Answer: Space fire. (When it's under control, at least). Members of the Burning and Suppression of Solids –II (BASS-II) investigation aboard the ISS study how things burn and extinguish in microgravity. This photographed was captured yesterday by astronaut Reid Wiseman, during one of the team's experiments.
Wiseman is doing an excellent job as the unofficial historian of ISS Expedition 40. Just look at this picture of the constellation of Orion rising above the curvature of the earth, which he captured earlier this week:
Who's the star of this photograph? We can't decide. Is it the International Space Station, zipping around the planet at 17,500 miles per hour? Is it the Earth's horizon, beautifully lit and clearly visible in its enveloping embrace of our planet? Or is it Mintaka, Alnilam and Alnitak, the stars, from left to right, that comprise Orion's belt, shining blue and bright through the haze of that same atmosphere?