Drifting smoke plumes from the launch of Space Shuttle Atlantis swirl above the Vehicle Assembly Building (right) and NASA News Center (left) near sunset.

Shuttle exhaust is 97 percent water resulting from the combining of liquid hydrogen and liquid oxygen. Roughly half of the shuttle's exhaust water goes into the thermosphere, the highest layer of the atmosphere above about 55 miles (88 kilometers). There, it settles in the next lower layer, the mesosphere, and actually forms clouds over the poles which are visible at night. These clouds, called noctilucent ("night-shining"), are not visible in daylight due to their thinness. When the sun's rays hit them from below the horizon while the lower atmosphere is dark, they become visible. Long-term effects of this cloud formation has yet to be studied.

Liftoff of Atlantis on mission STS-117 to the International Space Station from Launch Pad 39A was on-time at 7:38:04 p.m. EDT, Friday, June 8. 2007. The shuttle is delivering a new segment to the starboard side of the International Space Station's backbone, known as the truss. Three spacewalks are planned to install the S3/S4 truss segment, deploy a set of solar arrays and prepare them for operation. STS-117 is the 118th space shuttle flight, the 21st flight to the station, the 28th flight for Atlantis and the first of four flights planned for 2007.

• Rocket Exhaust Leaves Mark Above Earth
• Last Shuttle Flight Made Clouds Over Antarctica
• Interactive Space Shuttle: The Orbiter

--NASA and SPACE.com Staff

Credit: NASA/Ken Thornsley