The paper describes a semi-autonomous control system on board a small mobile robot intended to explore planetary environments with minimal human supervision. The algorithm performs dead reckoning, hazard mapping, trajectory planning, motion control, health monitoring, and supervisor telemetry. A remote operator station provides a graphical interface for real time telemetry, guidance, and performance tuning. Design emphasis was placed on processing efficiency and robust performance for implementation with inexpensive sensors and microcontrollers. Of novel contribution are a high-level behavioral task architecture and a reactive trajectory planning routine that explicitly incorporates steering and safety constraints. Simulation and field test results under various operating conditions are presented.