Here's What Happens When You Light a Fire in Space

By Megan Garber

"When it comes to fire," says physics professor Forman A. Williams, "we're just getting started."

Take the flame of a candle, the kind you might find on a birthday cake. The flame takes the familiar shape -- as, basically, a gaseous teardrop -- because of gravity. The hot air rises and draws fresh, cool air behind it. The buoyancy makes the flame shoot up and flicker.

So we know all that. What we know less about, though, is the way fire behaves ... IN SPACE. In microgravity, according to the NASA video above, flames behave differently than they do here on Earth. Instead of the tear shape we're used to from our birthday cakes or Yankee Candle samples or what have you, the gravity-challeged flames form ... fire balls. Seriously. Unlike Earth-bound flames, "which expand greedily when they need more fuel," the video says, "flame balls let the oxygen come to them." Oxygen and fuel combine in a narrow zone at the surface of the sphere, rather than "hither and yon throughout the flame."

Which is knowledge that might have practical applications here on Earth -- and, specifically, to Earth-bound automobiles and the combustion process they rely on for their energy. Williams and a colleague were conducting an ISS-based experiment called the Flame Extinguishment Experiment (yep: FLEX) when they made an accidental discovery: in space, droplets of fuel continued to burn. Even after their flames had extinguished.

Yep: the fuel burned without flames.

Or, more accurately, the fuel burned with flames that were so faint that they were almost impossible to see. The FLEX experiment ended up testing the behavior of "cool flames" in space. The flame balls (fueled, in this case, by heptane) started out hot-burning. As they cooled, however, a different kind of burning seemed to take over. Cool flames burn at a relatively low-temperature 400 - 1,000 degrees Fahrenheit, and their chemistry is completely different from that of a standard flame. While teardrop-shaped flames produce carbon dioxide, water, and soot, cool flames produce carbon monoxide and formaldehyde. And, importantly, while cool flames on Earth extinguish immediately, the cool flames tested on the space station burned for at least a minute.

Which could have, as the video points out, big implications for cars and other vehicles that rely on combustion. In the meantime, though, it's worth remembering: if you light some birthday candles in space, you will receive the gift of flame balls.

For more videos from NASA, visit

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