Spacecraft are key components of military operations and everyday life. To achieve space situation awareness, human operators must monitor large numbers of parameters. During off-nominal conditions (e.g. severe space weather storm or onboard anomaly), it is difficult to cohesively monitor the data to develop an accurate tactical picture. To aid operators, we have developed a prototype Multi-Agent Satellite System for Information Fusion (MASSIF) for event detection and characterization. This system integrates a fuzzy logic system for semantic data processing and a Bayesian belief network system for multi-source data fusion and situation assessment. This paper describes initial research results.