Wednesday, April 21, 1999 Published at 12:39 GMT 13:39 UK
Sound of shooting stars
The sounds were recorded in Mongolia
From BBC Science reporter Dr Chris Riley in San Francisco
A Croatian astronomer thinks he has cracked a meteor mystery that has baffled the scientific world for over two centuries.
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.
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.
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.
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.
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.
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.
Alternatively you could click on our RealAudio recording. The sound to listen out for is a thud heard in between a couple of sharp clicks.