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(meteorobs) Clues to Mysterious Meteor Sounds Sought In Iridium Satellite Suicides



http://www.space.com/science/iridium_sound_000328.html

Clues to Mysterious Meteor Sounds Sought In Iridium Satellite Suicides

By Robery Roy Britt and Mary Motta
28 March 2000

While playing stickball one evening in the early 1970s in Dover, Delaware,
Chuck Bonner and his playmates heard a loud, abnormal whistling noise
accompanied by a buzz. The teenagers ran around the house, expecting to find
an airplane plunging earthward.

Instead they spotted a green fireball as large as a full moon streaking
across the sky.

Ever since, Bonner (now a software designer who runs a website called Lunar
Colony) has wondered how he could have heard the apparent meteor before he
saw it, since sound travels much slower than light.

Scientists have been wondering the same thing -- for a lot longer. And now
they have the opportunity to study this phenomenon in a giant lab experiment
in the sky.

At Bonner's suggestion, researchers will keep their eyes and ears open to
monitor what happens when 66 Volkswagen-sized Iridium spacecraft will be
deorbited into a fiery suicide dive to Earth. Scientists hope the artificial
meteors will simulate real space rocks burning up in the atmosphere,
yielding clues to what causes this mysterious cacophony of sounds.

Mystery meteor sound

There are only a handful of "earwitness" accounts of these melodious
symphonies in the sky. One account from China in 817 AD tells of a noise
"like a flock of cranes in flight."

A common and curious thread in these tales is that the sound seems to travel
at the speed of light -- even faster in some cases, arriving before there
was anything to see.

The accounts have confounded scientists for centuries. Assuming a meteor is
many miles away as it burns up -- most begin vaporizing around at 60 miles
(97 kilometers) high -- light would arrive several seconds before sound.

In the early 1700s Sir Edmund Halley, who has a comet named after him,
attributed the reports to the observer's imagination. More recently, one
scientist spent decades developing an obscure theory suggesting that these
electrophonic sounds, as they are now called, are the results of magnetic
spaghetti.

Magnetic spaghetti

Australian researcher Colin Keay uses the term to describe a theory he
developed in the early 1980s.

As the theory goes, when a space rock plunges earthward, friction caused by
the atmosphere creates a trail of electrically charged particles, or plasma,
in which Earth's invisible but potent magnetic field lines become trapped,
tangled and twisted like strings of cooked spaghetti.

This magnetic spaghetti is thought to generate very low frequency radio
waves, says Keay, a researcher at the University of Newcastle who, though
not famous like Halley, does have an asteroid named after him.

The waves are thought to travel at the speed of light and are converted into
sound when they interact near the ground with what are called "dielectric
media" or "transducers," which can be massive ordinary objects or electrical
activity in the lower atmosphere.

Sound crazy?

"It's coming out of the realm of myth and into the realm of possibility,"
said Donald Yeomans, a respected and typically moderate voice on matters of
meteors and asteroids. "But there are some serious doubters."

Yet evidence favoring Keay's idea is mounting, and Yeomans said he senses a
consensus building among serious scientists that electrophonic sounds from
fireballs are something that should be looked into.

That's where Dejan Vinkovic, head of the Global Electronic Fireball Survey
at the University of Kentucky, enters the arena. Vinkovic says there are
some 700 known reports of electrophonic fireballs. His survey has collected
five since the beginning of this year, and he's tracking down another 20
he's heard about.

Because the odds of witnessing the phenomenon are about as likely as hitting
it big in the lottery, Vinkovic is going to use Iridium's necklace of
satellites to simulate the experience.

Based on Chuck Bonner's suggestion via a scholarly newsletter known as
CCNet, Vinkovic will suggest to his colleagues around the world that they
point their video cameras and radio receivers at the falling satellites,
most of which Iridium plans to bring down over the Pacific.

Motorola, which operates the Iridium satellites, is hatching a plan with the
input of NASA to bring the giant birds back toward Earth sometime this year.

The satellites, primarily made up of graphite epoxy, fiberglass and
aluminum, will be yanked from orbit one by one.

The satellites' on-board propulsion system, which helped it place it in low
Earth orbit, will be used to bring it back down. By slowing its orbit, the
satellite is pulled closer to Earth. Once it gets to a couple thousand feet
above the surface, aerodynamic forces cause a drag, putting stress on the
spacecraft and causing it to break up and burn.

While much of the spacecraft will vaporize, there is a chance some pieces
will survive and fall to the planet.

"You can never be sure how a satellite is going to break up," said George
Levin, director of the Aeronautics and Space Engineering Board at the
National Research Council. "You can only make an educated guess because
these satellites were not built as reentry vehicles."

But Vinkovic points out the positive side of deorbiting Iridium's
satellites.

"One of the problems with meteors is that we don't know what exactly they
are," Vinkovic explained. "So, with the Iridium satellites we already know
the properties, and we can reduce the number of unknown parameters in our
modeling."

By monitoring reentries from around the world, researchers will be able to
study whether the local environment or ground configuration affects any
possible sounds, Vinkovic told SPACE.com.

How's that sound?

Chuck Bonner would love it if Vinkovic and the other scientists could solve
the mystery that has dogged him since the early 70s. Though he struggles to
remember his exact age when he saw and heard the apparent meteor, he is
clear about details of the streaking green object.

Bonner tells of ridges and depressions on the object, from which green dust
or smoke emanated.

"This dust remained as a glowing green trail across the sky that rapidly
faded to black or gray, then dissipated," Bonner recalled the other day.
"The sound continued long after the fireball was out of sight. I would guess
that from the time I first heard it until it faded to inaudibility was about
30 seconds."
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