How flames behave in space

Research on the International Space Station is helping scientists to understand how fire spreads and behaves in different environments and learn how to prevent and extinguish fires in space.

Microgravity in space dramatically influences flames and provides a unique environment for studying combustion. For example, on Earth, hot gases from a flame rise and gravity pulls cooler, denser air to the bottom of a flame, creating the classic shape and flickering effect. In microgravity, this flow doesn't occur and on the space station, low-momentum flames tend to be rounded or even spherical. By removing the effects of buoyancy, microgravity provides researchers a better understanding of specific flame behaviours.

Differences between flames on Earth and space

Space flames

But in a microgravity environment, like the one astronauts experience while in orbit, hot air still expands outward — but it doesn’t move upward, because there is no “upward.” Instead, fires in space are fed only by the random oxygen molecules that stumble into them by chance.

It’s a process called molecular diffusion, and it produces spherical flames that are different from their Earthly counterparts in more ways than one. Not only do they burn much slower and for longer periods of time, but they also survive on less oxygen and clock in at less than 900 degrees Fahrenheit — a fraction of the heat given off by most terrestrial flames.

 In 1997 a fire ignited aboard the Russian space station  MIR .  It originated in an oxygen generator, filling the station’s modules with toxic smoke and cutting off access to an escape vehicle during the several minutes it lived.

One of the reasons fire is so dangerous in space is its lack of predictability. Unlike on the ground, where gravity forces flames upward, flames in a microgravity environment can spread in any direction . The same goes for smoke, making the placement of smoke detectors in a space station (typically on the ceiling in most buildings) much more difficult.

Though the Mir crew quickly doused the errant flame with a fire extinguisher, preventing its growth, extinguishers that use gases to snuff a flame are less effective in space than on Earth. For one, the apparatus can literally fan the flames of a fire by directing air — and therefore, oxygen — toward it!

In the end, the flame extinguished itself only when the oxygen generator had emptied. 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 either themselves or the station’s structure.

During the FLEX experiments on the space station, scientists made a discovery in question? After certain liquid fuels are extinguished in space, they spontaneously reignite. In these cases, the subsequent flame — called a “cool flame” — burns at lower temperatures and is invisible to the naked eye. 

FLEX, which analyzed the effectiveness of fire suppressants, led to the discovery of a type of cool flame, where the fuel continued "burning" under certain conditions after extinction of the visible flame. Typical flames produce carbon dioxide and water, but cool flames produce carbon monoxide and formaldehyde. Learning more about the behavior of these chemically different flames could lead to the development of more-efficient, less-polluting vehicles. 

Scientists aren’t sure exactly why this happens, but from a practical point of view, we could use such low temperature combustions hypothetically to produce fewer air pollutants in diesel engines back on Earth. We’re far from that reality, though research conducted in June 2021 took another great leap when it repeated the phenomenon using gaseous fuels, rather than liquid ones.

The next step in NASA’s research is SoFIE, or Solid Fuel Ignition and Extinction. This set of experiments, launched to the ISS in February of 2022 and expected to continue until 2025, will help the administration choose the best fire-retardant materials and designs for "spacesuits, cabins and habitats." 

A whole host of materials is in line to be tested, including plexiglass and cotton-based fabrics. Afterward, SoFIE’s results will even be applied to mathematical models that forecast how those same materials might burn in conditions outside of microgravity — including on the moon, Mars or elsewhere in our solar system. 

Combustion investigations contribute to the safety of crew members, equipment, and spacecraft by guiding selection of spacecraft cabin materials, improving understanding of fire growth, and identifying optimal fire suppression techniques. This research also contributes to fire safety on Earth and some studies improve our understanding of combustion for uses such as producing electricity and powering vehicles on the ground.

The Combustion Integrated Rack (CIR), developed and operated by NASA's Glenn Research Center, provides a secure and safe environment for a wide range of combustion experiments. Different chamber inserts that enable a variety of investigations include the Multi-user Droplet Combustion Apparatus, which supported FLame Extinguishment Experiments (FLEX), the Advanced Combustion via Microgravity Experiments (ACME) insert, and the Solid Fuel Ignition and Extinction—Growth and Extinction Limit (SoFIE) chamber.