Earths Trojan Asteroid
"Trojan" Asteroid Caught in a Synchronized Orbit with the Earth

Earths Trojan Asteroid
"Trojan" Asteroid Caught in a Synchronized Orbit with the Earth

A team of Canadian scientists have discovered a "Trojan" asteroid that is caught in a synchronized orbit with the Earth. Jeffrey Brown discusses what this discovery means with the California Institute of Technology's Mike Brown.

2010 TK7 is the first Earth Trojan asteroid to be discovered and it comes close to sharing the Earth's orbit around the Sun. Such objects had previously been observed only in the orbits of Mars, Jupiter, Neptune and several moons of Saturn. 2010 TK7 has a diameter of about 300 meters (1,000 ft).

Its path oscillates about the Sun-Earth L4 Lagrangian point (60 degrees ahead of Earth), shuttling between its closest approach to Earth and its closest approach to the L3 point (180 degrees from Earth), with a period of about 395 years.

The asteroid was discovered in October 2010 by astronomers from Athabasca University, UCLA, and the University of Western Ontario using NASA's Wide-field Infrared Survey Explorer.

The Wide-field Infrared Survey Explorer, a space telescope launched into Earth orbit in December 2009, imaged 2010 TK7 in October 2010 while carrying out a program to scan the entire sky from January 2010 to February 2011.

Spotting an asteroid sharing Earth's orbit is normally difficult from the ground, because their potential locations are generally in the daytime sky. After follow-up work at the University of Hawaii and the Canada-France-Hawaii Telescope, its orbit was evaluated on 21 May 2011 and the Trojan character of its motion was published in July 2011.

2010 TK7 has an absolute magnitude of luminosity (determinable because of its known location) of about 20.6. Based on an assumed albedo of 0.1, it would have an estimated diameter of about 300 meters. No spectral data are yet available to shed light on its composition.

2010 TK7 would exert a surface gravitational force of less than 0.00005 times that of Earth.

The asteroid has an orbital period of 365.389 days on its path around the L4 point, comparable to Earth's 365.256 days around the Sun.

On its eccentric (e = 0.191) orbit, 2010 TK7's distance from the Sun varies annually from 0.81 AU to 1.19 AU. It orbits in a plane inclined about 21 degrees to the plane of the ecliptic.

Trojan asteroids in actuality do not orbit right at Lagrangian points but oscillate in tadpole-shaped loops around them (as viewed in a corotating reference frame in which the planet and Lagrangian points are stationary).

However, 2010 TK7's loop is so unusually elongated that it sometimes travels nearly to the opposite side of the Sun from the Earth. Its orbit does not bring it any closer to the Earth than 20 million kilometers (12.4 million miles), which is more than 50 times the distance to the Moon.

In 2010/2011, TK7 was at the near-Earth end of its tadpole, which facilitated its discovery. 2010 TK7's orbit has a chaotic character, making long-range predictions difficult. Prior to A.D. 500, it may have been oscillating about the L5 Lagrangian point (60 degrees behind Earth), before jumping to L4 via L3. Short-term unstable libration about L3, and transitions to horseshoe orbits are also possible.

Since Earth Trojans share Earth's orbit and have little gravity of their own, it can sometimes be less energetically costly to reach them than the Moon, even though they are dozens of times more distant.

However, 2010 TK7 is not an energetically attractive target for a space mission because of its orbital inclination: It moves so far above and below the Earth's orbit that the required change in velocity for a spacecraft to match its trajectory coming from Earth's would be 9.4 km/s, while some other near-Earth asteroids require less than 4 km/s.