German astronomer Jens Hackmann shot this picture of what is thought to be the last waste water dump from the space shuttle Discovery on March 8 before re-entry. This waste water would, of course, include astronaut urine:

"When observing ISS and space shuttle Discovery with unaided eye, there was nothing special for me," Hackmann wrote in the caption of his video on YouTube. "But when I examined the video clip I wondered what that tail on Discovery was…Then I read in the internet about other sights."

Such water dumps are a usual event, allowing the spaceship to unload its dirty water and urine before returning to Earth. A similar disposal gave skywatchers a nice view during the shuttle Discovery's STS-128 mission in September, 2009 as well.

Link via Alphecca