Three bodies, and the future stops being writable.
Two masses under gravity trace a perfect, eternal ellipse — Kepler solved that in an afternoon of the mind. Add a third, and you break something that cannot be repaired: the closed-form solution. There is no equation that hands you the positions at time t. There is only the next tiny step, and the next, forever.
In 1889 Henri Poincaré entered a prize competition to prove the solar system stable. He found the opposite. In the three-body problem, orbits could fold back on themselves in a tangle so intricate that no drawing could contain it — the first sighting of what we now call chaos.
Chaos is not randomness. Every body here obeys one clean law, run deterministically. The trouble is sensitive dependence: shift a starting position by a hair and the paths, identical at first, diverge without limit. The universe is exactly predictable and completely unforecastable at once.
The rate of that divergence has a name — the Lyapunov time. Beyond it, memory of the initial conditions is gone. For the inner solar system it is around five million years; for the toy overhead, a few seconds. Wait long enough and a system always forgets how it began.
And yet inside the mess hide islands of grace: Lagrange points where a small body rides for free, tight binary pairs that survive the storm, and rare closed choreographies — three equal masses chasing each other forever around a single figure-eight. Order is not the absence of chaos. It floats within it.
Give it a star, a few planets, and a rogue — then look away. When you look back, it will have written something you could never have predicted, and can never undo.