It was realized in 1772 that small bodies can stably share the same orbit as a planet if they remain near ‘triangular points’ 60° ahead of or behind it in the orbit1. Such ‘Trojan asteroids’ have been found co-orbiting with Jupiter2, Mars3 and Neptune4. They have not hitherto been found associated with Earth, where the viewing geometry poses difficulties for their detection5, although other kinds of co-orbital asteroid (horseshoe orbiters6 and quasi-satellites7) have been observed8. Here we report an archival search of infrared data for possible Earth Trojans, producing the candidate 2010 TK7. We subsequently made optical observations which established that 2010 TK7 is a Trojan companion of Earth, librating around the leading Lagrange triangular point, L4. Its orbit is stable over at least ten thousand years.
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We thank the WISE team and JPL and NEODyS (University of Pisa) data services. Support came from Canada’s Natural Sciences and Engineering Research Council and Research Chairs. Problems with some 2010 TK7 positions reported in ref. 23 were pointed out by T. Spahr and D. Tholen. We are grateful to them for data reductions provided, and to D. Tholen, M. Micheli and G. T. Elliot for observations made in support of this study.
The authors declare no competing financial interests.
This file contains Supplementary Figures 1-2 with legends and the full legend for Supplementary Movie 1. (PDF 897 kb)
This movie shows an artist's conception of the Sun, Earth and asteroid 2010 TK7 (right) as seen by an observer revolving around the Sun near the Earth (see Supplementary Information file for full legend). (MPG 6257 kb)
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Connors, M., Wiegert, P. & Veillet, C. Earth’s Trojan asteroid. Nature 475, 481–483 (2011). https://doi.org/10.1038/nature10233
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