Combustion, or fire, is a rapid chemical reaction that releases heat and light. It requires three fundamental components to begin and sustain itself, traditionally represented by the fire triangle: fuel (the material that burns), heat (the energy to start the reaction), and an oxidizing agent, typically oxygen. If any one of these elements is removed, the fire cannot exist. The presence of gravity is so fundamental to fire on Earth that removing this factor fundamentally changes the entire chemical process, leading to strange behaviors.
Gravity’s Role in Earth Combustion
On Earth, the familiar, flickering, teardrop shape of a flame is a direct consequence of gravity. Combustion produces hot exhaust gases, which are less dense than the surrounding cooler air. This difference in density causes the hot gases to be pushed upward by the buoyant force of the heavier, colder air sinking to replace them, a process known as natural convection.
This continuous upward flow of hot air creates a current that constantly pulls fresh, oxygenated air toward the base of the flame. The convection current efficiently feeds the fire with the necessary oxidizer and sweeps away the inert combustion products. This vigorous process allows terrestrial fire to burn brightly, rapidly, and at a high temperature.
Clarifying Combustion Environments in Space
A common misconception is that fire is impossible in space due to a lack of oxygen. The deep vacuum of space lacks the atmospheric oxygen required for standard combustion to occur, meaning a campfire or a burning log would be impossible. However, fire can and does occur within the pressurized, oxygen-rich environments of spacecraft, such as the International Space Station (ISS).
These environments are carefully regulated to provide breathable air for astronauts, meaning all the necessary components for fire—fuel, heat, and oxygen—are present. The fire hazards studied in orbit are not in the vacuum, but within the man-made atmosphere of the spacecraft. Rockets rely on combustion in space by carrying their own oxidizer, often liquid oxygen, mixed with the fuel in a self-contained system. Understanding how fire behaves under microgravity conditions is a matter of importance for crewed missions.
The Unique Physics of Microgravity Flames
In the microgravity environment of an orbiting spacecraft, the buoyant force that drives convection is essentially eliminated. The hot exhaust gases no longer rise because there is no significant difference in weight between the hot and cool air. Without the upward draft to pull in fresh oxidizer, the flame loses its familiar teardrop shape and instead forms a perfect sphere.
This spherical shape is a physical manifestation of a different gas transport mechanism taking over: molecular diffusion. The flame must now rely on the slow, random movement of individual oxygen molecules to drift inward toward the fuel source. Simultaneously, the waste products of combustion must diffuse outward through the same slow process. This reliance on diffusion makes the flame much less vigorous.
Microgravity flames burn significantly slower and at cooler temperatures than their terrestrial counterparts. Because the hot exhaust gases are not rapidly swept away, they tend to smother the flame, limiting its lifespan and heat output. Researchers studying combustion on the ISS have discovered “cool flames,” a distinct, low-temperature form of chemical reaction. These cool flames hover around 950 degrees Fahrenheit (500 degrees Celsius), a fraction of the heat of a typical flame, and are often nearly invisible.