NASA / JPL-CALTECH / UCLA / SCIENCE PHOTO LIBRARY NASA / JPL-CALTECH / UCLA / SCIENCE PHOTO LIBRARY
Earth Trojan asteroid. This is the orbit (green line) of 2010 TK7, the first Earth Trojan asteroid to be discovered. Earth's orbit is a blue dotted line. Trojans are astronomical objects trapped in the Lagrangian points (gravity wells) formed by two larger bodies, in this case the Earth and the Sun. This asteroid is at the L4 Lagrangian point, and was discovered in 2010 by NEOWISE, the asteroid-hunting portion of NASA's WISE (Wide-field Infrared Survey Explorer) mission. TK7 is about 300 metres across, and takes a year to trace each small loop, and 395 years to trace a larger libration loop (taking it behind the Sun).
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