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.
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).
The Center for NEO Studies website has a list of recent and upcoming asteroid close approaches, as well as all other data on the orbits of known NEOs.
For asteroid news and updates, follow AsteroidWatch on Twitter.
Or subscribe to Minor Planet Daily from the International Astronomical Union. They’ve been doing an awesome job on daily updates about asteroid close approaches.
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.
Deborah Byrd created the EarthSky radio series in 1991 and EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.