News Space 17 June 2016
1 minute read 

Don't look now, but we're being followed...


...by an asteroid that tracks our every movement. Bill Condie reports.


As we orbit the Sun, a little asteroid orbits us, scientists have discovered, in a dance that will go on for centuries.

The new asteroid, named 2016 HO3, is too distant to be considered a satellite of our planet like the Moon, but astronomers designate this constant companion a “quasi-satellite”.

It “loops around our planet, but never ventures very far away as we both go around the sun”, says Paul Chodas, manager of NASA's Center for Near-Earth Object (NEO) Studies at the Jet Propulsion Laboratory in Pasadena, California.

“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 on to 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.”

During a full orbit round the Sun (a year), the asteroid 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 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.”

Asteroid 2016 HO3 was first spotted on 27 April 2016, by the Pan-STARRS 1 asteroid survey telescope on Haleakala, Hawaii, operated by the University of Hawaii.

Astronomers aren’t quite sure how big the asteroid is but think it is likely to be between 40 metres and 100 metres across.

The Center for NEO Studies website has a complete list of recent and upcoming close approaches, as well as all other data on the orbits of known NEOs you can sue to track them yourself.

You can also get asteroid news and updates from AsteroidWatch on Twitter.

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Bill Condie is publisher of Cosmos.