Flame Extinguishment Experiment in space. Fuels burning in space behave very differently compared to flames on Earth because of the microgravity conditions on spacecraft in Earth orbit. This image was obtained as part of the Flame Extinguishment Experiment (FLEX) that examined the combustion of liquid fuel droplets on the International Space Station (ISS). Here, a 3-millimetre diameter droplet of heptane fuel (yellow) burning in microgravity is producing soot (green). The image was coloured to show the features highlighted by a bright, uniform backlight. This image is a composite of multiple video frames. The FLEX experiments took place in 2009.