Presets
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Comet
Eccentricity decides how different perihelion and aphelion are.
Time and activity
One orbit takes 90 seconds at 1x, whatever its real period.
Composition
Water ice, CO, a dust-covered nucleus or a nearly dead one.
Environment
Solar flux scales the heating; dust pressure scales how strongly sunlight pushes grains.
Camera
Inner system is fixed on the Sun; follow keeps the comet centred.
Events
A jet burst briefly raises the outgassing.
Layers
Model checks
The science
Orbit
The orbit is a Keplerian ellipse with semi-major axis a and eccentricity e. Mean anomaly M = n t with n = 2π/P, Kepler's equation M = E - e sin E is solved by Newton iteration, r = a(1 - e cos E), and the speed follows from the vis-viva equation v² = GM (2/r - 1/a). Kepler's third law P = 365.256 a3/2 days (a in AU) fixes the period.
Sublimation
The subsolar surface balances absorbed sunlight with thermal emission and the latent heat of the vapour it loses: S0(1 - A)/r² = εσT&sup4; + L Z(T), with Z(T) = Pv(T) √(m / 2πkT) (Hertz-Knudsen) and the vapour pressure of water ice from the Murphy-Koop fit (CO ice from Clausius-Clapeyron through its triple point). Production is Z times the active hemisphere area, so it falls off steeply beyond the snow line near 2.7 AU where the surface is too cold for water to sublimate quickly. CO sublimates at 20 AU or more.
Dust tail
Each grain feels gravity reduced by radiation pressure, (1 - β) GM/r², and flies on its own Kepler orbit (Finson-Probstein model); small grains (large β) are pushed furthest and form a curved fan, while big grains stay near the orbit as a dust trail. The ion tail is drawn straight away from the Sun because the solar wind drags the ions at hundreds of km/s.
Honest simplifications: a two-dimensional orbital plane, a spherical nucleus with a fixed radius estimated per comet, subsolar rather than rotation-averaged heating, no gas drag on the dust, no non-gravitational forces, a tail length exaggeration slider, a qualitative ion-tail length and a fixed ejection speed. Hale-Bopp's period listed on the 3D page (252 yr) disagrees with its own semi-major axis (186 AU gives about 2,540 yr), which the Kepler graph shows; this page uses the semi-major axis.
What to try
- Pick Halley and watch the production graph: the marker climbs sharply as it crosses the snow line, and the speed peaks at perihelion.
- Switch to CO-rich: activity starts far beyond Jupiter and the ion tail grows.
- Turn the dust pressure up and see the fan sweep away from the orbit.
- Fire a jet burst near perihelion, then change to the Kreutz sungrazer to see an extreme eccentricity.