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One of the most thrilling surprises for a skywatcher is a bright fireball. Aside from the unexpected shock, there's the visual variety of colors, flares, a trail, and perhaps even sounds.
Sometimes the fireball is manmade, a falling satellite or on occasion a deliberately descending manned spacecraft. Their special features include a flatter flight path and a wider variety of materials to burn (and occasionally explode) spectacularly.
These artificial fireballs may be a treat when viewed from the ground. But the view from inside the manmade meteor is even better, and may provide even more unique testimony about details of the atmospheric entry heating process that all objects--natural and artificial--endure.
These close-up views include bizarre colors, shapes, and motions of the flames. And they provide some intriguing new mysteries, too.
Several hundred people--astronauts and cosmonauts--have ridden descending spacecraft through the flames of atmospheric entry. Mostly this happened over the sunlit side of the Earth, since the spaceships wanted to land in daylight. In those cases, the fiery outside patterns were subdued, and often the crew had other things to concentrate on, or they weren't near convenient windows.
But sometimes they glance outside. Based on interviews with John Glenn, National Geographic writer Robert Voas described what America's first man in orbit saw during his Mercury-6 entry in 1962: "All around him glows the brilliant orange color. Behind, visible through the center of the window, is a bright yellow circle. He sees that it is the long trail of glowing ablation material from the heat shield, stretching out behind him and flowing together." Glenn's own comments over the radio were pithier: "This is Friendship 7," he had reported. "A real fireball outside!"
Later, Gemini and Apollo astronauts viewed and sometimes filmed the fiery trail behind them, since these spacecraft had small backwards-facing windows (Russian manned space vehicles do not have windows in these positions). But none of the reports from these small capsules prepared engineers for what astronauts were able to see out of the bigger windows of the much bigger space shuttle vehicle.
It's important to realize that the shuttle enters the atmosphere at a high "angle of attack"--in earthside terms, it's making a big "belly flop" with its nose very high in relation to its forward motion. The compression shock wave covers the entire underside of the spacecraft, but is hottest at the leading edges of the wing and under the nose. The fireball trail streams backwards in about the same direction the shuttle's tail is facing.
The shuttle's windows face in many directions. Forward, the six windows allow the pilots more than 240 degrees of view. Overhead, two windows at the back of the cabin allow observation of targets in space which the shuttle is docking to. Aft-facing windows show the payload bay--but of course, during landing the payload bay doors are closed so no external scenes are visible through them. On the middeck, there is a large window in the main hatch on the spaceship's left side.
The first shuttle entry partially in darkness was STS-5 in November 1982. Gazing forward, mission commander Vance Brand later described how "the sky turned rust-colored." Copilot Bob Overmyer compared the view to "leaning into a blast furnace." Watching the lights from the middeck, Joe Allen recalled how "it's like being inside a neon sign--we could see this rose-colored glow for a long time."
The first shuttle night landing was STS-8 in September 1983, so they had darkness the entire length of their atmospheric descent. All five astronauts got good views of the flames surrounding their spaceship. Commander Dick Truly described how the color started out as "salmon-colored," then shifted to "white hot."
But astronaut Dale Gardner--sitting in the aft right seat that had been installed after the flight of STS-5--noticed something entirely different. He recalled being puzzled by light flashes from above and behind the Orbiter, coming in through the overhead windows. "I leaned forward and craned my neck around," he recalled, "but I never could get in the right position to see anything." Instead, Gardner held a 16-mm camera up, pointed it "in the blind" towards the light, and triggered off several seconds of exposures.
The film captured a bizarre glowing structure, just like the inside of Joe Allen's "neon tube" extending backwards (downstream) from the Orbiter. On a later flight, as two crewmen deliberately stayed out of their seats to watch the apparition, it reminded them of a glowing Easter Island statue. "Should we photograph it," one wondered out loud, "or bow down to it?"
The "light show" clearly progressed through stages. At the end of his first space flight, aboard STS-11 in February 1984, "Hoot" Gibson kept careful track of the show. The color surrounding the Orbiter changed from dull pink to a bright orange pink, which then became supplemented with a bright white flickering from behind. The period ranged from one to three seconds. Looking backwards from his seat, through the overhead window, Gibson recalled seeing "two sheets of flame interacting, moving back and forth from one side to the other."
Describing a video sequence out the overhead window during the STS-94 entry in July 1997, Don Thomas reported: "It starts off just as bright flashes like a giant flame." After about five minutes, the "Easter Island statue" appears: "It changes into this little inverted mushroom structure you can see glowing there," he explained. Finally the trail widens into a bright yellow-white "structure," then slowly fades.
What the astronauts were probably seeing was the unstable transition between laminar and turbulent regimes, as the airflow flip-flopped back and forth at a certain point during the descent. It was a totally unexpected visual phenomenon.
From an entirely different perspective, ground viewers also saw the shuttle flames first flickering, then growing steady, and finally dying out--but over a track thousands of kilometers long, so no single observer could see the entire sequence as the crew could. For example, the STS-72 entry (January 20, 1996) was entirely across the dark side of Earth, and clear weather allowed alerted observers to watch the entire length of the glowing portion.
From Maui in the Hawaiian Islands (with the shuttle near orbital velocity at an altitude of116 km), observer Ken Harris reported: "The glow was barely visible, but I could see bright flashes of light." From Tucson, Arizona (the shuttle was at mach 22 at 70 km), Alan Varner wrote: "There was an orange spot that came out of the west with a very bright trail streaming behind it. The light from the shuttle was bright white in the center and deep red at the edges." Added observer Carl Holmberg: "The glow of the trail was florescent white, while the fireball was orange."
