Lagrange points
/ lah-GRANZH /
In the gravity of two big bodies orbiting each other — the Sun and the Earth, say — are there any places where a small object can sit and simply keep pace, never falling behind or rushing ahead? Surprisingly, yes: there are exactly five such balance spots. They are the Lagrange points, parking places in a moving gravitational field where the pulls and the orbital motion conspire to hold a small body steady relative to the two giants.
At a Lagrange point the combined gravity of the two large bodies provides exactly the force needed to orbit at the same rate as the pair, so a spacecraft placed there co-rotates with them and stays put in the rotating frame. Three of the points (L1, L2, L3) lie along the line through the two bodies — L1 between them, L2 just beyond the smaller one, L3 on the far side of the larger one. The other two (L4 and L5) sit 60 degrees ahead of and behind the smaller body, at the tips of equilateral triangles with the two masses.
These points are prime cosmic real estate. L1 between Sun and Earth gives an unobstructed view of the Sun (home to solar observatories); L2 gives a stable, shaded perch facing away from the Sun (the James Webb and Gaia telescopes live there). Crucially, L4 and L5 are stable — small nudges leave a body gently circling the point rather than drifting off — which is why swarms of Trojan asteroids are trapped at Jupiter's L4 and L5, sharing its orbit forever.
More than a million Trojan asteroids share Jupiter's orbit, herded into two great clouds at its L4 and L5 points — 60 degrees ahead of and behind the planet — held there stably for the lifetime of the Solar System.
Nature uses the stable Lagrange points as long-term parking lots for whole populations of asteroids.
Only L4 and L5 are truly stable; L1, L2, and L3 are unstable saddle points, so spacecraft there must make small periodic course corrections to keep from drifting away — they are balanced like a pencil on its tip, not resting in a bowl.