The Wonder of Fire Without Gravity

It’s about to get lit on the International Space Station.

A flame in microgravity, filling with burning soot particles
A flame in microgravity, filling with burning soot particles (NASA)

A fire on the International Space Station, high above Earth and far from help, could be catastrophic, even deadly. Space agencies try to reduce the risk as much as possible. The station is equipped with smoke detectors, and astronauts practice fire drills. But astronauts have been setting fires in space for years. On purpose!

They do it in the name of science, at the careful instruction of researchers back on the ground. The experiments unfold in small facilities inside the station, safe from other equipment, the crew, and their precious supply of breathable air.

To study combustion on the ISS, and before that, the Space Shuttles, astronauts have set fire to a variety of materials and observed how this distinctly earthly phenomenon unfolds in microgravity. And they love doing it.

“They get very excited,” Paul Ferkul, a scientist at the Universities Space Research Association, told me recently. “They’re all very glad to burn stuff.”

Researchers usually guide astronauts in real time as they feed samples into a special chamber and watch them burn. “The astronauts really enjoying doing it because it is so hands on,” Ferkul said. “This is one [experiment] where they can see the results immediately and really feel like they’re doing current research.”

The results are pretty bonkers. In space, a flame is shaped like a sphere instead of a teardrop, and it doesn’t flicker. It just hovers, a small, ghostly orb, until it goes out. Such orbs are called “flame balls,” a term that is both extremely accurate and delightfully deranged. “It’s kind of mesmerizing to see this burning without gravity present,” Ferkul said.

Ferkul and his fellow researchers have devised a new fiery experiment for astronauts. The experiment, which was scheduled to launch to the ISS on a resupply mission today, will study how fire spreads in small, confined spaces. A blaze inside a building or a spaceship behaves differently than it does in open spaces, and the researchers hope this work could inspire better infrastructure design and fire-safety codes.

Studying fire in space is actually easier than doing it on Earth. Fire is complicated here. A single candle flame contains thousands of chemical reactions. The heat of the flame vaporizes wax and breaks down its molecules into carbon and hydrogen, which combine with oxygen in the air to produce light, heat, carbon dioxide, and water vapor. As hot air rises to the top, fresh, cooler air is drawn in at the bottom, providing fuel for the flame. This flow molds the flame into a teardrop and makes it flicker.

On Earth, the flickering hides the subtle dynamics of fire. In space, this flow doesn’t exist. Air travels in all directions, and the flame rounds out. Without interference from gravity, scientists can get a better look.

“Flame balls are to combustion scientists what fruit flies are to geneticists,” Paul Ronney, a combustion researcher, once said. “It’s not that we want more fruit flies, or flame balls, but they provide a simple model for testing hypotheses and checking computer models.”

Ronney first observed the strange phenomenon in the 1980s. The researcher dropped a can of burning hydrogen down a shaft designed to simulate the weightlessness of microgravity and filmed what happened. When he saw the footage, “I thought I had done something wrong,” he said. “It was ridiculous. No one had ever seen anything like it.”

The flames cracked into tiny floating balls. Computer analyses had predicted flames would be small and only last a few minutes. Ignited, the weightless flames produced a tiny fraction of the thermal power of a birthday candle. But they turned out to be two to three times bigger than predicted, and burned for longer, only dying out when the system automatically extinguished them. One of the most destructive forces on the planet appeared delicate, suspended in weightlessness.

Trapped in a space station, though, free-floating fire will still spark fear. In 1997, a faulty oxygen canister on a Russian space station erupted into flames. Dense smoke filled the spacecraft. “My natural reaction was to want to open a window,” Aleksandr Lazutkin, a Russian cosmonaut, recalled later. “And then, I was truly afraid for the first time. You can’t escape the smoke. You can’t just open a window to ventilate the room.” The crew—four Russians, one American, and one German—rushed to put on oxygen masks and deploy their fire extinguishers, spraying the blaze with foam and water. The fire eventually exhausted itself. The crew, shaken but unharmed, resumed their mission.

Crews today would take similar action. They could be forced to evacuate to a Soyuz capsule, the Russian spacecraft that could take them home. If the fire spreads there, astronauts would be left with a terrifying but necessary option. “The Soyuz does not have any fire extinguishers,” British astronaut Tim Peake has explained. “The way to fight a fire in the Soyuz is to close your helmet and depressurize the whole spacecraft. No oxygen, no fire.”

These days, fires are confined to special facilities on the ISS and empty spacecraft that deliver supplies and science experiments. After astronauts unpack these vehicles, they stuff them with trash and send them to burn up in Earth’s atmosphere. Scientists have remotely ignited fires inside three departing spacecraft—a safe distance from the ISS, of course—and collected the resulting data before the vehicles disintegrated in a fiery descent.

Playing with fire in today’s spacecraft is good preparation for potential missions beyond Earth’s orbit. Fire is one of the earliest technologies mastered, and humanity could someday deliver it to places with far less gravity than we’re used to. The surface gravity on Mars is about 38 percent that of Earth’s. On the moon, it’s 17 percent. To keep astronauts healthy and alive, scientists will need to understand how all kinds of earthly phenomena behave when removed from the planet where they were first tamed.