About the cover
More than 200 years ago, mathematician Joseph-Louis Lagrange predicted the existence of what became known as Trojan asteroids — small bodies that can stably share the orbit of a planet if they remain near triangular points' 60° ahead of or behind it in its orbit. Jupiter has thousands of Trojans; Mars and Neptune have some too. Now Earth is shown to have a Trojan. A search of data collected by NASAs Wide-field Infrared Survey Explorer (WISE) satellite revealed the asteroid 2010 TK7 as a strong candidate, and subsequent optical observations confirm its status as a Trojan companion of Earth, oscillating around the L4 (leading) Lagrange triangular point. The cover imagines 165 years of motion of 2010 TK7 seen from a vantage point outside of, and above, Earths orbit. Cover image: Paul Wiegert.
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