The term 'Trojan Asteroids' was first used by the Piedmontese astronomer and mathematician Joseph-Louis Lagrange to describe (at that time theoretical) asteroids traveling in the same orbit as Jupiter, but 60° degrees ahead and behind, which he predicted to be stable points. In total he came up with 5 'Lagrange Points' (now named in his honor); points at which the gravity of two bodies cancel one another out, resulting in no net force. L₁ is between a satellite and the body it is orbiting (bearing in mind planets are satellites of the sun), L₂ is on the opposite side of a satellite to the body it is orbiting, L₃ is 180° ahead of (or behind) an orbiting body, in the same orbit, L₄ is 60° ahead of an orbiting body in the same orbit and L₅ is 60° behind an orbiting body in the same orbit.
Jupiter's Lagrange Points.
Strictly speaking the term 'Trojan Asteroid' should be restricted to those sharing an orbit with Jupiter. Traditionally these are named after figures from the Trojan wars, those at the L₄ point being named after Greeks and those at the L₅ point being named after Trojans. Bodies orbiting in the same orbit as other planets should be referred to as 'Lagrangian Asteroids', but this is seldom adhered to.
In the 28 July edition of the journal Nature, a paper by a team lead by Martin Conners of of the Centre for Science at Athabasca University and the Department of Earth and Space Sciences at the University of California Los Angeles, report the discovery of a Trojan Asteroid at Earth's L₄ Lagrange point. The asteroid, 2010 TK₇ was discovered using the NASA Wide-field Infrared Survey Explorer space telescope in October 2010. 2010 TK₇ implies the 185th object discovered during period 'T' (1-15 October) in 2010. It is a rock with a diameter of about 300 m, which migrates back and forth between the Earth's L₃ and L₄ Lagrange points over a period of about 400 years.
NASA animation of the orbit of 2010 TK₇.
2010 TK₇ is not the only asteroid to have an orbital relationship with Earth.
3753 Cruithne was originally discovered in October 1986 by Duncan Waldron of the UK Schmidt Telescope at Sliding Spring Observatory. In 1997 a paper in the journal Nature by a team lead by Paul A. Weigart of the Department of Physics and Astronomy at York University and two co-authors described the orbital relationship between Earth and 3753 Cruithne. Essentially 3753 Cruithne is in an elliptical orbit about the sun intersecting the Earth's, with an year of 364 days, very similar to that of Earth. Seen from the Earth 3753 Cruithne seems to track a kidney shaped path ahead of us in our orbit, though due to the slightly shorter year it is slowly drawing ahead of us, and will eventually catch up with us from behind. When it does this an exchange of gravitational energy will slow it in its orbit so that it falls behind again, till Earth eventually catches up with it, when another exchange of gravitational energy will cause it to speed up again.
The orbit of 3753 Cruithne as seen from outside the system.
The orbit of 3753 Cruithne as seen from Earth.
See also Visiting Vesta, and Asteroid 2011MD.