A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

If you lit a match in zero gravity, would it smother in its own smoke?

February 19, 1993

Dear Cecil:

If a match is lit in an atmosphere like earth's but outside of gravity's pull, will it suffocate? Will it snuff out from its own gases faster than if it had a steady earthbound updraft to refresh it with more oxygen? I heard the space shuttle astronauts were doing tests with fire in microgravity. I missed reading any results so all I can do is guess: it snuffs. My earthist rivals insist fire will burn in zero Gs; some say heat rising will start a draft which in turn ventilates the reaction. I immediately ask: which way is up? Duh!

Cecil replies:

It is just this kind of restless curiosity, John, that is at the heart of all great science. They've been doing a lot of fire experiments on the space shuttle lately, and while the official line is that they're trying advance the frontiers of knowledge, etc., one suspects the scientists just want to know, among other things, whether a flame would smother in its own smoke.

As it turns out, a match is not the ideal experimental medium for this purpose. The head may contain oxidizers and whatnot that would queer the results. What you really want is a candle. The space shuttle astronauts brought a couple along on a mission last summer and lit them inside a sealed chamber having an earth-type atmosphere. (This is not something you would want to do in the open space shuttle cabin, where for obvious reasons an exposed flame is on a par with leaving the front door open.) One candle burned for about two minutes, the other for 20 seconds. Then — here's the vindication you've been waiting for — they snuffed. The flames were weak, spherical, and pure blue.

As you realized and your earthist friends obviously didn't, in a zero-gravity environment you don't have convection. Convection is the familiar process whereby heated air over a candle flame rises, carrying smoke and waste gases with it. Cool, oxygen-rich air rushes in to replace the departing warm air and in the process keeps the flame going. Convection works in normal gravity because warm air is less dense and thus lighter than cool air and so rises above it. But in a weightless environment the exhaust gases basically hang around the candle flame until all the oxygen in the immediate vicinity is exhausted, at which point the flame goes out.

So there's your basic answer. Now for the hedging. It turns out that with a little tweaking you can get stuff to burn in zero Gs just fine, in some cases better than it would burn on earth. NASA has been burning paper and other stuff in zero Gs both on the space shuttle and in "drop towers" on earth, where you can get a few seconds of weightlessness while something is in free fall. If there's the slightest breeze, as opposed to the dead still air in the candle-flame chamber, enough oxygen gets through to sustain combustion. Pump in 35 to 50 percent oxygen instead of the normal 21 percent and you'll get a self-sustaining flame even in still air. Adjust the air flow just so and you can get some weak flames to burn longer in zero Gs than they would on earth, where too-hearty convection currents can cause a flame to blow itself out. Dunno that it's worth spending six jillion dollars a year on the space program to find this all out, but it's nice to have the definitive answer.

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