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It is very difficult to justify the moon as a primary goal for human spaceflight -- there's not enough new to find out said  Robert Zubrin  president of the  Mars Society  and a critic of renewed lunar exploration .
 top members employees: Robert Zubrin, Mars Society
Given options, please tell me the relationships of all the listed entity pairs.Output format is "relation1: word1, word2; relation2: word3, word4". 
Option: cities of residence, top members employees, members, alternate names, origin, state or provinces of residence, title of person, spouse, city of headquarters, NA, founded by, state or province of headquarters, employee of, countries of residence, country of birth, founded, subsidiaries, country of headquarters 
Text: {0} 
 Entity Pairs:  Robert Zubrin, Mars Society 
Answer:
 top members employees: Robert Zubrin, Mars Society