Expert: Philip Stahl - 5/19/2005

Question
OK time for some theory. I have always wondered this, and even asked straightdope.com (no answer of course) but I am curious If you lit a match in space (in a spaceship with oxygen) how would the flame be shaped? I assume some sort of a ball because if hot air rises then it must have direction. There is no up or down in space, so would the flame exist? I think it would radiate outwards uniformly, but dosen't a flame need the upward morement to maintain oxygen flow. Would it only burn for a second and snuff itself out? Have any experiments been done on this topic? Thanks for your time, and please forgive the typos.
-Nathan S

Answer
Hello.

According to the General Theory of Relativity and its ‘Principle of Equivalence', inertial and gravitational forces are treated exactly the same in any space craft as they would be in a system (say on Earth) subject to Newtonian dynamics, etc.

Simply put, this would mean that - for a spacecraft that can be imparted an acceleration of gravity g (say by the use of rotation via centripetal acceleration) and assuming an atmosphere the same as on Earth - any flame would burn the same as on Earth. (All other elements, variables taken into account - or to put it another way, with the same *controls* used in each case).

Now beyond this, and altering all the variables  - all sorts of things are possible, of course. For example, in zero g conditions yes, the flame would *tend* to assume a spherical shape. The duration etc. would largely be a matter of 'how much oxygen' is present in the ship's atmosphere. If there's a high % it will last longer than if there's a very low percentage.

Re: 'up' or 'down' in space - these are purely relative referents anyway (though yes, 'down' is usually defined in the direction of 'g'), so have no bearing on whether the flame exists.

As a basic plasma, the flame exists - or continues to do so- so long as conditions are right for ionization (e.g. of oxygen molecules) to be sustained. And for the temperature for such ionization to be adequate. Once the temp. falls below the critical threshold (T_c) ionization halts, and the plasma disspates. I.e. the flame winks out.

So far as I am aware, no experiments have been done on this, or - if they have- I haven't seen the results published anywhere.