Flames in space

Considering it’s one of mankind’s oldest tools, you might assume we know everything there is to know about fire. And of course, we know this much: As the hot air near the base of the flame rises, gravity pulls in cooler, denser air to replace it. It is the circulation of air that provides fresh oxygen and gives the flames their characteristic teardrop shapes.

But in a microgravity environment like that experienced by astronauts while in orbit, all bets are off. Here, the hot air is still expanding outward – but it’s not moving up, because there’s no “up”. Instead, fires in space are fueled only by the random oxygen molecules that happen to fall into them.


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This is a process called molecular diffusion, and it produces spherical flames that differ from their terrestrial counterparts in more ways than one. Not only do they burn much more slowly and for longer periods of time, but they also survive on less oxygen and maintain a temperature below 900 degrees Fahrenheit—a fraction of the heat given off by most terrestrial flames.

And yet there’s still a lot scientists don’t understand about how fire works in microgravity. Are some materials more flammable than others? What is the best way to extinguish a rogue flame?

These questions are critical to the safety of the astronauts who already live and work in the International Space Station (ISS) and will become increasingly important as people prepare for longer space trips. Fortunately, NASA scientists are on the case.

Fire on board

To be clear, the threat is not merely hypothetical. In 1997, for example, a fire broke out on board the Russian space station Mir; originated from an oxygen generator, filling the station’s modules with toxic fumes and cutting off access to a rescue vehicle for the few minutes it lived.

One of the reasons fire is so dangerous in space is its unpredictability. Unlike on earth, where gravity forces flames upward, flames in a microgravity environment can spread in any direction. The same goes for smoke, which makes placing smoke detectors on a space station (usually on the ceiling in most buildings) much more difficult.

Although the Mir crew quickly extinguished the strange blaze with a fire extinguisher, preventing it from spreading, fire extinguishers that use gases to extinguish flames are less effective in space than on Earth. For one thing, the apparatus can literally fan the flames of a fire by directing air—and therefore oxygen—at it.

Mir (Credit: Rawpixel Ltd/CC BY 4.0/Flickr)

In the end, the flame only went out when the oxygen generator ran out. Over the next several hours, the station’s life support systems cleared Mir’s atmosphere of all smoke, and the crew escaped the incident without significant damage to themselves or the station structure.

Playing with fire

OK, so we’ve found that filling these gaps in our knowledge of fire behavior is obviously important. Now, how exactly do scientists do this?

Well, in 2008, NASA created theirs Integrated burner stand (CIR) and sent it to the ISS. Used to safely operate controlled burns in microgravity, its hardware includes a 26-gallon combustion chamber and five different chambers that have been put to good use in thousands of tests over the past 15 years.

Many of these tests were part of Flame Extinction Experiment, or FLEX, which started about a year later. As the name suggests, they revolve around putting out fires in space and ultimately improving fire suppression systems on future spacecraft. Using the CIR, researchers aboard the ISS will ignite tiny droplets of heptane or methanol fuel and record the results.


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Daniel Dietrich, a scientist at NASA’s Glenn Research Center, told the administration that “one of the biggest discoveries, not only in the microgravity program, but probably in the last 20 [to] 30 years of combustion research was during the space station’s FLEX experiments.

The discovery in question? After certain liquid fuels go out in space, they spontaneously re-ignite. In these cases, the subsequent flame – called “cool flame” — burns at lower temperatures and is invisible to the naked eye.

Scientists aren’t sure exactly why this happens, but from a practical standpoint, we could use such low-temperature combustion hypothetically to produce less air pollutants in diesel engines on Earth. We are far from this reality, although research conducted in June 2021 made another great leap when it replicated the phenomenon using gaseous fuels instead of liquid ones.

Bringing the flames home

Probably the best part about studying flames in space, though, is that the lack of gravity just… makes things simpler in a lot of ways. While the candle on your coffee table may flicker, for example, as a result of buoyancy-induced instability, this is not the case in microgravity.

Another series of NASA studies, Advanced burning through microgravity experiments (ACME), took advantage of this to study what makes a good flame—one that’s efficient but doesn’t emit a lot of pollutants like soot. In early 2017, ACME took a closer look more than 1500 flames in CIR, for more than four years.

A flame ignited in microgravity as part of ACME’s investigation. The yellow spots are soot clusters. (Credit: NASA)

The next step in NASA’s research is SoFIE, or Ignition and extinguishing of solid fuel. This set of experiments, launched on the ISS in February 2022 and expected to continue until 2025, will help the administration select the best flame-retardant materials and designs for “spacesuits, cabins and habitats.”

A range of materials are to be tested, including Plexiglas and cotton-based fabrics. The SoFIE results will then even be applied to mathematical models that predict how these same materials might burn in conditions outside of microgravity — including at moon, mars or elsewhere in our solar system.

But we’re not stopping there: the results could be felt all the way down to Earth if they potentially revamp fire-safe materials for our homes, offices and airplanes. You might say it’s NASA’s version of fighting fire with fire.

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