In the late 1950s and early 1960s, space science became a topic of research in the wake of the launch of Sputnik I. Urey helped persuade NASA to make unmanned probes to the moon a priority. When Apollo 11 returned moon rock samples from the moon, Urey examined them at the Lunar Receiving Laboratory. The samples supported Urey's contention that the moon and the Earth shared a common origin. While at UCSD, Urey published 105 scientific papers, 47 of them about lunar topics. When asked why he continued to work so hard, he joked, "Well, you know I'm not on tenure anymore."