotted it.

A tiny asteroid has been orbiting us for years and is likely to continue doing so for centuries to come, according to scientists.

Although the rock is too distant from us to really be a new moon, Nasa says that it is stable enough in our orbit to be considered a “near-Earth companion” or a “quasi-satellite”.

The asteroid is called 2016 HO3 – taking its name from the year in which it was discovered – and orbits the sun but also stays as a constant companion to Earth.

"Since 2016 HO3 loops around our planet, but never ventures very far away as we both go around the sun, we refer to it as a quasi-satellite of Earth," said Paul Chodas, manager of NASA's Center for Near-Earth Object (NEO) Studies at its Jet Propulsion Laboratory. "One other asteroid – 2003 YN107 – followed a similar orbital pattern for a while over 10 years ago, but it has since departed our vicinity.

“This new asteroid is much more locked onto us. Our calculations indicate 2016 HO3 has been a stable quasi-satellite of Earth for almost a century, and it will continue to follow this pattern as Earth's companion for centuries to come."

2016 HO3 spends about half of the year closer to the sun than to us, passing ahead of Earth. But the rest of the time it falls away. Nasa describes its orbit as a game of leap frog with Earth – though one that’s further complicated because the asteroid also moves up and down in its orbit.

“The asteroid's loops around Earth drift a little ahead or behind from year to year, but when they drift too far forward or backward, Earth's gravity is just strong enough to reverse the drift and hold onto the asteroid so that it never wanders farther away than about 100 times the distance of the moon," said Mr Chodas. "The same effect also prevents the asteroid from approaching much closer than about 38 times the distance of the moon. In effect, this small asteroid is caught in a little dance with Earth."

The mini Moon was first spotted in at the end of April by a telescope based in Hawaii. Its size hasn’t yet been confirmed, but it’s probably between 40 meters and 100 meters, Nasa said.