Millionth photograph from International Space Station. NASA image.

When astronauts go up into space, they admit, they spend a lot of their free time looking back down at Earth. It is the most colorful and most beautiful thing in sight.

Now, astronaut Don Pettit has posted what he says is the one millionth photo taken from the International Space Station since its first components were launched in 1998. Pettit is one of the six crew members currently on board the station (there are two Americans, three Russians and one Dutch astronaut) and they're not quite sure who took the picture.

"1 millionth ISS photo. Part of time lapse series. Not sure who took it, Dan Burbank or myself. We can't remember http://pic.twitter.com/MjnkRm2S," said Pettit on his Twitter feed. (NASA does keep track of how many pictures it has catalogued since the station began, which is why they know this particular image is No. 1,000,000.)

The picture - looking down like so many others - shows two Russian supply ships in the foreground and the Earth beyond. The eerie green is an aurora, the northern or southern lights, seen near the poles at night.

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The shot was part of a time-lapse sequence, taken by a camera mounted in a station window and left there to shoot one of the gorgeous sped-up movies NASA has been showing of Earth as the station races 240 miles above it at a speed of about 17,300 mph. Here's one recent example:

Pettit, who is on his second tour as a space station astronaut (he flew a space shuttle mission as well), has been orbiting since December and is due home in May. He has been keeping a blog, titled " Letters to Earth."

"From my orbital perspective, I am sitting still and Earth is moving," he wrote in one post. "I sit above the grandest of all globes spinning below my feet, and watch the world speed by."