The iconic line has become so familiar that it might be hard to imagine it any other way: as the first man on the moon put it, his achievement was “one small step for man.” But what if the first human being on the lunar surface had been a woman? That’s a central question asked by the Netflix documentary Mercury 13, directed by David Sington and Heather Walsh, premiering Friday — and, say the filmmakers, a question that’s still worth asking even decades after the last moon landing. The “13” in question were a group of experienced female pilots — Jerrie Cobb, Janey Hart, Jan Dietrich, Marion Dietrich, Rhea Hurrle, Irene Leverton, Bernice Steadman, Jean Hixson, Gene Nora Stumbough, Jerri Sloan, Myrtle Cagle, Sarah Lee Gorelick and Mary Wallace Funk — who in the early 1960s were invited by Dr.