Astronauts looking down on Earth from space have long said the view is tremendous, but it is also comes with the revelation that of all the planets in the cosmos, there is only one world that humanity calls home.

"Our planet is our spaceship," said NASA astronaut Sandra Magnus, who recently returned to Earth after spending about 4 1/2 months in space. "It looks very fragile from here, and it's very easy to take it for granted when we're living on it, when it seems so big and so massive. But it's not. It's very small and very fragile."

Magnus returned home in late March with a new perspective of her home planet, one that came just in time for Earth Day today.

"When you look out the window, you notice how incredibly thin our atmosphere is, how such a fragile shell of air we have that surrounds our planet and makes it habitable," she said before leaving the station. "And you can read that in a book, but until you see it it doesn't strike home."

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This island Earth

Magnus is not the only astronaut to marvel at the sight of her home planet from afar.

The first astronauts ever to see the entire planet as a distant orb in a sea of black space were the three Apollo 8 astronauts, who took the iconic image of Earth rising over the limb of the moon in December 1968.

"The amazing public perception of that stunning photo gave everybody an awareness that the Earth was an oasis out there in a very barren, harsh cosmos," said former astronaut Thomas Jones, a planetary scientist and co-author of the book "Planetology." "I think those images became the icon of the environmental movement in its earliest phase."

Other missions and robotic probes have beamed home views of Earth from more distant realms, including the surface of the moon and Mars, through the rings of Saturn and from more than 4 billion miles away, which revealed the planet as a "pale blue dot" in space.

"That's what the space program gives us is the ability for everybody to share in the astronauts' vantage point," Jones, a four-time spaceflyer, told SPACE.com.

Meanwhile, Japan's Kaguya lunar orbiter has been recreating the Apollo 8 Earth rise view using its high-definition cameras since it arrived at the moon in 2007.

A station with a view

There are three astronauts aboard the International Space Station right now: Russian station commander Gennady Padalka and flight engineers Michael Barratt of NASA and Koichi Wakata of Japan. Their Expedition 19 mission began in late March.

While his nearly seven-month mission is just beginning, Barratt said the impact of seeing his native planet far below has already had an impact.

"There's no doubt, when you look down at the Earth from here, you're just overwhelmed by how beautiful it is," Barratt said this week, adding that two things immediately jump out. "One is how much you miss it, and two, is how much you really want to take care of it as best you can."

Magnus said that when a person gazes at the Earth, there is a sense that humanity and all life as we know it are completely dependent on a single planet and its thin atmosphere.

"It makes you think about our planet as a whole system," Magnus said. "We're all there together living together as human beings and other organisms and we have to take care of each other."

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