It's a little early for me to start thinking about my annual Top Ten Astronomy Pictures, but I have a feeling this one will make the cut: the actual glowing trail of plasma left in the wake of Atlantis as it entered Earth's atmosphere, as seen from space by astronauts aboard the space station!
Amazing! Oh yes, you want to click to embiggen. Atlantis undocked from the International Space Station on July 19, and two days later the ISS was in position to coincidentally catch a view of the Orbiter as it made its final descent. This shot shows the plume of ionized gas left behind as Atlantis descended, as well as clouds, parts of the ISS itself, and atmospheric airglow: the faint glow of molecules and atoms high in the atmosphere as they slowly recombine with electrons and emit light. This shot is simply spectacular. Since the stars aren't trailed, this must be a fairly short exposure, not more than a few seconds. The trail you're seeing is therefore not actually the Orbiter streaking across the Earth! The plasma trail behind it fades with time, so the trail is brightest near the Orbiter's position and fainter as you backtrack along its path. Think of it as an afterglow of the passing of Atlantis. Why does this happen? The air gets heated by the Orbiter's ramming the atmosphere at 20+ times the speed of sound. And contrary to popular belief, it's not friction that heats the air, but compression. When you compress a gas it heats up (like when a bicycle pump gets hot when you use it a lot), and the Orbiter is screaming through the atmosphere at hypersonic speeds. That compresses the air a lot. A shock wave forms in front of the Orbiter, and the air begins to glow as it gets heated up to temperatures as high as 1260° C (2300° F). That's what you're seeing above: the shocked, rammed, and glowing air as Atlantis pounded through it at several kilometers per second. And it did this many, many times over its life... until this one final time, caught on camera by astronauts high above the Earth.