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Discovery's ring of fire

When the space shuttle Discovery soared through the night sky on Saturday night, a camera mounted on the craft's external fuel tank beamed back a light show that had never been seen before - at least from this particular perspective. Arcs of multicolored lights shimmered around the orbiter itself, looking for all the world like an aurora. You can watch the display by clicking on this video link.

But what caused the spectacle? NBC News space analyst James Oberg went to some effort to explain the physics behind the show:

"It was pitch black outside - even the moon had not risen. The main engines burn liquid hydrogen and liquid oxygen, and that flame is nearly invisible (remember chem lab, and earlier shuttle launches). The three main engines glow red-hot and bright, but the flame itself looks transparent.

NASA
The Discovery orbiter is bathed in greenish light

as it separates from its external fuel tank. Click

on the image to watch a video.

"But - wow!!! - the shuttle, as it approached orbital speed and engine shutdown, was bathed in greenish phantom light. And back behind the tail of the Discovery was a sharp-edged pulsating ring of fire that had never been seen from that angle before (there are no rear-view mirrors in the shuttle cockpit).

"Then, as the engines shut down and Discovery fired its thrusters to pull away from 'the world's largest disposable cold drink can' (as the external tank is jokingly referred to), the tank and the shuttle's underbelly were brightly illuminated by flashes of white light.

"Those flashes were the easiest to figure out - the shuttle's thrusters create bursts of white light when they fire, expelling glowing clouds of 'combustion products' from the chemical fuels they use. That, too, has been seen many times before.

"The greenish phantom light is, according to some physics types I've talked with, the chemical glow of molecules being ripped apart by high speed impact, and then recombining. Specifically, it's probably the oxygen molecules (oxygen gas, like nitrogen and hydrogen gas, exists in the air as paired molecules of two atoms each).

"They get torn apart as they are hit by the speeding shuttle, then quickly rejoin, giving off a distinctive flash of light at a specific wavelength (color). They do the same when they are run into by onrushing clouds of gas ejected from the shuttle's main engines - and that's probably the cause of the 'ring of fire.'

"During these moments, the shuttle is rushing straight-and-level about 55 miles up, not quite in 'space,' and still within the boundary of a small but 'sensible' (that is, 'feelable') atmosphere. This is a lot lower than normal satellites orbit, so the visual effect is not shared on other space vehicles. (The shuttle is given enough speed to cast farther out into space, where it fires its own onboard engine an hour later to stabilize at a higher altitude, normally about 150 miles.)

"Its speed is nearing 18,000 mph, and since the engine flame is expelling gas at about 8,000 mph, even with that backward speed those particles are still moving FORWARD at about 10,000 mph. As they depart from the engines into a narrow-cone-shaped spray pattern, the forward edge of that cone is ramming innocent oxygen molecules at this speed - and creating chemical luminesence around its circumference. Viewed from near the engines (that is, on the shuttle), that glow forms a ring of fire.

"The 'shock wave' illumination has been seen before - I've seen it myself - during night launches. Even if the burning rocket engine is only dimly visible, it appears at the tip of a glowing 'V' whose sides expand as it rises into thinner and thinner air. The glow is not of exhaust super-heated by the fires in the rocket chamber - it occurs at the outer edges of the plume, where it is running into thin air.

"The same 'impact illumination' can also be seen along the edges of the shuttle, in the camera views, and along the spray of gas ejected from the shuttle's small thrusters when it pushes itself away from the tank.

"It is this illumination - not the engine flame - that is seen during night shuttle flights up the East Coast. 

"It was also this illumination that panicked people in Russia and China over the decades when those countries secretly launched big rockets from undisclosed locations, sometimes at night - creating glowing clouds in the sky that frightened witnesses concluded were UFOs.

"That's another story - except that the physics is the same.

"For the very first time, we've had the opportunity to see this visual effect from aboard the space shuttle, during this night launch - the first one to carry external TV cameras looking backwards. That was the novelty of this launch, a mission that I didn't expect to be visually exciting - well, I'll eat those words.

"The same 'impact illumination' surrounds spaceships plunging back into the atmosphere, and this has been observed and photographed by astronauts for decades. On Gemini and Apollo flights, the crew was facing backwards because they had pointed their heat shields into the air blast (a good plan), and could see the trail stretching behind them. On shuttle missions, the winged orbiter enters belly-first, and the crew can see the glow dancing around their nose, out their front windows, and also look out the overhead windows in the back of the cabin to see the glow extending back along the path they have just followed.

"The glow behind a shuttle forms a sharp-edged cylinder with pulsating sides - a ghostly shape that has reminded some astronaut watchers of the Easter Island statues. 'Should we take photographs of it, or bow down and worship it?' one whispered to a companion at the windows, only half in jest.

"Seeing the eerie flickering glows of 'impact illumination' that surrounded the 'Discovery' during Saturday night's launch can help us appreciate the awe in that personal impression. If we had expected the flight to occur 'in the dark,' boy, have the results been illuminating!"

Later on, Oberg sent along an additional note about the shape of the ring of fire, plus a graphic to explain why it looked the way it did:

"I think the ring of fire is the result of our perspective from observing from on board the shuttle - and the actual 3-D shape of the glowing region is a hollow cone with the shuttle at its apex. The cone, widening with distance from the shuttle, is the region of exhaust gases from the main engines, and the boundary of the cone is where the interaction with the thin upper atmosphere is creating this glow. It is at some angular distance from the centerline, perhaps, because the shuttle's own physical shockwave is protecting the very tip of this cone from getting the full blast from the ambient air at 55 miles altitude. From the side - for observers on Earth, the visible light is strongest where the lines of sight pass through the greatest density of glowing plasma - along the top and bottom edges as viewed from a distance. Hence these observers see the same 3-D shape in side profile, and it looks like a letter 'V.'"

NASA / James Oberg
This graphic explains the features seen in the "rocket-cam" view of the shuttle Discovery's separation. The "lit" view is displayed in the small inset image.

Update for 7:57 p.m. Dec. 11: Discovery's ring of fire isn't the only curious twist to this night launch. On the way up to orbit, the shuttle shed its solid rocket boosters, as usual, and the two rockets parachuted down into the Atlantic Ocean for recovery. But once the rocket casings and their parachutes hit the water, they began drifting toward each other - and actually came close to colliding.

As it is, the rockets' parachute lines are twisted together underwater, posing a devil of a problem for the recovery team. For a picture of the bobbing rockets and further details from this tangled tale, check out this posting to the CollectSpace bulletin board.