- A Croatian astronomer thinks he has cracked
a meteor mystery that has baffled the scientific world for over two centuries.
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- He has managed to record what he believes
are the illusive, instantaneous sounds made by shooting stars as they crash
through the Earth's upper atmosphere.
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- The extraordinary thing about these noises
is that the observer on the ground hears them at the same time as the meteor
appears overhead. On the face of it, this would seem impossible because
it would require the noises to break the sound barrier!
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- The electrophonic sound is the low thud
in between two clicksHe made the recordings during last November's Leonid
meteor storm and presented his findings to a conference in San Francisco
this week.
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- Dejan Vinkovic, currently at the University
of Kentucky, conducted his enterprising experiment on an expedition to
Mongolia. He was joined by colleagues Slaven Garaj, Goran Zgrablic and
a number of others from the University of Zagreb.
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- They were perfectly placed to record
any sounds that might accompany the Leonids, the shower of shooting stars
that are seen when the Earth plunges through the dust debris left by Comet
Temple Tuttle as it circles the Sun.
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- Eyewitness accounts
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- Since Biblical times, stories have been
told of a low thunder-like noise instantly accompanying bright shooting
stars. But it was not until 1784 that proper research into the phenomena
began.
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- Sir Charles Blagdon, the then Secretary
of the Royal Society, collected eyewitness accounts of a bright fireball
that rushed across the UK. He noted surprisingly that all the witnesses
had heard the accompanying rumble at the same time, regardless of where
they were in the country.
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- Dejan Vinkovic: "It was complete
madness"Blagdon concluded that it could not be a proper acoustic sound
as it had travelled too fast. But he knew of no principle that could account
for what his witnesses had heard and declared that future observations
would no doubt explain it.
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- But it was to take almost two centuries
to explain the principle when a huge fireball streaked across Australia
promoting lots of reports of the "instant" sound phenomena again.
This time astronomer Colin Keay gathered the eyewitness accounts and proposed
that the twisting wake of the fireball might trap its magnetic field -
creating very long radio waves which would travel to the ground below at
the speed of light.
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- Generating sounds in this way is still
somewhat controversial. And most astronomers believed that these instant
sounds which accompanied bright meteors were just something that eyewitnesses
were imagining.
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- Freezing temperatures
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- The proof to the contrary has had to
wait until now. Away from civilisation, 20 kilometres south of Ulan Bator
and battling with temperatures below minus 30 degrees centigrade, Dejan
Vinkovic and his team set out to record these enigmatic X-sounds.
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- The Microphones were buried in sealed
wooden boxes"Our equipment was not sophisticated," he admits.
"To protect the microphones from wind noise, we just buried them in
sealed wooden boxes with a partition of aluminium or paper foil to act
as a kind of drum skin. Then we just covered them in snow and waited."
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- Their wait was rewarded after two frigid
nights when the fireballs started falling over the Mongolian steps. "We
were just chatting around midnight and suddenly a few happened," recalls
Vinkovic.
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- "Then they just came, brighter and
brighter. We stood there amazed. It was complete madness. I've never
seen fireballs so bright. You could see your shadow - night turned to
day."
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- Vikovic estimates they saw more than
30 fireballs in about five hours. But this was a freak night that probably
will not be repeated in our lifetimes. "And the accompanying sounds
are very rare," he stresses.
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