China's Shenzhou 16 astronauts conducted an eye-raising experiment in space involving open flames aboard the Tiangong space station.
Astronauts Gui Haichao and Zhu Yangzhu lit a candle during a live lecture broadcast from China's Tiangong space station on Sept. 21 to demonstrate how flames burn in microgravity. Strikingly, the flames appear nearly spherical, rather than the teardrop-shaped flames we're familiar with back on Earth.
Lit candles on Earth produce flames shaped through buoyancy-driven convection, with hot air rising and cold air descending. That combustion convection current is weak in the microgravity environment of low Earth orbit, however. This means flames diffuse in all directions, resulting in spherical fireballs.
The livestreamed lecture was the fourth so-called "Tiangong classroom" hosted on China's space station. The astronauts interacted with students in five classrooms across China, demonstrating a number of microgravity phenomena. As with previous classrooms, the astronauts demonstrated that many physical processes behave differently than they do on Earth.
However the candle experiment — in which Gui strikes a match to produce an open flame to light the candle — would likely be met by surprise by International Space Station participants, who have strict rules regarding flammable materials and open flames.
Combustion in microgravity has been the subject of numerous experiments on the ISS, but usually using a specially-designed combustion integrated rack, keeping fire isolated and contained.
Tiangong also has its Combustion Experiment Rack (CER) for serious research in this area.