Press and drag in space to launch a body: drag length and direction set its velocity. Bound orbits have total energy below zero; escape is above zero; zero is the parabola exactly between. LIT Newtonian gravity, F = Gm₁m₂/r², integrated with a symplectic leapfrog; the self-check watches the total energy and momentum drift. MODEL E = 0 as the balance point, like Dave's Σ = 0. Units are scaled (G = 1). ← SaPpH{{i}}on