There are a variety of ways to approach this. One of the most direct sentences that can be given is that "Neil Armstrong was the first man to set foot on the moon." This is a sentence that is factual. It is a sentence that summarizes the achievement for which Armstrong is most known. In Armstrong's recent passing, this is the sentence that helped to define and craft his legacy. Another sentence that can apply to Armstrong would be that Neil Armstrong provided a sense of vision for an entire generation. The symbolic action of landing on the moon and walking on its surface, according to President Nixon, "taught men how to reach for the stars." There was something powerfully symbolic about Armstrong's actions, something that reflected the capacity for what can be out of what is. Finally, I think that a sentence that can be used to explain the importance of Armstrong's contribution would be the line itself that has been recorded over time as almost iconic:
“That’s one small step for man, one giant leap for mankind.”
Certainly, this statement can be a description of Armstrong, himself. Interesting enough, the line might not have been said as it was intended. "Man" and "Mankind" are the same thing. Rather, Armstrong took "one small step for a man" and in doing so, too a "giant leap for mankind." In this, a sentence that can be used is how iconic moments in history might not pay attention to specific detail. The sentence here could be that Armstrong's contributions were iconic enough to allow exact details to be rendered as unimportant in comparison to the magnitude of the moment. In this, history and fame are defined by instants in time, and to capture these is what defines historical consciousness. Armstrong represents this with his moment.