The β-factor expresses the gain in a momentum transfer exerted by an impacting spacecraft on a Near-Earth Object (NEO) due to ejecta momentum escape. Accurate knowledge of the β-factor allows insight to the physical properties of the NEO and bears potential for cost reduction of future mitigation missions by lowering the required impact momentum. First, the definition of the known scalar momentum enhancement β-factor is generalized to a matrix definition which is proven to overcome the shortcomings inherent to the scalar variant. Moreover, a clear distinction between the linear and angular momentum matrices is introduced. The new definition is shown to be consistent with the previous definitions in the literature [6][9]. Second, a measurement strategy for the β-factor in the close proximity of very small bodies based on heliocentric orbit determination is suggested. An operational spacecraft observation concept for very small bodies (d<300 m) is suggested. The paper ultimately shows the projected performance for this measurement strategy and highlights the critical elements in this measurement chain. The concept and operational scenario is directly applicable to future missions to small bodies (e.g. joint AIDA mission of ESA/NASA and NEOShield-2).