South of Dallas, Texas (with the shuttle at mach 16 at 60 km altitude), Jerry Matulka had loaded three generations of his family into a van for a midnight road trip to see the shuttle's fireball, and none of them was disappointed. "The trail stayed illuminated for some time," he reported, adding he thought its color was orange. "It was like a huge neon tube from horizon to horizon." Other observers also noted that the trail ("white with a slight orange hue" or "maybe a tint of yellow") tended to broaden (to half a degree, according to one) and then slowly fade.
By the time it passed Bay St. Louis on Mississippi's Gulf Coast (at Mach 11 at 52 km), the trail had shortened to about 60 degrees, but the fireball was still leaving an "incredibly bright orange-yellow streak," according to a report from observer Terry Jones. Further, "the pulsing of the light at the heat shield was clearly visible." From Pensacola, Florida (mach 9 at 48 km), Ted Kirchharr saw the shuttle as "a blinking star" ("there was a bit of yellow tint in the light"), while the trail "was only visible through binoculars." By this point, the fireball phase was clearly drawing to a close.
This yellow-orange light--I've seen it myself and described it as a "golden spark"--is a common description of the trail from all the way back to John Glenn's first flight. Why should it be that color?
In describing visual effects associated with spacecraft entries, observational astronomer Paul Maley concluded that "the best candidate" was the combination of nitrous oxide (NO) formed by the shock wave and naturally-occurring ozone (O3), to form stable nitric oxide (NO2). In the laboratory, that reaction "produces a yellow color, although initially it appears to be orange." That's in small amounts--but on a shuttle entry, up to six tons of nitrous oxide are produced along its entry path.
"As the reentry wake grows with time," Maley wrote, "it moves outward into ozone already present in the air. The wake forms a cylinder which spreads laterally from the flight path." This is exactly what returning spacefarers have noticed since the Mercury days.
Just why the trailing "neon tube" should thicken into the "mushroom cap" (or "Easter Island statue effect") reported by shuttle astronauts is still not clear. It remains a puzzle for the "fluid dynamics" experts.
There's one more mysterious physical process involved in high-speed atmospheric entry, and space vehicles may have accidentally helped solve one of astronomy's most baffling mysteries. This is the puzzle of "electrophonic meteor sounds."
During atmospheric entry, the plunging object literally smashes apart the gas atoms it runs into. Electrons are torn loose, creating super-hot ions. This creates the "plasma sheath" which blocks radio transmissions due to its electrical conductivity. It also apparently does something far, far stranger.
For at least two hundred years, astronomers have documented eyewitness reports of bright meteor falls in which people insist they heard noises from the fireball in real time. Even though the object may have been a hundred kilometers away or more, many people described instantaneous "hissing" or "whooshing" sounds.
Most scientists dismissed these accounts as imaginary delusions conjured up by perceptual parallels with fireworks. But others weren't so quick to disregard them, even if there was no known physical explanation. Some accounts described witnesses first hearing the sounds in a house, then coming outside to see the fireball. Since this perception, if real, had to be caused by something moving at the speed of light, it was dubbed "electrophonic sound"--but without any true understanding of the physical basis for it.
Then, in the mid-1980s, as space shuttles began landing in Florida after a fiery entry across the southern U.S., thousands of people in Texas deliberately went outside to watch the scheduled overflights, especially when it happened in darkness. They thrilled to the sight of the manmade meteor and the knowledge there were humans inside the fireball.
And many of them heard the same instantaneous hissing noise that had baffled meteor experts for centuries. It was variously described like "a skier moving down a slope," or a "motorboat cutting through water." The sound, furthermore, did not come from the object in the sky, it seemed to emanate from all around the observer.
Many of these observers "knew" it was impossible to hear sounds from space, and some thought they were crazy. One man called the NASA public affairs office in Houston to describe how the shuttle overflight "nearly shook the leaves off the trees," and the press official who talked to him--I was standing nearby and accidentally overheard the conversation--thanked him for the description, hung up, and then laughed out loud to his colleagues about the "idiotic redneck delusion." Since the reported phenomenon was known to be impossible, many assumed it could not have happened.
But the phenomenon was real, and the shuttle entries unexpectedly helped find the explanation at last.
It turned out that the bright natural meteor fireballs--and entering space shuttles too--can produce random bursts of radio noise in the 5 to 8 kilohertz radio spectrum. These signals move outwards at the speed of light, and are sometimes powerful enough to vibrate physical objects near ground observers, which creates the hitherto mysterious sounds. The physical origin of the radio noise is still obscure--it probably is connected with distortions to Earth's magnetic field and subsequent collapse back to normal conditions--but the radio noise has now been recorded, and its auditory effect on objects has been reproduced in laboratories. Leaves and pine needles are especially noisy, along with dry hair, eyeglasses, metal and metal foil, and other materials often near unsuspecting witnesses. That's where the sound was really coming from.
So one more aspect of meteors that most people assumed was true--that the reported sounds were delusions--was actually false. And artificial meteors helped explain the mystery.
And as with fireballs throughout history, such mysteries add to the thrill of those who watch them from the ground, and to the thrill of those who actually ride these manmade meteors through the sky.
is a 22-year veteran of Mission Control in Houston, and a lifelong sleuth
of Soviet space secrets. He is now a full-time writer and consultant.
His home page is www.jamesoberg.com.
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