Live demo → gaploid.github.io/cosmic-collisions
The Moon forming — the canonical Theia impact:
earth-simulation-thea.mp4
Shatter — half an Earth at 2.6 escape speeds:
earth-simulation-shatter.mp4
Impacts, simulated. Two planets of real mass fall together under their own gravity and collide — the splash, the tidal arm, the debris disk, and the moon that gathers out of it. Or a fourteen-kilometre asteroid comes down on the Yucatán at twenty kilometres a second and opens a ninety-kilometre bowl in eighty seconds. A page each, over one engine.
Giant impact — a proto-Earth and an impactor from 0.02 to 1 M⊕, iron core to mantle to crust, 16k to 262k particles falling together under their own gravity: the splash, the tidal arm, the disk, and the Moon that gathers out of it. Five what ifs — the canonical theia; a Theia grown to Earth's own mass (twins); one coming in flat and dead-on (head-on); one grazing too fast to be caught (hit & run); one fast enough to take both bodies apart (shatter) — and mass, angle, how far above the plane, speed, bounce, core and density are all knobs, so the next what-if is a restart away. The readout holds still: what stays, what orbits, what escapes, the Moon that disk would make, and the books on momentum, angular momentum and energy. Drag rotates · wheel zooms · Space pauses · R restarts · F follows; a phone opens at 33k particles and gives back pixels, never physics.
Chicxulub — the impact that ended the Cretaceous, at two scales. The crater: Yucatán carbonate over crystalline basement, a carbonaceous asteroid at 20 km/s and 60° from the north-east; the curtain goes up and the transient bowl opens in about eighty seconds, with presets from 1 km to 30 km and a readout of energy, ejecta, melt and seismic magnitude. The first day: the crater is a dot, but the plume outruns the atmosphere, arcs over half a world and is down everywhere within two hours — that, not the crater, is what ended the Cretaceous. Between them sits a breakable Earth, honest about what it cannot show: a grain is 218 km across and the rock was 14, so it is drawn as the one grain it is worth, does nothing, and says so. Sources, as on the page: Collins 2005 and 2020, Housen & Holsapple 2011, Melosh 1989, Expedition 364, Fischer-Gödde 2024.
What runs:
- Gravity, from the particles themselves. A 64³ particle mesh with a block-to-block far field, and everything loose — the arm, the disk, the escapers — corrected pairwise against it, P³M-style, so moonlets bind instead of smearing. The mesh is a box of ±6 R⊕ that rides the largest body, and a second box rides a second body of a twentieth of an Earth or more once it is leaving the first — a hit & run's survivor, the twin a graze lets go — the two pulling on each other as one mass each. The Chicxulub crater page swaps it for a constant g and down: a patch of ground too small to pull on itself.
- Contact. A spring-dashpot along the line between two touching grains, with no tension: the material resists being squeezed, and can be pulled apart for nothing.
- Heat. The dashpot's work stays in the rock as a temperature, conducts from grain to grain, and lights the scene by it — deep red past 900 K, white past 40 000.
- Structure and spin. Differentiated bodies, each material with its own density and heat capacity, built as onions of Fibonacci shells, relaxed and crept into equilibrium, arriving as the Maclaurin spheroid an eight-hour day asks for, on a two-body approach solved on the CPU.
- The books. Symplectic Euler, and momentum, angular momentum and energy kept and shown to a fraction of a percent.
- The first second at Chicxulub. A contact model has no shock, so the excavation flow is given to it — Housen & Holsapple's speeds, Maxwell's Z-model directions, stopping at the transient radius π-group scaling asks for. Everything after that is the sim's own.
What does not:
- No SPH, no equation of state — no pressure, no shock, no vaporization. The deformation, the tidal arm, the disk and the re-accretion are all there; the phase change is not.
- No strength — no cohesion, no tensile strength, no fracture, no friction across a contact. The rock is a pile of grains that resists compression and nothing else.
- No cooling, no latent heat — over the hours a run covers nothing radiates away, and melting and freezing cost nothing.
- No fluid air or ocean — the atmosphere a body wears is drawn, not solved, and what a giant impact strips off it is a rule of thumb.
- No chemistry, no climate — Chicxulub's sulfur and the winter after it want a hydrocode; they are quoted on the page, not run.
- Resolution — a grain of Earth is a couple of hundred kilometres across, so anything smaller is a number on the readout rather than a thing on the screen.
- The Moon itself — the disk is simulated; the Moon it would eventually make is the Ida–Canup–Stewart scaling, not a run.
