Earth trojans are asteroids that orbit in the vicinity of the Earth-Sun Lagrangian points L4 and L5. They are named after the Trojan asteroids that are associated with the analogous Lagrangian points of Jupiter.
Their location in the sky as observed from Earth's surface would average about 60 degrees east or west of the Sun, and as people tend to search for asteroids at much greater elongations few searches have been done in these locations.
The 300 m diameter asteroid 2010 TK7 has been determined to orbit in association with the Earth L4 Lagrange point, leading the orbit of the Earth, by Martin Connors and colleagues of Athabasca University using the Wide-field Infrared Survey Explorer (WISE) satellite. It is the first confirmed Earth trojan.[1][2]
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The orbits of Earth trojans could make it less energetically costly to reach them than the Moon, even though they are dozens of times more distant. Such asteroids could one day be useful as sources of elements that are rare near the Earth's surface. On Earth, siderophiles such as iridium are difficult to find, having largely sunk to the core of the planet shortly after its formation. A small asteroid could be a rich source of such elements even if its overall composition is similar to Earth's; because of their small size, such bodies would lose heat much more rapidly than a planet once they had formed, and so would not have melted, a prerequisite for differentiation. Their weak gravitational fields also would have inhibited significant separation of denser and lighter material; a mass the size of 2010 TK7 would exert a surface gravitational force of less than 0.00005 times that of Earth.
Earth has a second companion, asteroid 3753 Cruithne, about 5 km across, in a peculiar type of orbital resonance called an overlapping horseshoe. It is probably only a temporary liaison.[3] Several other small objects have been found in similar orbits. While these objects are in 1:1 orbital resonance, they are not Earth trojans because they do not librate around the L4 or L5 Lagrangian points.
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