Earth trojan asteroid
Earth trojans are asteroids that orbit the Sun a similar orbital path as that of Earth in the vicinity of the Earth–Sun Lagrangian points L4 (leading 60°) and L5 (trailing 60°). 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° east or west of the Sun. Because people tend to search for asteroids at much greater elongations, few searches have been done in these locations.
The 300-meter-diameter asteroid 2010 TK7 has been determined to be associated with the Earth L4 Lagrangian point, leading Earth's orbit, by Martin Connors and colleagues of Athabasca University using the Wide-field Infrared Survey Explorer (WISE) satellite. It is the first confirmed Earth trojan.
- Currently, there are no confirmed objects, nor are there any objects suspected of being L5 trojans of Earth.
The orbits of Earth trojans could make them less energetically costly to reach 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.
Other companions 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. Several other small objects have been found in similar orbits. Although these objects are in 1:1 orbital resonance, they are not Earth trojans because each of them does not librate around a definite Sun–Earth Lagrangian point, either L4 or L5.
See also 
- Choi, Charles Q. (27 July 2011). "First Asteroid Companion of Earth Discovered at Last". Space.com. Retrieved 2011-07-27.
- Murray, C., “The Earth’s secret companion”, Nature 387, 651–652 (1997)