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Earth's second moon discovered

We all know that our planet has only one moon. Right? Well, the correct answer might depend on your definition of the word 'moon'.

In April 2016, astronomers discovered an asteroid, duly called 2016 HO3, that orbits the sun in such a way that it never strays too far from the Earth, making it a 'quasi-satellite' or 'quasi-moon' of our planet. The size of this object has not yet been firmly established, but it is likely between 40 and 100 meters in diameter[1].

This newly discovered satellite is firmly locked onto the Earth. Calculations indicate that 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.
As the space rock circles the sun, it loops around Earth as well, zooming ahead of the planet half of the time and trailing behind the other half, NASA officials said. 2016 HO3's orbit is also tilted slightly relative to that of Earth, so the asteroid also bobs up and down through our planet's orbital plane.

The path of 2016 HO3 tends to twist and drift over time, but Earth's gravitational pull keeps the asteroid contained: It never comes closer than 14.5 million kilometers to our planet, and it never gets more than 38.6 million kilometers away, researchers said. In effect, this small mini-moon is caught in a little dance with Earth.

As our close companion, 2016 HO3 should get a proper name. In Greek mythology, Selene is the goddess of the moon. Ersa is the daughter of Zeus and Selene. She's is the goddess of dew. Say 'goodbye' to 2016 HO3 and say 'hello' to Ersa.


[1] NASA: Small Asteroid Is Earth's Constant Companion – June 15, 2016. See here.

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