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.
