"Great Balls of Not Fire"
Some weeks ago I was chatting with a couple members of COSI's floor
faculty team about meteors (they were doing a tabletop meteorite demo,
so this conversations wasn't totally unwarranted), when the subject of
fire came up. Meteoroids that come screaming through the Earth's
atmosphere are very bright, emitting a lot of heat and light, so it
naturally leads to the question: are they on fire?
When a piece of space rock intersects the orbit of the Earth at the exact wrong time, it's typically traveling at a few tens of thousands of miles per hour, and that only increases in its final moments. And even though our upper atmosphere is only a tiny fraction of the density of our surface-level air, that speed is sufficient to cause a ton of pressure and friction.
Friction from the rubbing of air against the meteor, and pressure as the rock pushes on the air directly in front of it during its passage. Both of these processes generate a lot of heat; enough heat to ionize both the air and the meteor material, ripping electrons off molecules and causing them to glow.
The air will stay ionized for a few moments after the meteoric passage, forming the familiar trail. Bits and pieces of the meteor itself will get stripped away - a process called ablation - leaving a glowing aftermath in its wake as well.
But fire is not involved here. Fire is a chemical reaction between oxygen and a fuel source, and the physics on display in a meteor shower are based on different, and far more violent, processes.