of astrodynamics and its applications. His two-volume work entitled Space Flight is probably the most complete and surely the most useful introduction to this complex subject ever written. It focuses on methods for exploration of the solar system. Although he was not the first, he clearly demonstrated the so-called "gravity assist" method for utilizing hyperbolic encounters with an intermediate planet to increase (or decrease) the velocity and orbital elements of a space vehicle. This technique had opened the entire solar system to robotic exploration by using what he called "Instrumented Comets." Examples include the Voyager missions to the outer planets and the recent successful New Horizons mission to Pluto. His contribution to this important field of exploration has been neglected for many decades and incorrect claims of "invention" of what is now called gravity assist were made by Minovitch. Extraterrestrial Imperative Ehricke promoted a philosophical concept called the "Extraterrestrial Imperative." This idea refers to Ehricke's belief that it was the responsibility of humanity to explore space and exploit the resources of the Solar System, in order to sustain the development of the species. There are no external "limits to growth," Ehricke insisted, because while the Earth is a "closed system," the exploration of space opens the universe to humanity. For Ehricke, human creativity has no limits. Further reading References External links www.hq.nasa.gov www.daviddarling.info www.tdf.it Ehricke, Krafft@ Astrophysics Data System Krafft Arnold Ehricke (in German) from the archive of the Österreichische Mediathek 1917 births 1984 deaths 20th-century German inventors V-weapons people German aerospace engineers German emigrants to the United States German spaceflight pioneers NASA people Engineers from Berlin People from Huntsville, Alabama German rocket scientists Space advocates American technology writers German technology writers Technical University of Berlin alumni Space burials Deaths from leukemia Operation Paperclip German male non-fiction writers 20th-century American essayists 20th-century American male writers American male non-fiction writers