Astronomers have recently discovered an asteroid dubbed 2016 HO3 that, whilst technically in orbit around the sun, is also spinning in its own merry dance around the Earth. Somewhere between 120 ft (40 m) and 300 ft (100 m) in size, our little traveling companion is estimated to have been in step with our orbit for about 100 years now, and will keep us company for many hundreds more. Too far away to be identified as a satellite to our planet, it is stable enough at the moment to be seen as a near-Earth companion body, or "quasi-satellite."
"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 NEO Studies at NASA's Jet Propulsion Laboratory (JPL). "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."
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Our relatively recent companion was spotted by the Pan-STARRS 1 asteroid survey telescope in Hawaii, that is funded by NASA's Planetary Defense Coordination Office. This is the same telescope that alerted the world to a sizable chunk of rock that whizzed past us at about 1.5 lunar distances on Halloween 2015, and continues to form an integral part of NASA's Near-Earth Object (NEO) Program.
From calculations and observations of the trajectory of its year-long elliptical loop around the sun, it appears that for around half of that orbit 2016 HO3 is nearer to the sun than Earth is, passing ahead of us, and then for the rest of the time it is farther away, so that it then ends up behind us. It also moves up and down relative to our planet as its orbit tilts in relation to ours, causing it to move through Earth's orbital plane once a year.
"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 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."
No news on its likelihood of ever colliding with the Earth, but for those who might like to keep a constant vigil on this and other near-Earth objects they can visit the NEO Program Website for regular updates.
The video below shows an animation of 2016 HO3's orbit around us and the sun.
Source: NASA JPL