Blue Fire in Space

Marianne Dyson, October 2021

As the holiday season approaches, many of us decorate our tables with candles. If we did that in space, they would burn round and blue, not pointy and yellow. Why?

Candle flames heat the air around them. The hot air rises because it is less dense and thus weighs less than cold air. This air movement shapes a candle flame into a cone with a point at the top (see photo). Flames are blue at the base where they are hottest, and yellow above because as the hot air rises it takes some of the unburned wax with it in the form of smoke and soot.

Flames on Earth are cone shaped and blue at the base and yellow at the top because heated air rises. Marianne Dyson photo, 2021.

In space, hot and cold air weigh the same: nothing. Heated air just expands outward in a sphere. Because the air doesn’t rush away in one direction (up) like on Earth, the fire burns hotter and doesn’t produce as much smoke and soot, making the whole flame blue.

space flame
In space, hot air doesn’t rise, so flames are round and blue like this candle lit during a Shuttle flight in 1992. NASA photo STS050-232—011, cropped by author

If there’s a fire on Earth, we quickly head outside into the fresh air. In space, there is no air outside. Fire can quickly fill spacecraft or space station modules with smoke and suffocate the crew. But fresh air is available via oxygen masks and tanks available in all modules. So the first thing astronauts do in case of a fire is to don an oxygen mask.

Luckily both on Earth and in space, flames can be put out by smothering them—depriving them of oxygen.

On Earth, firefighters remove brush that fuels fire and smother and cool the fire with water or chemicals. In space, when the space station’s smoke detector sounds the alarm, the station’s main computer automatically shuts off the fans and the flow of oxygen. This helps keep the fire from spreading and makes it easier for the crew to fight the fire.

Spraying water does not work well in space because water forms balls which wobble away in weightlessness. Using chemicals in a closed environment could be as harmful to the crew as breathing smoke and clog up the air filters. Both water and chemicals could also ruin expensive experiments and computers that are cooled by blowing air.

Instead of water or foam, crew members use fire extinguishers that spray carbon dioxide. The crew have to anchor themselves while spraying in free fall. Otherwise, the force of the spray will send them flying backward. After the fire is out, the crew continue to wear oxygen masks until the air system filters out the smoke particles and removes the extra carbon dioxide.

Whether on Earth or in space, fires goes out when deprived of fuel and oxygen. But the best way to put out a fire is to not let it start in the first place. This holiday season, please don’t leave a burning candle unattended!

Fire needs oxygen to burn. A birthday candle goes out in a few seconds when deprived of oxygen (via placing a glass jar upside down over it).  Marianne Dyson photo, 2021.

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Cover of Up in Space: we build a station by Marianne J. Dyson "Space Nanna"
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