First Trojan asteroid of Earth

Astronomers led by Martin Connors of Athabasca University have discovered a ‘Trojan’ asteroid that orbits the sun by traveling around one of Earth’s Lagrange points.  Let’s unpack that, shall we?

First, a Lagrange or libration point is the position where an object such as moon can remain stationary relative to two larger bodies.  Each two-body system has five such points.  In the diagram below, if Mass₁ is the sun and Mass₂ is the Earth, L₁ through L₅ represent the Lagrange points of that system.  A Trojan asteroid or moon orbits one of those Lagrange points.  Although the asteroid may lie in the path of the orbiting planet, it can never collide with that planet because it is always in front of or behind the planet.

Trojan asteroids have been seen in the orbits of Jupiter and Saturn, but never before in Earth’s orbit.  One would have to detect an exceedingly small object in the full glare of the sun to find one.  Thanks to the WISE telescope, astronomers were able to do just that.  At only 300 meters in diameter, asteroid 2010 TK7 is tiny indeed.  It travels around one of Earth’s Lagrange points, maintaining a distance of about 80 million kilometers from Earth.

This artist's concept illustrates the first known Earth Trojan asteroid, discovered by NEOWISE, the asteroid-hunting portion of NASA's WISE mission. The asteroid is shown in gray and its extreme orbit is shown in green. Earth's orbit around the sun is indicated by blue dots. The objects are not drawn to scale.

Image credit: Paul Wiegert, University of Western Ontario, Canada.   

If like me, you had difficulty visualizing the path a Trojan asteroid would take around the sun, the following animation may help.