It's a bird, it's a plane—no, it's astronaut pee! If you looked up on March 8, 2011, you might have a seen a bright streak across the sky as wastewater from the space shuttle Discovery was burned up in the atmosphere (which is acting as one huge incinerating toilet in this case). See the full video on YouTube.
On the International Space Station, where astronauts live long-term and water is scarce, urine is reclaimed to become drinkable water.
Solid waste, on the other hand, is brought back to Earth because chucking it out would require a dangerously large hole in the spacecraft.