MER: from landing to six wheels on Mars...twice

Application of the pathfinder landing system design to enclose the much larger Mars Exploration Rover required a variety of rover deployments to achieve the surface driving configuration. The project schedule demanded that software, design, engineering model test, and flight hardware build be accomplished in parallel. This challenge was met through (a) bounding unknown environments against which to design and test, (b) early mechanical prototype testing, (c) constraining the scope of on-board autonomy to survival-critical deployments, (d) executing a balance of nominal and off-nominal test cases, (e) developing off-nominal event mitigation techniques before landing, (f) flexible replanning in response to surprises during operations. Here is discussed several specific events encountered during initial MER surface operations.