In Space, Flames Behave in Ways Nobody Thought Possible

Combustion experiments conducted in zero gravity yield surprising results

  • By Ker Than
  • Smithsonian magazine, December 2012, Subscribe
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A composite false-color image of fire in space. The bright yellow traces the path of a drop of fuel shrinking as it burns producing green soot.
A composite false-color image of fire in space. The bright yellow traces the path of a drop of fuel, shrinking as it burns, producing green soot. (Paul Ferkul / NASA)

Recent tests aboard the International Space Station have shown that fire in space can be less predictable and potentially more lethal than it is on Earth. “There have been experiments,” says NASA aerospace engineer Dan Dietrich, “where we observed fires that we didn’t think could exist, but did.”

That fire continues to surprise us is itself surprising when you consider that combustion is likely humanity’s oldest chemistry experiment, consisting of just three basic ingredients: oxygen, heat and fuel.

Here on Earth, when a flame burns, it heats the surrounding atmosphere, causing the air to expand and become less dense. The pull of gravity draws colder, denser air down to the base of the flame, displacing the hot air, which rises. This convection process feeds fresh oxygen to the fire, which burns until it runs out of fuel. The upward flow of air is what gives a flame its teardrop shape and causes it to flicker.

But odd things happen in space, where gravity loses its grip on solids, liquids and gases. Without gravity, hot air expands but doesn’t move upward. The flame persists because of the diffusion of oxygen, with random oxygen molecules drifting into the fire. Absent the upward flow of hot air, fires in microgravity are dome-shaped or spherical—and sluggish, thanks to meager oxygen flow. “If you ignite a piece of paper in microgravity, the fire will just slowly creep along from one end to the other,” says Dietrich. “Astronauts are all very excited to do our experiments because space fires really do look quite alien.”

Such fires might appear eerily tranquil to people accustomed to the capricious nature of earthly flames. But a flame in microgravity can be more tenacious, capable of surviving on less oxygen and burning for longer periods of time.

NASA has practical applications in mind with its research. Scientists hope to learn if certain materials are more flammable in space, and thus to be avoided. Experiments suggest that space station fire extinguishers that squirt gases at a flame are less effective than on terra firma, since they direct air (and oxygen) to the fire, providing additional fuel.

Moreover, the data obtained aboard the space station—through experiments such as comparing how fire spreads on flat objects versus spherical ones—will help engineers better understand the behavior of fuel and flames on Earth, where approximately 75 percent of our power comes from some form of combustion.

NASA scientists are especially excited about the potential applications for a bizarre, unprecedented type of combustion they observed in space this past spring: When certain types of liquid fuel catch fire, they continue to burn even when the flames appear to have been extinguished. The fuel combustion occurs in two stages. The first fire burns with a visible flame that eventually goes out. But shortly afterward, the fuel reignites, taking the form of “cool flames” that burn at lower temperatures and are invisible to the naked eye.

Scientists do not yet have an explanation for this phenomenon. But engineers say that if this chemical process could be duplicated on Earth, the result could be diesel engines that use cool flames to produce fewer air pollutants.

NASA researcher Paul Ferkul says the microgravity experiments provide a unique opportunity to study the underlying dynamics of fire “from a more fundamental point of view” by looking at combustion processes “that would otherwise be masked or at least complicated by gravity.”


Recent tests aboard the International Space Station have shown that fire in space can be less predictable and potentially more lethal than it is on Earth. “There have been experiments,” says NASA aerospace engineer Dan Dietrich, “where we observed fires that we didn’t think could exist, but did.”

That fire continues to surprise us is itself surprising when you consider that combustion is likely humanity’s oldest chemistry experiment, consisting of just three basic ingredients: oxygen, heat and fuel.

Here on Earth, when a flame burns, it heats the surrounding atmosphere, causing the air to expand and become less dense. The pull of gravity draws colder, denser air down to the base of the flame, displacing the hot air, which rises. This convection process feeds fresh oxygen to the fire, which burns until it runs out of fuel. The upward flow of air is what gives a flame its teardrop shape and causes it to flicker.

But odd things happen in space, where gravity loses its grip on solids, liquids and gases. Without gravity, hot air expands but doesn’t move upward. The flame persists because of the diffusion of oxygen, with random oxygen molecules drifting into the fire. Absent the upward flow of hot air, fires in microgravity are dome-shaped or spherical—and sluggish, thanks to meager oxygen flow. “If you ignite a piece of paper in microgravity, the fire will just slowly creep along from one end to the other,” says Dietrich. “Astronauts are all very excited to do our experiments because space fires really do look quite alien.”

