Lu, who was a research astrophysicist before becoming an astronaut in 1994, estimates that he spent 100 hours watching the northern and southern lights during half a year in space. The auroral light show, which takes place well below the station's 380km altitude, shimmers and pulses depending on natural variations in incoming solar particles trapped by the Earth's magnetic field.

On three occasions - July 11, September 24 and October 12 - Lu saw something markedly different: flashes as bright as the brightest stars, which lasted only a second then blinked off again.

In one instance, he called crew-mate Yuri Malenchenko over to the window to see the bursts. Lu says they appeared very different from the random but harmless retinal flashes that many astronauts experience when heavy cosmic rays hit their eyeballs...