Drag a body to place it. Drag from a body to set its launch velocity.
Two bodies orbiting under gravity have a clean formula, an ellipse, worked out centuries ago. Add a third body and that formula breaks. No one has ever found one, and mathematicians now know no such tidy formula exists in general.
So we just simulate it, stepping gravity forward in tiny slices of time, same as the sandbox above is doing live.
Chaotic does not mean random. It means the future still follows exact rules, but a start you cannot measure perfectly, which is any real start, grows into a totally different future. Hit "nudge a copy" and watch two nearly identical systems peel apart.