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Exaggerating Jupiter speeds up what would take millennia.
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The science
Restricted three-body problem
Each asteroid obeys r̈ = -GM r/r³ + GMJ [(rJ - r)/|rJ - r|³ - rJ/rJ³] with Jupiter on a circular 5.2 AU orbit (indirect term included). A kick-drift-kick leapfrog with a 0.02 yr step keeps the energy of an isolated orbit stable.
Resonances
An asteroid whose period is TJ p/q meets Jupiter at the same orbital phase again and again and is pumped to large eccentricity. Kepler's third law T² = a³ places these at a = aJ (q/p)2/3: 2.50 AU for 3:1, 2.82 AU for 5:2, 2.96 AU for 7:3 and 3.27 AU for 2:1 (the Kirkwood gaps).
Honest simplifications: two dimensions, circular Jupiter, no Mars, Saturn or Yarkovsky drift, 2-3 orders of magnitude fewer bodies than the real belt. Real gaps need about 105 years or more at real Jupiter mass, so the slider exaggerates the mass; heavy settings mostly scatter the outer belt rather than reproduce the exact gap pattern.
What to try
- Tick the rotating frame: horseshoe and resonant families become static shapes.
- Run Strong Jupiter (x20) and watch the outer belt thin out and the mean eccentricity climb.
- Pick No Jupiter: nothing changes, proving the stirring comes from Jupiter alone.