- SpaceX launched a Dragon ship to the International Space Station just before dawn on Friday.
- The launch of the Falcon 9 rocket created a miles-high pillar of exhaust stretching toward space.
- One witness called the smoke trail a "Dragon's Tail."
- If the timing is right, rocket launch plumes can bounce high-altitude sunlight toward dark, pre-dawn locations.
Most rocket launches happen in early daylight, when the weather most often cooperates — and nervous engineers can keep a clear eye on their precious space vehicles.
But for those lucky enough to witness a dark, pre-dawn launch, an incredible sight sometimes awaits. Such was the case Friday morning, when SpaceX fired a Falcon 9 rocket from a launch pad in Cape Canaveral, Florida, and toward the International Space Station.
The SpaceX mission, called CRS-15 — NASA's designation for Commercial Resupply Service — sent a Dragon spaceship with nearly 6,000 lbs of supplies (including a floating robotic head) to the space station's crew. The launch of SpaceX's 23-story rocket was stunning, but a phenomenon that the mission left in its wake was even more spectacular.
Taylor Harris, a YouTube artist invited by NASA to watch the launch from a few miles away, described the launch plume as a "Dragon Tail."
"I'm glad I got the opportunity to see the Dragon's Tail in person," Harris tweeted about 45 minutes after the launch at 5:42 a.m. EDT.
How a 'Dragon's Tail' works
When rockets launch, they leave behind a trail of hot exhaust, also called a plume. The appearance of the plume depends on the fuel, in SpaceX's case it's RP-1 — a high-grade kerosene — burned by liquid oxygen.
Falcon 9 rockets can send payloads more than 250 miles above Earth, beyond the edge of space and where the space station orbits our planet.
At first, a rocket leaves behind a relatively thin plume. But as it climbs higher and higher toward space, the air pressure gets lower and lower. About a dozen miles up, the air pressure is less than 1% of that at Earth's surface, causing hot launch plumes to dramatically expand.
If atmospheric conditions are right, these billowing plumes can make water condense out of the air, which then freezes into tiny ice crystals. And if the timing is right, these crystals can reflect the sun's light from far over the horizon like a mirror, beaming it down to a dark, pre-dawn location (at least until high-altitude winds blow around the plume and ice).
The phenomenon is known to scientists as noctilucent or "night-shining" clouds, which form naturally and most frequently over the Arctic and Antarctic.