The International Space Station orbits Earth after the undocking of space shuttle Endeavour during the STS-123 mission.

There may be only one place in the universe which can be the subject of 300,000 and counting photos and still never get old. It is the same place that astronauts spend hours upon hours of their free time watching, for months, yet still can't get enough. It's not a distant galaxy, or a spectacular nebula. It's simply home -- our planet Earth.

Of all the things astronauts speak of after their flights, the view of Earth remains their most consistent, indescribable, awe-inspiring constant.

Imagine looking out your window and seeing the planet pass below at 17,500 miles per hour every day, circling it each 90 minutes. That's the view for astronauts living and working on the International Space Station for six months at a time, orbiting 220 miles above the ground. Every day is Earth Day aboard the station.

The station provides an incomparable vantage point from which to observe, monitor and even discover Earth. A high quality optical window, located in the U.S. Laboratory, Destiny, was designed just for that purpose.

"Astronaut photographs of Earth are taken from the human perspective from space,” said Sue Runco, Earth remote sensing scientist at Johnson Space Center. “Just the fact of seeing Earth as another human sees it, is why people often can relate to them much greater than they can to satellite imagery."

Astronauts are trained in meteorology, geology, oceanography and environmental science in advance of their mission to maximize their observations of Earth. They use an array of professional digital cameras and lenses to capture the images, and, more recently, high-definition video.

A team of scientists on the ground helps the crews identify upcoming photo opportunities. The scientists send daily messages to the crew with specific times, locations and background on the areas of interest. Those areas can range from coral reefs to alpine glaciers to smog over industrial regions.

The unique documentation has become a valuable asset to researchers who use the data to help illustrate changes over time. By comparing photos from space of areas of interest, they can develop maps of land cover change, identify changes in Earth’s atmosphere and document changes in water levels, vegetation or even urban sprawl.

Their photos also serve as the “eyes of the world” – giving us never seen before images of hurricanes from above or squall lines as they develop. Unlike satellites, astronauts can actively search and identify new developments below them. During Expedition 13, Flight Engineer Jeff Williams was the first person to identify an erupting volcano of which even ground scientists were unaware.

"Astronaut photography of Earth has some unique aspects that aren't found in most satellite imagery,” said Runco. “There is a person behind the camera, and they use their judgment and training to pick the features they will photograph and the angle they will use. Because of their orbit tracks and variable imaging times the lighting will be different which emphasizes different features. They operate in a mode of real-time discovery to see features of interest and document them in a way that is not possible with satellites.”

Because it must rely on as few supplies as possible, the space station uses several very green principles in its daily operations. Water aboard the complex is recycled, not for drinking use, but to provide air for the complex. The water is split into hydrogen and oxygen. The oxygen is then used for breathing air while the hydrogen is vented overboard. All the electrical power on the station is generated by more than a half-acre of solar panels. Future systems may also even recycle the crew's exhaled breath -- combining the carbon dioxide scrubbed from the cabin atmosphere with hydrogen to create additional water.

Learning to use resources onboard the ISS for sustainable living is a smaller version of learning about the larger space platform, Earth, its resources, changes, and effects on sustainable living