“A typical candle flame produces light, heat, carbon dioxide and water vapor. The heat causes these combustion products to expand, which lowers their density, and they rise due to buoyancy. Fresh, oxygen-containing air can thus get into the flame, further fueling combustion. Because gravity is necessary for density differences, neither buoyancy nor convection occur in a zero-gravity environment such as space. Combustion products accumulate around the flame, preventing sufficient oxygen from reaching it and sustaining the combustion reaction. Ultimately the flame goes out. Oxygen could still reach a flame if someone blew the gas into the flame. It could also possibly "diffuse" in; it is diffusion that spreads the scent of a perfume in a room without air circulation. The perfume slowly mixes with the air to try to achieve a uniform distribution. This process, however, is too slow to sustain a flame.”

Scientific American, November 2000

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