A page per scenario over a shared engine in src/, wired with plain script
tags — no dependencies, no build step, and a page still opens from disk. It is
WebGL 2 with no compute shaders to lean on, so the hard parts are tricks: the
contact grid is hashed and filled by depth-peeling, standing in for an atomic
counter; the gravity mesh does its far field block to block, so that it never
pushes itself; the particles are kept in Morton order, so a grain's neighbours
sit beside it in the texture. The picture is screen-space fluid rendering —
impostors, a bilateral blur that melts them into a skin, a coverage cut that
takes the beads off the limb — with a surface drawn in each grain's own
coordinates so it rides the material, and hot rock glowing by its temperature
and lighting everything else. Over that, the cinema: an atmosphere on each
body — off until the panel switches it on, since at speed the shell trails
the body — integrated along the ray through a shell keyed to the skin's own
silhouette, the steam a young planet wears until the giant impact blows it
off; the loose hot grains drawn once more as comets, streaked by their own
motion; the bodies' sun shadows on the disk and on each other; convection
cells on the melt; and a lens — the sun's glare walked past the bodies into
shafts, an anamorphic streak and ghosts off the brightest, a heat shimmer,
chromatic aberration, grain, a split tone. GGX, ACES and FXAA; every knob is
in __impact.look.
The starfield is the real sky: every star down to magnitude 6 — 5080 of them, each six bytes of right ascension, declination, magnitude and colour, 30 KB of base64 in the page — on the celestial sphere, turning with the world, with the Milky Way behind it where it really is. Positions from the Bright Star Catalogue, 5th Revised Ed. (Hoffleit & Warren), via the Harvard/SAO catalogue archive; public domain, free to use with credit.
Broad strokes, newest first; the commit history tells each one in full.
- 2026-09-02 — Findable. A title that says what the page is, a card for the link, an about panel with the page in words — what a crawler, or a reader without WebGL, gets of it — structured data, a sitemap and a robots.txt, and the favicons as files.
- 2026-08-30 — A box for each body. The gravity mesh was a fixed box of ±6 R⊕ about the barycentre, and whatever left it — a hit & run's survivor, a shatter's recoiling target — lost its own gravity and came apart from inside within the hour. Now a box rides the largest body and a second rides a second body big enough to keep, so a grazing pair of Earths flies apart whole, and the shatter's target, which the old box had broken up by the seventh hour, survives it at 0.95 M⊕. The energy balance is read against the run's kinetic and potential scale rather than its total, which for a shatter is nothing.
- 2026-08-30 — The readout's arithmetic. The moon a disk makes is capped at the disk there is to make it from, which the fit runs over past j = 1.13; the heat reads as a temperature at rock and iron's capacity together rather than rock's alone; and the speed beside the frame rate says it is the impact's. Theia leaves a 28° tilt, not the 23° this page claimed.
- 2026-08-29 — Surfaces and the melt. The craters are craters now — a floor, a narrow crest, an ejecta blanket, a central peak in the biggest, rays off a few fresh ones — in four octaves so that there are four times as many at half the size; Earth's crust wears an early ocean over basalt islands, with the sun's glint on it, and a cratered crust wears dark maria and a regolith grain. The magma ocean convects over its whole face, goes to a photosphere's granulation where it is hottest, darkens to its limb, and turns with the body it sits on.
- 2026-08-29 — The air, the axis, and keeping up. The atmosphere is keyed to a body's own centre and edge, and a giant impact takes the share of it the ground motion throws off — a tenth for Theia, a third for a hit & run, all of it for twins and the shatter — with a hole over the impact. Theia comes in from above the orbital plane and leaves Earth a 28° tilt, and the day and the axis on the readout are the planet's own rather than its group's. The picture keeps up at 4× and is whole from the first frame out from under the loader.
- 2026-08-28 — The cinema, and the Moon that stays. The bodies wear an atmosphere, the loose hot grains fly as comets, the planet shadows its disk, and the film has a lens — shafts, streak, ghosts, shimmer, aberration, grain — with thirty thousand faint stars and a camera that drifts until touched, for 0.3 ms a frame. The Moon the theia run makes stays in orbit at 4.6 R⊕ where it used to drift out of the box; the clock reads zero at first touch, the step runs twice as fast, and the approach is long enough to see who is coming from where.
- 2026-08-27 — A surface, and Chicxulub. The planet wears a surface
textured in the coordinates each grain carries from where it sat in its
body, so the relief turns with the body, stretches with the arm and goes
with the ejecta. Chicxulub is the second scenario, at the other end of the
scale: a crater on a patch of ground, the plume's first day over a globe,
and a breakable Earth built by the engine both pages now share in
src/. The readout became a table that holds still, with a phone's own version. - 2026-08-26 — Heat, light and the books. Every particle carries a temperature and the rock glows by it, lighting the disk and the second body with its own magma, under an ACES tone curve and a real sun. The bodies are onions of Fibonacci shells with a feathered skin, they arrive spinning as the Maclaurin spheroids their days call for, and the disk binds into moonlets with a pairwise correction under the mesh. The readout keeps momentum, angular momentum and energy to a fraction of a percent, the step runs at 4× on a desk, and a phone holds the same collision at a quarter of the particles.
- 2026-08-25 — Impact. Two planets of real mass — iron core, mantle, crust — collide under their own gravity on the GPU, from the canonical Moon-forming impact to a hit-and-run, under the real catalogue sky. A readout says what stays, what orbits, and what escapes.
The page began as a second page of stardust and moved into this repository on 2026-08-26, with its history.
This site uses Google Analytics to understand basic usage.
MIT — see LICENSE.