William Morrow & Co., 1977, p149 Von Braun, W.:Crossing theFinalFrontier, Colliers, March 22, 1952 Hermann Potočnik: The Problem of Space Travel (1929)
Mission Blog (页面存档备份,存于互联网档案馆) NASA biography from 1998 (页面存档备份,存于互联网档案馆) From the Last to theFinalFrontier - An Alaskan in Outer Space (页面存档备份,存于互联网档案馆)