Expert comments: "The most widely distributed picture ever connected to NASA is this iconic Apollo 17 image showing the full Earth. It could be considered the 'poster child' for the cliché, 'a picture is worth a thousand words.'" › View full caption
A blue marble in the window as the last crew headed to the moon. Mountains that appear to touch space. Volcanic fury seemingly so close to orbit it could be felt. A vivid green of life that somehow darkens the blackness of space.
These are some features of a top 10 list of the most often requested types of images of Earth taken by U.S. astronauts during the past 50 years. NASA has made astronaut imagery available to the public and media since human space flight began. The majority of the history of those requests predates the age of tabulating downloads off the Internet, but corporate memory remains.
Experts at NASA's Johnson Space Center media library have assisted with the distribution of astronaut imagery to the public and media since Apollo. Three of them cumulatively have more than 100 years of experience in filling literally millions of requests for that imagery from people all over the world.
Most of that distribution now is done via the Internet, although their expertise remains an integral part of that process. In honor of Earth Day a half century into the space age, using admittedly subjective techniques, they provided this list of the top 10 most requested Earth images taken by astronauts. Their comments about the photographs and their popularity are included.