The ACIS instrument on-board the Chandra X-ray Observatory (CXO) experienced pronounced degradation in spectrometric performance during the spacecraft's orbital activation and calibration phase. This damage was associated with a sharp increase in charge-transfer inefficiency combined with relatively constant dark current. Damage occurred only during passage through the earth's radiation belts, and only when ACIS remained in the focal plane during the passage. Subsequent measurements and analyses support the conjecture that the damaging radiation entered through the Observatory's High-Resolution Mirror Assembly (HRMA) aperture. A mechanism whereby low-energy magnetospheric protons and heavier ions are scattered through the HRMA and reach the focal plane with just enough energy to stop in the CCD's charge transfer channel provides a reasonably consistent explanation of all observed phenomena. In this paper, we shall describe analyses which support this conclusion. We simulated the mirror surfaces and various path elements using a standard ion transmission code to generate a bi-directional reflectance distribution function (BRDF). We then convolved the BRDF with the geometry using a ray-trace code. This paper presents damage estimates using measured proton fluences and ground measurements of ACIS-type CCD damage versus proton energy and compares them with observed on-orbit damage.