Decades later, Clarke was preparing a new edition of "Childhood's End" after the story had become dated. The initial chapter of the 1953 novel correctly foresees a race between the US and Soviet Union to first land men on the Moon (and the prominence of German rocket scientists in both space programs), but sets it later than it would actually happen (post-1975; the exact year is not given in the text, but 1945 is said to be more than thirty years ago). After the book was first published, the Apollo missions landed humans on the Moon in 1969, and in 1989 US President George H. W. Bush announced the Space Exploration Initiative (SEI), calling for astronauts to eventually explore Mars. In 1990, Clarke added a new foreword and rewrote the first chapter, placing it in the early 21st century, changing the goal from the Moon to Mars, and implying a joint effort rather than a race. Editions since have appeared with the original opening or have included both versions. "Guardian Angel" has also appeared in two short story collections: "The Sentinel" (1983), and "The Collected Stories of Arthur C. Clarke" (2001).