This study offers insight into the processes of expert designers at the Jet Propulsion Laboratory (JPL) and how they use heuristics in the design process. A methodology for the extraction, classification, and characterization of heuristics is presented. Ten expert participants were interviewed to identify design heuristics used during early stage space mission design at JPL. In total, 101 heuristics were obtained, classified, and characterized. The use of interviews to extract heuristics allowed for researchers to confirm those heuristics were indeed used by designers. Through the use of post-interview surveys, participants characterized heuristics based on attributes including source/origin, applicability based on concept maturity, frequency of use, reliability, and tendency to evolve. These findings are presented, and statistically significant correlations were found between the participant perceptions of frequency of use, reliability, and evolution of a heuristic. A positive correlation was found between frequency of use and reliability, while negative correlations were found between frequency of use and evolution, and reliability and evolution. Survey results and analysis aim to identify valid attributes for assessing the applicability and value of multiple heuristics for design practice in early space mission formulation.