Two planets of real mass fall together under their own gravity and collide: a proto-Earth and an impactor from a fiftieth of an Earth to a whole one, iron core to mantle to crust, 16k to 262k particles. The splash, the tidal arm, the debris disk, and the moon that gathers out of it — computed live in WebGL 2, in the browser, nothing to install.
Five what-ifs. Theia, the canonical Moon-forming impact; head-on, flat and dead-on; hit & run, grazing too fast to be caught; twins, an impactor grown to Earth's own mass; shatter, fast enough to take both bodies apart. Mass, angle, inclination, speed, bounce, core and density are knobs under advanced, so the next what-if is a restart away.
The readout says what stays, what orbits, what escapes, and the Moon that disk would make, and keeps the books on momentum, angular momentum and energy. Gravity comes from the particles themselves on a 64³ mesh, contact is a spring-dashpot, and the heat of the impact stays in the rock and lights the picture. No SPH, no equation of state: real masses in a toy, not a paper.
Drag rotates · wheel zooms · space pauses · R restarts · F follows. Source on GitHub, MIT.