If you lit a flame in an oxygen atmosphere in space, what direction would the flame burn in?

Asked by: Matthew Wilson

Answer

I suppose the "proper" question is as follows: If one lights a flame in an oxygen atmosphere in a place where there is effectively no gravitation, which direction would the flame burn in? On earth, as we all have seen and experienced, when you light a flame, such as a candle or a match, the flame points "up". But why? If we answer that question, we will be quite a bit closer to answering the actual question here. When you have combustion, such as a match, what happens is that oxygen gets consumed, and other gases (such as carbon dioxide) get produced. In addition, of course, energy is liberated in the form of heat and light. The produced gases are hotter than the environment due to the generated energy, and they expand, as heated gases do. Then, of course, since their density is lower than the surrounding atmosphere, the gas tends to rise. This has two effects. First, while it moves up, it carries energy with it, thus the flame ends up pointing "up". Second, since the products of combustion move up, they make more oxygen available to make the flame continue to burn. But, in case there is no gravity, the expansion of the produced gases does not cause them to go anywhere. They just stand there and expand spherically around the source. So we have neither of the two effects above. The flame does not point anywhere, and burns like a sphere (given the reaction point is small, like a match). Second, since the auto-oxygen-feeder mechanism also fails, the flame burns in rather dull colors, and will extinguish itself rather quickly if there is no flow of air (or if the match is not moved around) to feed oxygen into the reaction.
Answered by: Yasar Safkan, Ph.D., Software Engineer, GVZ., Istanbul, Turkey

Flames burn upwards on Earth. This is due to the changing density of the Earth's atmosphere. Flames are essentially hot gases, and hot things rise because they are less dense than their surroundings. It is the gravitational pull of the Earth that makes the air more dense nearer to the ground - so it is ultimately because of gravity that flames burn in the direction they do. The direction a flame burns is also determined by air currents: a strong wind or a puff of air will take a flame with it. If we could set something on fire in space there would be neither gravitational fields nor any winds to pull flames one way or another. Flames would flicker in all directions, just as a star sends out light in all directions. However, it really depends on how we set up our fire to begin with. This space flame, now not affected by gravity or winds, would be sensitive to the shape of the thing burning and the way it was set alight. Flames are essentially streams of particles escaping from the burning item, at the same time there are other particles that are moving inwards towards the fire. These currents are affected by the shape of the burning item. One final note to bear in mind is that space is actually mostly that - there are very few particles in the vast stretches of space between galaxies and between stars and no oxygen. Therefore getting the flame started is of course impossible without imagining a make-believe atmosphere of oxygen.
Answered by: Sally Riordan, M.A., Management Consultant, London

The simple answer is that it would burn in a sphere. This has been experimentally proven... and not in a good way. The astronauts aboard the space station Mir actually had a fire onboard the station, and they survived, and reported that as predicted, the fire was in a spherical shape. Now for the explanation why: What you see when you see a flame is actually the rapid and energetic reorganization of molecules, usually free oxygen (O2) combining with gaseous hydrocarbons (any combination of C's and H's). Most gaseous hydrocarbons at room temperature (and DEFINITELY when burning) are lighter than air. Hence, when you look at a flame, it always seems to burn in a direction opposite gravity. However... in a 0-G environment, there is no "up". The gas spreads evenly in every direction, burning as it goes. Fires in a 0G environment actually often burn themselves out, because they gobble up all the air in the immediate area, and the hot hydrocarbons on the inside of the "bubble" have nowhere to go.
Answered by: Frank DiBonaventuro, B.S., Air Force officer, Physics grad, The Citadel