Witness Chinese astronauts conducting a daring open-flame experiment on the Tiangong space station, a risky maneuver forbidden aboard the ISS

During a live lecture broadcast from China’s Tiangong space station on September 21, astronauts Gui Haichao and Zhu Yangzhu demonstrated the unique behavior of flames in microgravity. To illustrate this, they ignited a candle, resulting in the formation of nearly spherical flames, a stark contrast to the familiar teardrop-shaped flames seen on Earth.

On our planet, candle flames take their shape due to buoyancy-driven convection, where hot air rises while cold air descends. However, in the microgravity environment of low Earth orbit, this combustion convection current is considerably weaker. As a consequence, flames disperse in all directions, creating spherical fireballs.

This live-streamed lecture marked the fourth installment of the “Tiangong classroom” series, where astronauts engaged with students in five classrooms across China. The aim was to showcase various microgravity phenomena, highlighting how numerous physical processes differ from their behavior on Earth.

Notably, the candle experiment, where Gui ignited a match to produce an open flame for lighting the candle, would likely raise eyebrows among International Space Station personnel. This is because the ISS has strict regulations concerning flammable materials and open flames, partly stemming from a significant fire incident on the Russian space station Mir in 1997.

While combustion in microgravity has been the subject of numerous experiments on the ISS, these typically employ specially-designed combustion integrated racks to isolate and contain the fire. Similarly, the Tiangong space station has its Combustion Experiment Rack (CER) dedicated to rigorous research in this area.