Orionid meteoroids are grains shed by Comet 1P/Halley on its 76-year orbit. Earth crosses this debris stream every October, meeting it nearly head-on at ~66 km/s. The stream's approach direction is fixed on the celestial sphere near the Orion/Gemini border (RA ≈ 6h20m, Dec ≈ +15.6°) — the radiant — and stays fixed while the marked observer's local horizon rotates underneath as Earth spins:
altitude(t) = asin( ẑ(t) · R̂ )
R̂ = fixed inertial unit vector toward the radiant
ẑ(t) = observer's rotating local zenith
Only when the radiant is above the horizon can that observer see the shower, and the classic rule of thumb — rate scales with sin(altitude) — falls straight out of the geometry, which is why Orionids are best after midnight when Orion is high.
Each grain's ablation brightness follows kinetic-energy dissipation per unit path length, dE/ds ∝ ½·m·ρ(h)·v², with air density ρ(h) rising as the grain descends through the 100→0 km entry corridor. The grain vanishes once its kinetic-energy budget E₀ = ½mv² is spent — producing a rise-then-burnout light curve without needing to track deceleration, valid for the sub-gram grains typical of this shower.