there 's no subjunctive in sight in this sentence. The verb is est, which is the auxiliary verb to form the perfect tense with the participle parti(s).
On est parti à la conquête de la Lune. -- We (i.e., humanity) set out to conquer the moon.
The participle is in singular (i.e., parti, and not partis) because the "we" is a general we, denoting humanity and not a particular group.
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