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It hasn't been so famous since its discovery (in October 2010 by the Wide-field Infrared Survey Explorer satellite), and this is because it appears on the cover of the last July issue of the journal Nature. We're talking about 2010 TK7, an asteroid with a diameter of around 300 metres, that soon after its discovery was recognised as a rare Earth Co-orbital Asteroid (ECA), having an orbital period equal to the Earth due to a 1:1 resonance between the two bodies.
This asteroid is actually the most interesting of Earth's ECAs because its complex path (shown, exaggerated, in green above) is centred about the L4 Lagrangian point of the Earth's orbit. In other words the asteroid is a small Trojan, the name given to asteroids that share their orbit with planets in such a way that they always precede or follow the planet at a mean position of 60 degrees. Their stable location means that they do not represent an impact risk to the planet.
After the discovery of Trojans in the orbits of Jupiter, Mars and Neptune, starting with the Greek 588 Achilles (originally "Greeks" were distinguished from "Trojans" for those at +60 and -60 degrees from Jupiter) now the Earth has its first Trojan, though much smaller than its more distant cousins.
The orbital motion of 2010 TK7, that brings it no closer than 50 times the distance of the Moon, was determined by Martin Connors (Athabasca University, Alberta, Canada) and Christian Veillet (director of the Canada-France-Hawaii Telescope, Hawaii). Their observations were then used by Paul Wiegert (The University of Western Ontario, Canada) who simulated the complete orbital motion. The object (represented by the white disk in the illustration above) is
well over 20 million km away, and it will come no closer than this within the next 10,000 years.
We don't know for how long 2010 TK7 has been a Trojan of the Earth, nor for how long it will remain one, nor if there are others. It is likely, that on timescales shorter than hundreds of millions of years, gravitational perturbations by other planets (especially Jupiter) will render the orbit chaotic.
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