The default preset throws a body a tenth of Earth’s mass into the young planet at 45 degrees and a shade over escape velocity, and you sit and watch the splash spread into a debris disk that slowly gathers itself into a moon. Six presets — theia, head-on, hit and run, twins, shatter — and sliders for impactor mass, angle, spin and core fraction, all on 131,000 self-gravitating bodies in a browser tab.