How does fire behave in zero gravity?

We all know about how fire behaves in outer space: It doesn't. All those action movies were wrong. Fire needs oxygen and oxygen isn't abundant outside of Earth's atmosphere. Fires can happen inside a contained, oxygenated environment in space, but they don't behave the way they do on Earth. The Earth's gravity imposes a lot of conditions on fire that wouldn't happen in microgravity or zero g. Here's why astronauts can't burn a candle at both ends.

Image: NASA - LRC

Gravity it is a mixed bag. Sure, it helps maintain muscle tone, and bone density, and allows us to eat soup in a bowl without any mess. Unfortunately, it also tries to kill us by tripping us up and pulling heavy objects down onto us. It turns out that, not content with blunt force trauma, gravity is also trying to kill us with fire. When a fire starts, it heats up the air around it. This causes that particular section of air to become less dense. That low-density section of air heads upwards because gravity is pulling down on everything and colder, denser sections of air muscle it out of the way to get closest to the earth. This is lucky for the fire, because the only reason the air got hot was because the fire was consuming the oxygen. When the low-density air got pushed up and out of the way by the oxygen-rich air, the fire got a new delivery of fresh oxygen to burn.

Not so in zero gravity. The air that's is heated by the fire does expand, but with no gravity pulling denser air down into its space, it just . . . stays there. Meanwhile the fire continues consuming oxygen and putting out carbon dioxide until it smothers itself. Space travel is so complicated, with so many problems having such drastic consequences, fire is one of the few ways that astronauts really catch a break. Unless there is some kind of air circulation system blowing over the fire, giving it fresh oxygen, a fire in a space craft would put itself out. Which is not to say that fire isn't discouraged. Here an astronaut in the International Space Station nervously chuckles that he can't give a demonstration of how a lit match will put itself out up there. There are rules about that kind of thing.

Some fires, for example candle flames, also change color in space. Part of a burning candle is very endothermic - heat consuming. When a candle wick burns, it is being taken apart molecule by molecule. This separation takes up heat. However, when the molecules, in this case long strings of carbon, get pushed upwards, they're burned like charcoal and glow bright yellow. In zero gravity, the carbon strings don't get burned, and the flame is blue, cooler, and much much dimmer. As awesome as it might seem, fire in zero gravity is a very low-drama affair.

Via Nasa, Scientific American, Newton AAS, and About.com.