Holy Flaming APUs!

This video of Endeavour‘s picture-perfect landing at 2:35 a.m. today offers a little surprise, even for some veteran shuttle watchers. Can you guess what we’re referring to?

If you guessed the flickering light at the base of the vertical tail, you’re spot on. And if you stuck with the video to the 1:20 mark, the infrared footage that begins there shows what appears to be a large, pulsing flame that would alarm any unsuspecting layman, shuttle engineer or astronaut.

Turns out it’s not a new phenomenon, and was initially observed on STS-51, the first night landing at the Kennedy Space Center, back in 1993. NASA also addressed it here after it occurred on STS-106, pointing out that the auxiliary power units at the back of the orbiter vent hydrogen, helium, and ammonia from three vents at the base of the tail.

We’ve contacted NASA for further info on why it ignites on some night landings and not most others. Check back here for further details.