Post-collision orbital evolution of two particles in the a–e diagram. Each panel shows the constraint curves for a collision at a_col = 7 R_planet: the apoapsis constraint (e = a_col/a − 1; left branch) and the periapsis constraint (e = 1 − a_col/a; right branch). The green- and blue-shaded regions indicate orbits with periapsis below the planetary surface and the Roche limit, respectively. Blue and red symbols trace the successive positions of particle 1 (initially at apoapsis) and particle 2 (initially at periapsis), respectively. The left column adopts e₁ = e₂ = 0.65, while the right column adopts e₁ = 0.65 and e₂ = 0.40. From top to bottom, the coefficient of restitution decreases: ε_n = 0.9, 0.5, and 0.1. For high ε_n, particle 1 crosses from the apoapsis curve to the periapsis curve, whereas for low ε_n, particles remain on their respective constraint curves and converge toward the circular orbit at (a_col, 0).