Ley grew up in his native Berlin, and studied astronomy, physics, zoology, and paleontology at the University of Berlin. He became interested in spaceflight after reading Hermann Oberth's book Die Rakete zu den Planetenräumen ("By Rocket into Interplanetary Space"). After publishing Die Fahrt ins Weltall ("Travel in Outer Space") in 1926, Ley became one of the first members of Germany's amateur rocket group, the Verein für Raumschiffahrt (VfR - "Spaceflight Society") in 1927 and wrote extensively for its journal, Die Rakete ("The Rocket"). With Oberth, he also acted as a consultant on Fritz Lang's film Frau im Mond ("Woman in the Moon").

In 1935, Ley left Nazi Germany for Great Britain and ultimately the United States. In 1936, he supervised operations of two rocket planes carrying mail at Greenwood Lake, NY. Ley was an avid reader of science fiction, and began publishing scientific articles in American science fiction magazines, beginning with "The Dawn of the Conquest of Space" in the March 1937 issue of Astounding Stories. Ley had a regular science column called "For Your Information" in Galaxy Magazine from its premiere in October, 1950 until his death. He was a member of science fiction fandom as well, attending science fiction conventions, and was eventually a Guest of Honor at Philcon II, the 1953 World Science Fiction Convention.

His book "Rockets - the Future of Travel Beyond the Stratosphere" (1944) describes the early rockets at VfR and more futuristic projects to reach the moon using a 3-stage rocket "as high as 1/3 of the Empire State Building" - a very good estimate of the height of the Saturn V rocket designed 20 years after. His works from the 1950s and '60s are regarded as classics of popular science and include The Conquest of Space 1949 (with Chesley Bonestell), The Conquest of the Moon (with Wernher von Braun and Fred Whipple, 1953), and Beyond the Solar System (1964). His book, Rockets, Missiles, and Space Travel, (1957) was cited in the Space Handbook: Astronautics and its Applications, a staff report of the Select Committee on Astronautics and Space Exploration of the U.S. House of Representatives, which provided non-technical information about spaceflight to U.S. policy makers. He also acted as science consultant for the Tom Corbett, Space Cadet series of children's science fiction books and TV series, as well as the 1959 feature film entitled "The Space Explorers."

Ley was best known for his books on rocketry and related topics, but he also wrote a number of books about animals. One notable book was Exotic Zoology (1959), which combined some of his older writings with new ones. This is of some interest to cryptozoology, as Ley discusses the Yeti and sea serpents, as well as reports of relict dinosaurs. The book's first section (Myth?) entertains the possibility that some legendary creatures (like the sirrush, the unicorn or the cyclops) might be based on actual animals (or misinterpretation of animals and/or their remains).

He was a member of the all-male literary banqueting club the Trap Door Spiders, which served as the basis of Isaac Asimov's fictional group of mystery solvers the Black Widowers.

Ley died at the age of 62 on June 24, 1969 in his home in Jackson Heights, Queens, where he had lived with his family since the mid-1950s.

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