A Croatian astronomer thinks he has cracked a meteor mystery that has baffled the scientific world for over two centuries.

He has managed to record what he believes are the elusive, instantaneous sounds made by shooting stars as they crash through the Earth's upper atmosphere.

The display of shooting stars was amazing, says Dejan Vinkovic

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!

These strange sounds are very rare

He made the recordings during last November's Leonid meteor storm and presented his findings to a conference in San Francisco this week.

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.

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.

Eyewitness accounts

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.

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.

The team endured freezing temperatures while they waited for the Leonids

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.

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 phenomenon 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.

The display of shooting stars was amazing, says Dejan Vinkovic

In the lab Keay showed that such radio frequencies could produce an audible sound at ground level by interacting with anything from a tree to someone's hairstyle or pair of spectacles! He called his phenomena electrophonic sound.

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.

Freezing temperatures

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.

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."

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.

"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."

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.

Remember, it is the thud in the middle of the clicks that is the electrophonic sound

"The average person may expect to hear only one electrophonic fireball in a lifetime, as long as they spend most of their night hours outside."