Hey cool project! I built a solar system visualizer some years ago but just using the ephemerides data, no physics. This is ambitious. Noting your age I would say, invest some time in the basics - Euler, Improved Euler, finite differences, explicit vs implicit, multi-step methods etc.

Eg I see you've got a powerful adaptive Runge-Kutta method implemented in integrator.js. While that will do really well, for the sake of study you might make the solver implementation swappable and experiment with basic techniques. Some are very slow. Some maybe unstable and blow up the solar system. Why? Numeric methods are not one size fits all - see what the different tradeoffs are and how they respond to fiddling parameters. Understand the fundamentals.

Thank you! this is exactly the kind of feedback I was hoping for. Making the solver swappable is a genuinely great idea I hadn't considered. Watching Euler blow up the solar system while RK4 stays stable would make the tradeoffs immediately visible. Adding that to the roadmap.