Star trails and Earth, seen from space

One of NASA's more prolific space-based photographers is at it again.

Astronaut Don Petit, currently in his second stint aboard the international space station, has unveiled new photos he's taken of star trails -- long-duration nighttime exposures that show the motion of the stars though the sky.

Of course, the stars aren't moving; Petit is, at about 17,500 miles an hour. His unique vantage point 250 miles above Earth also means that the planet's rotation can be seen in the photos, manifested by rings of man-made light and natural lightning.

The images are actually compilations of several exposures, totaling 10 to 15 minutes. But it's not as simple as clicking the shutter and letting it go.

"With modern digital cameras, 30 seconds is about the longest exposure possible, due to electronic detector noise effectively snowing out the image," he explained via Johnson Space Center mission controllers. "To achieve the longer exposures, I do what many amateur astronomers do. I take multiple 30-second exposures, then ‘stack' them using imaging software, thus producing the longer exposure."

During Petit's last trip to the space station, he made a name for himself in photography circles by using spare parts to construct a 'barn door tracker' -- a special tripod for a camera that compensated for the motion of the ISS, allowing for sharper nighttime photography of the earth's surface at night.