Groff Conklin in 1954 found the novel to be "a richly textured and thoroughly mature tale" and the best of the seven Heinlein juveniles available.[1] Anthony Boucher and J. Francis McComas praised it for its "good character-development, rousing adventure-telling, and brilliant creation of several forms of extra-Terrestrial life."[2] P. Schuyler Miller ranked it "close to the the [sic?] best in mainline science fiction."[3]