The Rock Abrasion Tool (RAT) is an integral part of the Athena Science payload. Serving primarily as the geologist's rock hammer, the RAT will expose fresh surfaces of Martian rocks to other instruments on the payload. The RAT also brushes dust and debris from an excavated hole or unaltered rocks. To accomplish these tasks autonomously, the RAT, a sophisticated 3-axis precision-controlled device, was designed. Data products derived from RAT telemetry enable the RAT to also act as an important rock physical properties science instrument. The returned RAT grinding and penetration rate data will be inverted and compared to a rock library on the Earth. The design is also very compact and lightweight: the RAT is contained within a cylinder 128 mm long and 85 mm in diameter and has a mass of 687 g.