On Friday 15th February 2013, locals in the Russian city of Chelyabinsk were astonished to see a blinding light streak across the sky in the early morning glow, with a brightness rivaling that of the sun itself. Soon after, amid noises of rolling thunder, the ground shook, buildings swayed and windows shattered. It turned out later that some 1200 people had been injured, mostly by falling debris and flying glass from damaged buildings.
Reconstructing this extraordinary event, scientists concluded that the event was generated by a meteoroid exploding above the ground, producing a giant shock wave that blasted downwards and outwards and generating energy similar to that of a small nuclear explosion.
Planet Earth is enshrouded by a protective blanket - the atmosphere – that enables life as we know it, in all its forms, to exist. But of equal importance it provides an invisible shield that protects us from the fusillade of rocks called meteoroids that constantly bombard us from outer space.
When a meteoroid enters the atmosphere, travelling at speeds normally in excess of 50,000 km per hour, it begins to burn up because of friction with the atmospheric gasses. This produces a bright streak of light across the sky that is normally only visible at night. If you are to go outside on a clear night and observe the sky for a period of half an hour or more you will usually see one or more of these luminous, transient streaks that usually only last for less than a second. This streak of light is known as a meteor.
A meteor in the night sky
(Image form Wikipedia Commons)
In most cases the chunk of rock will burn up or vaporize before reaching the ground, and it is this protective property of the atmosphere that has played a big part in the survival of life on the planet. Without it we would look like the Moon that has no such protection and is pockmarked with impacts from many thousands of meteoroids.
In some cases fragments will reach the surface of the Earth as a shower of rocks called meteorites and these are always interesting to scientists as they are ancient objects from the far flung reaches of outer space and often have a different structure and composition to rocks we find on Earth.
The surface of the Moon - scarred by countless meteoroid impacts.
(Image from Wikipedia Commons)
On rare occasions much larger meteoroids will enter the atmosphere, with a size big enough to prevent complete vaporization and these will reach the lower levels of the atmosphere partially intact and still travelling at tremendous speed. One of two things usually follows.
The first is what probably happened over Chelyabinsk – a massive mid air explosion that generates a blindingly bright flash, together with a shock wave and sound of rolling thunder. Smaller rock fragments will then shower across the area as meteorites.
Russia was also the scene of another such event in 1908 when a monstrous air burst explosion over the Tunguska area produced a massive shock wave that flattened huge tracts of pine forest in the area, affecting a zone of around 2000 square kilometers. This was a much more powerful event that Chelyabinsk incident.
But what can also happen is when the rock remains largely intact all the way to the surface, eventually striking the ground with colossal force and producing a large impact crater. It has long been believed that an event such as this some 65 million years ago produced a cataclysmic explosion of such a scale that the atmosphere around the world was affected, with the extinction of the dinosaurs a consequence.
As a testament to this type of event, large impact craters can be found today, including “Meteor Crater” near Winslow, Arizona that was created about 50,000 years ago when a large meteorite slammed into the area. The crater is about 1.2 km across and 170 m deep.
Above: Meteor Crater, Arizona. (Image from Wikipedia Commons; Click to enlarge)
In Australia, Wolfe Creek is a large crater located in the desert of Western Australia about 150 km to the south of Halls Creek. It is estimated that this crater was produced by a meteorite that had a mass of some 50,000 tonnes striking the surface around 300,000 years ago. The crater is about 900 metres across and 60 metres deep.
Above: The Wolfe Creek Crater, Western Australia
(Image from Wikipedia Commons; click to enlarge)
What would happen if such an object were to stroke a modern city – such as Sydney or Melbourne? Well, the short answer is - disaster. The actual impact crater would take out a suburb, and the associated shock wave and impact vibrations would likely destroy much of the city. So should we be concerned?
Well the short answer to this is no. Events of such as Meteor Crater and Wolfe Creek probably only occur only about once every 50,000 years or so, and in the great majority of any future cases the actual impact point would be in non-urban area. You have more of a chance of being struck by lightning or kicked to death by a donkey that dying through meteorite impact.