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Is asteroid 2016 HO3 a second moon?

Here’s a word about asteroid 2016 HO3 – first spotted earlier this year – which astronomers say is a “constant companion” of Earth. That doesn’t mean it’s a second moon. It doesn’t orbit Earth; it orbits the sun. But its orbit keeps it as companion to Earth, and it will remain so for centuries to come. What’s more, as it orbits the sun, this asteroid appears to circle around Earth as well. That’s why the astronomers at NASA’s Center for Near-Earth Object (NEO) Studies at the Jet Propulsion Laboratory in Pasadena, California wrote about the object:

It is too distant to be considered a true satellite of our planet, but it is the best and most stable example to date of a near-Earth companion, or ‘quasi-satellite.’

Paul Chodas, manager of the Center for NEO Studies said:
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. 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.

Image of asteroid 2016 HO3 taken on June 10, 2016 by Denise Hung and Dave Tholen of the University of Hawaii. The asteroid is the bright dot near the center. During this 5-minute exposure, the telescope tracked the slowly moving asteroid, making the background stars appear trailed.

The Pan-STARRS 1 asteroid survey telescope on Haleakala, Hawaii first spotted asteroid 2016 HO3 on April 27, 2016. Since then, astronomers have learned that, in its yearly trek around the sun, the spends about half of the time closer to the sun than Earth and passes ahead of our planet, and about half of the time farther away, causing it to fall behind. Its orbit is also tilted a little, causing it to bob up and then down once each year through Earth’s orbital plane. The Center of NEO studies said:
In effect, this small asteroid is caught in a game of leap frog with Earth that will last for hundreds of years.

The asteroid’s orbit also undergoes a slow, back-and-forth twist over multiple decades. Paul Chodas explained:
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.
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 size of this object has not yet been firmly established, but it is likely larger than 120 feet (40 meters) and smaller than 300 feet (100 meters).

Animation of the discovery images of asteroid 2016 HO3, taken on April 27, 2016 by the Pan-STARRS NEO search program. Pan-STARRS is located on Haleakala, Maui, and run by the University of Hawaii’s Institute for Astronomy.

Bottom line: Is asteroid 2016 HO3 as second moon? It isn’t because it orbits the sun. But its orbit keeps it as a constant companion of Earth, and it’ll remain so for centuries to come.

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