Complex sentence: The American Surveyor program (since 1966) was designed to determine where Apollo could land safely; thus these robotic missions required soft landers to sample the lunar soil and determine the thickness of the dust layer, which was unknown before Surveyor.
Split into simpler sentences: The American Surveyor program (since 1966) was designed to determine where Apollo could land safely. As a result, these robotic missions required soft landers to sample the lunar soil and determine the thickness of the dust layer, which was unknown before Surveyor.