Fifteen consecutive U.S. unmanned lunar missions over a six - year period from 1958 to 1964 all failed their primary photographic missions ; however , Rangers 4 .
6 successfully repeated the Soviet lunar impacts as part of lunar missions ' secondary missions .
Fifteen consecutive U.S. unmanned lunar missions over a six - year period from 1958 to 1964 all failed their primary photographic missions ; however , Rangers 4 and 6 successfully repeated the Soviet lunar impacts as part of their secondary missions .