Science Puzzle
What Shape Is a Candle Flame in Space?
On Earth a candle flame is a teardrop, pointing upwards, with a bright yellow tip. Astronauts have carefully lit candles in orbit, inside sealed boxes, to see what happens.
What shape is a candle flame in space, and why?
The Answer
Round. In orbit a candle flame is a small, dim blue dome around the wick, because without gravity there's no rising hot air to stretch it upwards.
On Earth, the hot gas from a flame is less dense than the cool air around it. Gravity pulls the denser cool air down underneath it, pushing the hot gas upwards. That steady draught stretches the flame into a teardrop and keeps sweeping fresh oxygen in at the bottom. The flame burns hot and fast, and tiny specks of soot glow yellow as they're carried up.
In orbit, everything is falling together, so nothing is heavier than anything else and hot gas doesn't rise. With no draught, the flame spreads out evenly in every direction. Oxygen can only reach it by slowly drifting in, so it burns gently and cooler, makes almost no soot and glows a faint blue.
In 1996 the astronaut Shannon Lucid burned about 80 candles on the Mir space station. The flames were about 1.5 centimetres across and some kept going for 45 minutes. Earlier tests on the Space Shuttle had died out in under a minute. Flames in space are a serious subject: spacecraft fire safety depends on knowing how they behave.
The principle: Convection needs gravity. Hot gas only rises because gravity pulls the denser cool air beneath it; without gravity a flame gets no draught and spreads into a dim ball fed by slowly drifting oxygen.