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We’ve already shown you how water behaves in zero gravity, but what does fire look like? Tests aboard the International Space Station show that flames in space are quite different. They are less predictable, and potentially more hazardous than down here on Earth.

On Earth, a flame from a candle is shaped like a teardrop. That’s because the heat causes the surrounding air to expand and become less dense. Gravity pulls the colder, denser air down, while hot air rises and pulls combustible material upwards. This process also supplies fresh oxygen to the fire which allows it to continue to burn.

In space however, a flame burns in all directions and looks like a blob. No gravity means that hot air doesn't move upwards, nor does smoke. That makes it difficult for astronauts to find the source of a fire, and placing smoke detectors is not as straightforward as on earth since there will be no smoke trail traveling up towards the ceiling. No upward draft also means that the fire is either shaped like a dome or spherical.

Slow and steady wins the race

A flame in micro gravity gets less oxygen than on earth. It survives thanks to random oxygen molecules drifting into it. It doesn't burn as bright or fast as you might imagine. If you were to set a piece of paper on fire, the flame will slowly creep along from one end to the other.

Experiments on the now defunct Russian spacecraft Mir however found that candles that burn for 10 minutes or less on earth lasted up to 45 minutes in space.

The fire is a very faint, almost invisible blue color as the temperature of the flame is too low for the yellow color seen on earth. When the flame is extinguished, a white ball surrounding the candle tip remains, which may be a fog of flammable wax vapor.

Smoke and soot production is also different. In the absence of any airflow, the slower gas exchange from diffusion can produce a soot-free flame. When burning stops however, at the tip of the flame, soot production begins. Smoke production depends on the fuel flow rate.

Fire extinguishers that shoot gases are less effective than on earth. Instead they direct air and oxygen to the fire and provide additional fuel to the flames. Since March of 2009,  The ISS’ Flame Extinguishment Experiment, or FLEX, has conducted more than two hundred tests to better understand fire in microgravity. 

During their work, they made a baffling accidental discovery... The cool flames can be used on Earth to produce cleaner auto-ignition. To understand how, check out the video below at 3:38. 

Invisible Fire

Another peculiar - and quite creepy - behavior that was picked up on by NASA was that when certain types of liquid fuel catch fire, they continue to burn even when the flames appear to have been extinguished. In this case combustion happens in two stages. First, the fire burns with a visible flame, but shortly after the fire goes out, the fuel reignites and takes a form they call “cool flames” that burn at lower temperatures and is invisible to the naked eye.

Scientists haven’t been able to come up with an explanation for this phenomenon. But engineers hope that this chemical process could be duplicated on earth. Their plan is that if diesel engines use cool flames they will produce fewer air pollutants.

Scientists have many other ongoing experiments on space stations that could help engineers better understand the behavior of fuel and flames on earth. Data on such matter is highly valuable since approximately 75% of our power comes from some form of combustion.

And now, the moment we've all been waiting for...Video footage: