Mission
Artemis II — the first crewed lunar mission since Apollo 17 (1972). Four astronauts aboard the Orion MPCV (Integrity) flew a free-return trajectory around the Moon, reaching 6,546 km above the lunar surface at perilune before returning to Earth. Launched April 1, 2026 from Kennedy Space Center LC-39B. Splashdown April 11, 2026, Pacific Ocean.
Simulator
This simulator computes Orion's trajectory from first principles using 3D RK4 integration under Earth + Moon + Sun gravity with J2 oblateness and solar radiation pressure perturbations. Spacecraft state is injected from JPL Horizons at 30-minute cadence — effectively ground truth between injection points.
Physics Model
IntegratorRK4, 5-second inner step
Moon ephemerisChebyshev DE440 (sub-mm)
Sun modelCircular Keplerian
Earth oblatenessJ2 = 1.08263×10⁻³
Solar radiationCR = 1.3, A = 32 m²
State injectionHorizons 30-min cadence
Burn modelState injection (not Δv)
Atmospheric entryExponential ρ, L/D = 0.05
Accuracy
Position RMS vs Horizons0.05 km
Position max error0.08 km
Perilune altitude6,545 km (NASA: 6,546)
Splashdown offset880 km (bank guidance N/A)
Trajectory samples12,845 at 60s cadence
Known Limitations
Atmospheric entry: Orion uses closed-loop bank-angle modulation to steer toward the landing target. This guidance law is not public. Our constant L/D model places splashdown 880 km from the published point.
Launch phase: The pre-tracking trajectory (T-10s to T+3.4h) is a Hermite spline interpolation from KSC to the first Horizons sample — not physics-integrated.
Missing perturbations: Solid-body tides, ocean tides, relativistic frame-dragging, solar wind variability, spacecraft outgassing.