Wireless Sensor Networks (WSNs) are made of a usually large number of nodes deployed over an area of interest in order to monitor specific phenomena. WSNs constitute a promising technology for planetary exploration, since they can be deployed in order to monitor the environmental conditions on a planet's surface, also in view of possible manned missions. This paper deals with the design of node and network architectures in a WSN targeted for planetary exploration, with a particular focus on the challenges, the driving principles and the design solutions adopted at WSN node and network level. Also, the paper introduces the basic architecture supporting data fusion at node and network level, which plays a fundamental role in order to increase overall WSN performances.