Plume Power

It was a bridge worth returning to, and it yielded something new. Photographer James Vernacotola had stood on the Palm Valley Bridge, which crosses the Intracoastal Waterway near Jacksonville, Florida, to capture night, dawn, and dusk launches from NASA’s Kennedy Space Center, more than a hundred miles to the south. Most recently, he had come in the middle of the night on February 8, 2010, where he nailed an awe-inspiring time exposure of the space shuttle Endeavour rising from the launch pad 115 miles away (see “Sightings” Jun./Jul. 2010). That 4:14 a.m. launch was the last scheduled night launch of the shuttle program. Vernacotola went back a couple months later for the 6:21 a.m. launch of Discovery on STS-131. With the sky brightening in the east, and no moon or stars in the frame, it wouldn’t quite rival the STS-130 shot.

Think again. Six and a half minutes into the ascent, with the solid rocket boosters having long since fallen away but with its three main engines still cooking, the orbiter was suddenly high enough in the northeastern sky to be in sunlight. This revealed a gossamer plume of water vapor expelled by the burning of the liquid hydrogen and liquid oxygen propellants. The exhaust instantly condensed to ice crystals in the frigid vacuum of near-space almost a hundred miles high. Vernacotola swiveled his camera on its tripod and made this 15-second exposure (see a larger version here). “That’s the first time I’d ever seen anything like it,” he says. “When we first saw it, we were wondering if something was wrong. Now I suppose the vapor trail is always there, but maybe I’ve never seen it because it was never backlit by the sun like that.”

Four-time shuttle flier Tom Jones confirms these suspicions. He says that the plume gets lost in the fiery booster exhaust during the first two minutes, and would be too diffuse to be seen in daylight or darkness anyway. But the main engines continue to produce it until they shut down about eight and a half minutes after liftoff. Jones has seen the plume from inside the shuttle during ascent. “The steam from the nozzles, at high vacuum [occurring at high altitude] begins to creep up the side of the stack,” he says. “With no ambient pressure, the plume expands to engulf the [external fuel] tank and orbiter sides, so much so that the crew can see the flickering of the steam plume reflecting the luminosity at the engine nozzle through the top windows in the flight deck. The pulsing plume gets your attention—is it a fire?!!! Is everything OK back there? But it’s normal. Just unexpected to see your exhaust racing along with you.”