Space explorations history is a popular topic, as record numbers of First-Day covers for postage stamps with space themes will attest. The Project Mercury issue of 1962 had more than three million 'First Day of Issue' cancellations, while the average number of First-Day cancels for other commemorative issues at that time was around 500,000. In 1969, the Apollo VIII issue received 900,000 First-Day cancels while others received less than half this amount. As the advent of U.S. space exploration grew, so did the topic of Space Exploration on stamps.