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Q & A: Candles in Microgravity

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Most recent answer: 08/31/2011
Q:
If a candle burns in micro-gravity it *might* form a sphere and snuff itself out because there’s no convection. However, if by chance it starts to stream off in some direction, might that not be a self-sustaining situation?
- Colin
Solipsys Limited, England
A:
Colin -

Yes, you are absolutely correct. When a candle burns on earth, the flame heats up the air around it, causing that air to rise. This allows fresh oxygen to move in towards the candle flame so the flame can keep burning.

In microgravity, there is no such effect as 'hot air rises' so this can't happen. The hot air tends to stay very close to the candle flame, and without any sort of outside ventilation to keep the air moving, the flame will burn at a much lower temperature than on Earth  and the candle will eventually burn itself out. And since the hot air doesn't rise away, the flame is spherical in shape:


(from )

Another interesting thing about this is that the typical candle flame in space will produce virtually no smoke, because the flame's temperature is too low.

But this all relies on the air in the room being perfectly still. If there is a breeze, then the flame will be 'blown' into a different shape, and it will have plenty of fresh air so it will not burn itself out. Of course, once it burns up its fuel it will die out just like on Earth, but that may take a while.

For more information on research and experiments currently being performed on flames in microgravity, check out .

-Tamara

p.s. Your idea about a self-sustaining pattern is very reasonable. Such things do exist. However, I gather from Tamara's links that in practice that sort of instability isn't found here. /mw

(published on 10/22/2007)

Follow-Up #1: microgravity and free fall

Q:
Why does everybody use the term microgravity. There is no such thing as microgravity, only gravity fields. It is either being on ground or being in free fall. Astronauts in space are inside many gravity fields (the Moon's, the Earth's, the Sun's, the other planets', the other stars', our supermassive black hole's, and so on) but they are also in a perpetuous free fall. On Earth, you are inside those same gravity fields, but not in a free fall anymore. Being in a no gravity zone is being outside our universe since gravity fields extend indefinitely at the speed of light. Lighting up a candle in a elevator set loose (cutting the cables) would produce the same flame as it does while in orbit until air resistance becomes so great that it stops the elevator's acceleration (or until the elevator crashes).
- Anonymous
A:
You're right that free-fall is free-fall, giving zero effective gravitational field in the free-fall coordinates. Unless there are some very strong tidal fields that conclusion holds generally. By micro-gravity people usually mean some situation in which the object isn't quite in free-fall. For example, even on an artificial satellite your gravitational pull toward the satellite itself will be resisted by some sort of force from the satellite material. Of course that deviation from free-fall is small compared to that experienced by someone standing on the earth or even someone standing on the moon. As you mention, even in a falling elevator, air friction on the elevator causes a definite deviation from free-fall, and thus some effective gravity.

Mike W.

(published on 08/31/2011)

Follow-up on this answer.