Why Jupiter Has Two Asteroid Camps
🛰️ Orbital mechanics
Question
Why do two clouds of asteroids share Jupiter's orbit without colliding with it?
Jupiter has two clouds of asteroids riding along with it, one 60° ahead in its orbit and one 60° behind - the stable L4 and L5 points. They're named after the two sides of the Trojan War in the Iliad: the 'Greek' camp leads, the 'Trojan' camp trails. The first, 588 Achilles, was spotted in 1906. They don't crash into Jupiter because they're locked in a 1:1 resonance with it - each Trojan laps the Sun in the same ~11.86 years Jupiter does - and the 60° offset parks them at the two stable Lagrange points. Thousands are catalogued and millions more are estimated to exist, making the Trojans one of the largest asteroid populations in the Solar System.
The math
1:1 resonance: the Trojan orbital period equals Jupiter's (~11.86 yr), so they share the orbit without colliding. They sit at L4/L5, 60° ahead/behind Jupiter - the tips of equilateral triangles with the Sun, which is why those two points are the stable ones.
Why it matters
These are leftovers from the Solar System's birth - primitive bodies that never got folded into a planet, parked safely by Jupiter's gravity. Because they've been sitting undisturbed for ~4.5 billion years, they're a natural archive of the early Solar System, and NASA's Lucy mission, launched in 2021, is flying out to visit a handful of them up close.