Garry Trethewey
Cherryville, South Australia
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A candle flame isn鈥檛 a thing, but a process. Burning heats air, which expands and becomes less dense. Because of gravity, the non-heated air around it falls, but in common parlance, we say the hot air rises. That creates a flow, bringing fresh oxygen to support combustion.
Without gravity, or a fan to maintain gas flow, the candle flame would surround itself with a sphere of oxygen-free burnt gas, and the process would end in a small fraction of a second.
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Richard Klingbiel
Friday Harbor, Washington, US
A flame in zero gravity would be spherical. A flame is simply hot gas, heated by the oxidation of its fuel. Being hot, it expands and is therefore of a lesser density than the air around it, forcing it upwards in an environment with gravity. In zero gravity there would be no such upward movement. The flame would expand in a uniform way and be spherical except for where it makes contact with its fuel (the candle wick).
鈥淓xperiments with flames are perhaps not ideal in the limited, air-conditioned environment of a spacecraft鈥�
Its size would increase depending on how rapidly its heating from combustion could either radiate or conduct to the surrounding air.
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Tony Compton
Hexham, Northumberland, UK
A candle relies on convection to clear away the combustion products, so the obvious answer is this won鈥檛 work in zero gravity. However, if the candle were sitting in a tube of slightly greater diameter than its own and containing a gentle flow of air, it should operate normally 鈥� with a flame of the usual shape.
Perhaps not an ideal experiment in the limited, air-conditioned environment of a spacecraft!
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Alex McDowell
London, UK
The flame would be spherical and blue, as astronauts found when they burned candles on NASA鈥檚 Skylab space station in the 1970s.
There are no convection currents without gravity, hence the supply of oxygen is limited by diffusion. The convection current elongates the candle flame when gravity is present, and cools it. Hence candle flames burn hotter and blue without gravity.
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Mike Follows
Sutton Coldfield, West Midlands, UK
Not only is a flame spherical in microgravity, but the colour is also blue, different to that on Earth.
A candle is made of wax, a hydrocarbon, and the products of its combustion are water vapour and carbon dioxide. Carbon dioxide is normally colourless and transparent. However, in the hot flame, the electrons are excited and, when they return to their ground state, emit photons corresponding to the blue part of the electromagnetic spectrum.
Apart from blue close to the wick, a candle flame on Earth is mainly yellow. This is because some tiny particles of candle wax escape as soot in the convection current before they can be completely combusted. These particles are tiny black body radiators whose radiation peaks in the yellow part of the electromagnetic spectrum. Without convection, this process doesn鈥檛 happen.
A fire in a confined space like a spacecraft is rightly feared and a blue flame makes them difficult to spot. In microgravity, a flame in undisturbed air would normally burn itself out. However, if only to prevent electrical appliances overheating, fans are used to circulate air inside spacecraft.
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@PhilMorey8
via Twitter
If you are lighting candles in apparent zero gravity then your spaceship is in deep trouble.
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Robert Gallop
Sydney, Australia
I had this exact question in 1970 in an Imperial College London university entrance exam. I got the answer wrong.
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