Such fires might appear eerily tranquil to people accustomed to the capricious nature of earthly flames. But a flame in microgravity can be more tenacious, capable of surviving on less oxygen and burning for longer periods of time.

NASA has practical applications in mind with its research. Scientists hope to learn if certain materials are more flammable in space, and thus to be avoided. Experiments suggest that space station fire extinguishers that squirt gases at a flame are less effective than on terra firma, since they direct air (and oxygen) to the fire, providing additional fuel.

Moreover, the data obtained aboard the space station—through experiments such as comparing how fire spreads on flat objects versus spherical ones—will help engineers better understand the behavior of fuel and flames on Earth, where approximately 75 percent of our power comes from some form of combustion.

NASA scientists are especially excited about the potential applications for a bizarre, unprecedented type of combustion they observed in space this past spring: When certain types of liquid fuel catch fire, they continue to burn even when the flames appear to have been extinguished. The fuel combustion occurs in two stages. The first fire burns with a visible flame that eventually goes out. But shortly afterward, the fuel reignites, taking the form of “cool flames” that burn at lower temperatures and are invisible to the naked eye.

Scientists do not yet have an explanation for this phenomenon. But engineers say that if this chemical process could be duplicated on Earth, the result could be diesel engines that use cool flames to produce fewer air pollutants.

NASA researcher Paul Ferkul says the microgravity experiments provide a unique opportunity to study the underlying dynamics of fire “from a more fundamental point of view” by looking at combustion processes “that would otherwise be masked or at least complicated by gravity.”

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Comments (24)

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This isn't new. I saw the same thing on a Grateful Dead poster back in the 70's.

Posted by Ricardo on December 3,2012 | 03:59 PM

@Jimmy "I'm no" Einstien: No climate scientist -ever- predicted that "half the world was supposed to be under water in year 2000". Heck, even IF all the ice in the world melts, sea level rise would only amount to about 70 meters, perhaps 30 meters more with heat expansion. That much may flood Florida, the Amazon, parts of Siberia, and expand the Black Sea, but that's nowhere near "half the world". If you want to be taken seriously, you really have to stop making things up, or at least stop listening to fundamentally reactionary denialist "news" sources.

Posted by Moonhare on November 25,2012 | 12:51 PM

Hurrhurr...you said flames.

Posted by Beavis on November 22,2012 | 08:32 AM

They are just now figuring this out? C'mon on... am I the only one not buying this? Didn't they figure this out before sending a rocket into space even? No wonder we haven't gone back to the moon when humans are devolving exponentially every day.

Posted by shaun on November 22,2012 | 05:07 AM

With regard to the comment concerning the ignorance of science and gthe world being half submerged by 2000---isn't it 2/3 water already? Scientists would be the first to admit ignorance, that's why the keep researching! I tell my studentsl, "you are supposed to be ignorant, that's why you are here. It's my job to dispel that ignorance. If you don't ask questions then you are stupid."

Posted by dr.. william schweiker on November 21,2012 | 08:48 PM

With regard to the comment concerning the ignorance of science and gthe world being half submerged by 2000---isn't it 2/3 water already? Scientists would be the first to admit ignorance, that's why the keep researching! I tell my studentsl, "you are supposed to be ignorant, that's why you are here. It's my job to dispel that ignorance. If you don't ask questions then you are stupid."

Posted by dr.. william schweiker on November 21,2012 | 08:48 PM

I've seen a fire in space or two and didn't even know it...

Posted by MKULTRA on November 21,2012 | 08:46 PM

Wouldn't it be possible to suck up a fire in space with some sort of vaccuum cleaner?

Posted by Sander on November 21,2012 | 02:46 PM

Why does it appear symmetrical in the photograph?

Posted by Linda Gardner on November 21,2012 | 01:51 PM

WHERE IS THE VIDEO? Nobody took a video of this incredibly interesting phenomenenon?

Posted by Alan on November 21,2012 | 11:10 AM

I want video!

Posted by Matt on November 21,2012 | 10:50 AM

Since when is there oxygen in space????

Posted by C on November 21,2012 | 10:26 AM

Lets see some pictures

Posted by Prove-it Patrick on November 21,2012 | 09:41 AM

Didn't the russian MIR space station have fires break out? I'm surprised that there is still this much mystery over fire in zero g. I'm guessing NASA and Russia didn't share notes that well regarding fire. This makes what happened on MIR that much spookier.

Posted by andrew on November 21,2012 | 09:26 AM



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