Humans continue to explore space so as to learn its secrets.

so 在这里是副词，与 as to 构成固定结构 so as to，表示目的，本身不单独表义。 as 是连词，与 so 搭配构成 so as to 结构，无独立含义。 to 是不定式符号，后接动词原形 learn，构成不定式短语。 so as to 是固定短语，意思是“以便、为了”，用于表达目的，相当于 in order to。