Jupiter drags two rival camps of asteroids named for the Trojan War
Sixty degrees ahead of Jupiter and sixty degrees behind it, two gravitational sweet spots hold more than a million asteroids. Astronomers named those ahead after Greek heroes of the Iliad and those behind after Trojans, so the two camps chase each other around the Sun forever, never meeting, like armies frozen in formation.
Those sweet spots are Lagrange points, places where a small object can keep a fixed position relative to two big bodies orbiting each other. Every such pair, whether Sun and Earth or Earth and Moon, has five. Three sit on the line through the two bodies: one between them, one beyond the smaller, one beyond the larger. The other two form equilateral triangles with the pair. Leonhard Euler found the three in-line points around 1750, and Joseph-Louis Lagrange found the triangular pair a decade later, publishing his essay on the three-body problem in 1772.
At the in-line points the balance is precarious. Take the point between Sun and Earth: anything orbiting closer to the Sun would normally circle faster than Earth, but Earth's pull there cancels just enough of the Sun's to stretch its year to exactly match ours. Nudge an object off such a point and it drifts away, so natural bodies rarely stay, and spacecraft must make small but constant corrections. The triangular points, by contrast, are stable when one mass exceeds the other by a factor of more than about 25; a disturbed object loops around them in a kidney-shaped path.
That stability gathers debris. Mars has four accepted trojans, Neptune several dozen, and Earth at least two asteroids plus dust. Saturn's moon Tethys shepherds two tiny moons at its triangular points. One version of the Moon's origin story even proposes that a body called Theia formed at an Earth-Sun triangular point before its orbit destabilised and it smashed into our planet.
The unstable points are prized real estate for spacecraft, since holding position there takes little fuel. About 1.5 million kilometres sunward, DSCOVR watches the solar wind and photographs Earth. The same distance in the opposite direction, the James Webb Space Telescope can shade itself from Sun, Earth and Moon all at once without turning its sunshield, while Gaia and Euclid orbit nearby.
Source: Lagrange point
