This technical white paper presents the Momentum Ledger (Π∞), a global momentum-flux framework derived from the integral form of energy–momentum conservation, ∇aTab=0,\nabla_a T^{ab} = 0,∇aTab=0, extended to a spacecraft control volume and its boundary at null infinity. Π∞ provides a unified method for modeling all untracked momentum leaving a space vehicle through thermal radiation, radio-beam recoil, ion-thruster plume asymmetry, reflectivity imbalance, outgassing, and other small but cumulative effects. These effects—typically of order mm/s in flybys and 10−10 m/s210^{-10}\ \mathrm{m/s^2}10−10 m/s2 in deep-space cruise—become mission-relevant for long-arc navigation, Mars transfers, solar-sail operations, high-power communications, and electric-propulsion architectures. This white paper focuses on practical engineering implementation for flight dynamics and GNC systems, including: real-time thermal and RF recoil modeling plume asymmetry estimation solar-photon imbalance detection integration of Π∞ into EKF and batch-least-squares navigation filters case studies relevant to SpaceX Starship interplanetary missions, EDL timing, and deep-space operations All equations and schematics are rendered natively in LaTeX/TikZ for clarity and reproducibility.This document is intended to accompany Module D: Boundary Momentum Flux in Radiative and Low-Thrust Dynamics (Zenodo DOI: 10.5281/zenodo.17624470), which contains the full mathematical derivation of Π∞.