# Accretion Sandbox

> Accretion Sandbox (https://space-sandbox.cc) is a free, browser-based space physics sandbox. Players explore the real Solar System, the Milky Way, the future Milky Way–Andromeda collision and the stars orbiting the black hole Sagittarius A*, or build their own star systems from dust and gas, smash planets together and feed black holes. It runs entirely in the browser (HTML5 Canvas, no install, no sign-up) on desktop and mobile.

## What you can do
- Watch the complete Solar System with the planets where they are today (computed from NASA JPL orbital elements), 11 major moons, the dwarf planets, Halley's Comet, the asteroid belt, the Kuiper belt and the Jupiter Trojans.
- Explore the Milky Way: central black hole Sgr A*, rotating bar, spiral density waves, the Sun 26,000 light-years from the centre, the Magellanic Clouds and the Sagittarius dwarf galaxy being torn into a stellar stream.
- Fast-forward about 4 billion years to see the Milky Way and Andromeda collide, throw out tidal tails and merge into one elliptical galaxy.
- Follow the S-stars around Sagittarius A*, including S2's 16-year orbit, with general-relativistic orbital precession.
- Visit real exoplanet and stellar systems: TRAPPIST-1 (seven Earth-sized planets), Alpha Centauri A, B and Proxima with its planets, and Saturn's rings and moons at true scale.
- Run physics experiments: planet formation from planetesimals, the Theia impact that made the Moon, Roche-limit ring formation, a red giant swallowing planets, a black hole shredding a star (tidal disruption event), circumbinary planets and a rogue star flyby.
- Build your own systems from a library of 15+ star types (red dwarfs to blue supergiants, white dwarfs, neutron stars, stellar, intermediate and supermassive black holes), 15+ planet and small-body types, and fields such as asteroid belts, comet swarms, rings, gas clouds and stellar nurseries.
- Tools: drag bodies to move them, aim velocities, paint composition onto planets, measure distances and light-travel time, an impact lab with slow-motion replay, undo, quick save and a command palette.
- Time control from slow motion (0.01×) to 10^16× real time (317 million years per second) in galaxy scenarios.

## Physics model
- Direct N-body gravity (adaptive leapfrog integrator) for up to 220 bodies plus up to 24,000 tracer particles.
- Collisions follow the Leinhardt & Stewart (2012) regimes: perfect merge, partial accretion, erosion, super-catastrophic disruption, graze-and-merge and hit-and-run, with debris and fragments.
- Roche-limit tidal disruption, tidal heating (volcanism, as on Io), a two-layer thermal model with magma oceans, atmospheric escape (Jeans parameter), habitable zones and snow lines.
- Black holes with Schwarzschild radius, accretion-powered luminosity capped at the Eddington limit, tidal disruption events and a first post-Newtonian (general relativity) correction.
- Galaxies as central black holes inside cored isothermal dark-matter halos with flat rotation curves, rotating bar and spiral density-wave potentials, dynamical friction, satellite accretion, galaxy mergers and star formation along spiral arms.
- Overlays: gravity well, temperature map, tidal stress, Lagrange points, Roche/Hill/capture zones, habitable zone, magnetic fields, composition, and a planet cutaway with pressure and density profiles.
- Simplifications: the simulation is two-dimensional; galaxy discs are test particles in fixed potentials; accretion can be sped up with an inflated collision cross-section.

## Good for
- Students and teachers of astronomy and physics (orbits, Kepler's laws, tides, planet formation, galaxies, black holes).
- Space enthusiasts who want to see the Solar System, the Milky Way or the Andromeda collision in motion.
- Anyone curious "what happens if…" (a black hole passes through the Solar System, Jupiter is ten times heavier, two planets collide).

## Links
- Play: https://space-sandbox.cc/
- Sitemap: https://space-sandbox.cc/sitemap.xml