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In this paper, we propose a stratified agent architecture for an autonomous earth imaging spacecraft to achieve higher levels of autonomy. The autonomous spacecraft should possess the self-adaptation and the self-healing capabilities, in addition to other fundamental autonomy requirements including reflex, awareness and self-regulation. The architecture is configured as stratified agent architecture with two strata. The upper stratum is concerned with the adaptive behaviour, which is achieved through an on-board mission planner. This stratum is in turn configured as hierarchical agent architecture. Hierarchical planning in a Petri net framework involves refinement of a transition in the upper layer using a Petri net model and searching for a valid plan using the reachability analysis technique. The lower stratum deals with the execution aspects and provides the reactive behaviour. It is configured with three peer level agents to achieve the reflex and the awareness properties. These agents follow a generic structure with perception, action, and communication components. The agent achieves the required autonomy capabilities by using an appropriate knowledge model. The uniqueness of the architecture is the utilisation of Petri net models, with varying degree of abstraction, in the various layers to represent the planning knowledge, diagnostic knowledge and the appropriate world model. In this paper, we present how the various capabilities for autonomy are achieved by the agent organisation by exploiting the Petri net models.