Primer vector theory is used to investigate a specific class of minimum-fuel spacecraft trajectory problems in which high- and low-thrust propulsion systems are utilized sequentially. The problem considered assumes a spacecraft initially on-station in an established orbit about the Earth. It is desired to intercept a pre-determined position in space in a timely manner for collision avoidance or platform surveillance, using an optimal high-thrust program. The spacecraft then returns to the original orbit station using optimal low-thrust propulsion. Fixed-time minimum- fuel solutions are obtained using the Clohessy-Wiltshire linearized dynamic model. In the time-open case, the optimal final time is unbounded. For this case a composite performance index involving both fuel consumption and the final time is minimized to obtain optimal finite-time solutions.