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Trojan asteroid[‚trō·jən ′as·tə‚rȯid]
(also Trojan planet, Trojan), in astronomy, one of a group of asteroids that revolve around the sun in such a way that their mean celestial longitudes are always approximately 60° more or 60° less than the celestial longitude of Jupiter. The sun, Jupiter, and each of the Trojan asteroids form an approximate equilateral triangle in space.
The periods of revolution of the Trojans around the sun and, consequently, the Trojans’ mean distances from the sun are almost exactly equal to the period of revolution and the mean distance, respectively, of Jupiter. Asteroids with these characteristics of motion are named for heroes of the Trojan War; hence the term “Trojans.” The study of the Trojans is of great interest since the motion of each such asteroid corresponds approximately to the motion in the Lagrange problem, a special case of the three-body problem that has been completely solved (seeCELESTIAL MECHANICS).
In all, 15 Trojan asteroids are known; they were discovered between 1906 and 1950. Their mean eccentricity is 0.096, and their mean inclination to the plane of the ecliptic is 16.4°. The Trojans are faint objects, having stellar magnitudes of 12.6 to 15.0. The photometric mean diameter of the Trojans is 140 km, which has been determined on the assumption that their albedo equals the arithmetic mean of the albedos of Mars and Mercury.