Astronaut Don Pettit has been taking long exposures of star trails and city trails during Expedition 31. In addition to stars, he captures an aurora and lightning flashes. In speaking of his technique, Don says, "My star trail images are made by taking a time exposure of about 10 to 15 minutes. However, with modern digital cameras, 30 seconds is about the longest exposure possible, due to electronic detector noise effectively snowing out the image. 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.” [Flickr]