The photograph coined the Blue Marble was taken on 7 December 1972 by the crew of Apollo 17 on their way to the Moon on what turned out to be the last manned lunar mission. The iconic image was seen by many as a depiction of Earth's frailty and vulnerability. No humans since that mission have cruised that far out in space. For those of us earthbound denizens destined never set eye on it from above, we'll just have to see it up close, by exploring the Blue Marble.