In celebration of Earth Day, a collection of thoughts about how our planet looks from orbit

Just over 500 of the more than seven billion human beings that live on Earth have seen with their own eyes what the planet looks like from space. The experience, they say, is profound and humbling.

"In one direction there's all of human history laid out in front of us. And we turn 180 degrees, there's nothing, forever and infinity," explained Andy Turnage, executive director of the Association for Space Explorers, an international professional organization for astronauts.

Dispatches about Planet Earth See full coverage

Turnage has not been to space himself, but works with astronauts from across the globe. He says seeing Earth from space -- apart from being hard to explain -- is invariably a sobering experience.

"They realize that as an individual you really are a unique creature and you really can have an impact," he says on behalf of his astronaut members. "You really do have an influence on what's happening in this huge milieu that's happening on a palette before you."

Astronauts frequently describe the planet as appearing incredibly fragile. "They all remark about different aspects - some comment more about the wildfires they see, or the deforestation they are able to discern from above," Nicole Cloutier-Lemasters, NASA's public affairs officer for flight crew operations, wrote in an email. "But they all seemed very humbled by the view. It definitely seems to give them a pretty universal sense of just how small our delicate planet is." 

Culled from journal entries, interviews, Twitter accounts and NASA records, the slideshow above presents a sampling of thoughts and images from space, which have changed the way we think about the third planet from the sun.