Flickering flame: spooky spirits or serious science?

Casting a swirling shadow, a glowing candle flame sways eerily before flaring up once more. As ghostly as it looks, this flame dance is not the result of any force – paranormal or otherwise – but rather the lack of one: gravity.

This short clip captures a scientific experiment that took place during the European Space Agency’s parabolic flight campaign in September, showing what happens to a candle flame when gravity is taken out of the equation.

Parabolic flights are a way of temporarily creating a microgravity environment without venturing so far from Earth. The ‘Zero-G’ plane follows a curved trajectory called a parabola – first climbing steeply to gain altitude, before reaching the top of the parabola and falling back down.

During the controlled fall, all passengers on board – both humans and experiments – experience about 22 seconds of microgravity, similar to the sensation you get at the top of a rollercoaster. And then it all repeats.

Each parabola takes about one minute to complete and is repeated 31 times in one flight, providing about ten minutes of microgravity in total.

This candle flame experiment was one of the many scientific experiments hosted on ESA’s 87^th parabolic flight campaign that took place this September in Bordeaux, France.

During 93 parabolas spread over three flights, candles made from different waxes and with various wicks were lit, placed in a sealed box, and closely observed by the experiment operators.

Three types of cameras captured what happened inside the box – a conventional visual camera recorded the flame, a set of infrared cameras unveiled the heat radiating from it, and a shadowgraphy camera tracked changes to the surrounding air. This animation snippet reveals what happened to the flame during the few seconds of microgravity, showing the flame on the right and its shadow on the left.

Author of the experiment, Jack van Loon of ESA’s Life Support and Physical Sciences Laboratory, explains: “We have all seen a candle burning at some point in our lives, but few of us stop to think about how it works.

“Many forces are involved in something seemingly so simple. Convection forces are creating an air circulation pattern which gives the flame its characteristic droplet shape; capillary forces are pulling the molten wax up the candle’s wick, competing with the force of gravity.

“We wanted to create a model where gravity plays a role, and to which people can relate. A candle is a perfect example.”

“It all started in 2017, when ESA’s Large Diameter Centrifuge (LDC) turned ten years old,” Jack recounts. “Everybody knows that when there’s a birthday, there are candles. So, to celebrate the LDC’s first decade in operation, we decided to set up a little experiment – lighting birthday candles in the centrifuge’s gondolas, and spinning them at 20 g, until the hypergravity force extinguished them.”

“Now, the idea is to explore what happens to a candle flame across the entire gravity range that we are able to access or simulate at ESA – from microgravity to hypergravity.”