12,500-tonne wake-up call
People claiming to have seen a dazzling streak of light and stones falling from the sky were once dismissed as fantasists.
And no wonder – even now, the images of the Chelyabinsk meteor flying across the skies of Russia in February defy belief.
The ubiquity of smartphones and security cameras has now given even the most sceptical compelling evidence that our planet faces a cosmic threat.
But the images have proved more than merely spectacular. Videos posted on YouTube have provided scientists with unprecedented insights into the object that shot through the Earth’s atmosphere in the early hours of February 15.
By scouring the images from hundreds of cameras, researchers have been able to extract amazingly detailed information about the speed, size and identity of the object. The results have been published in the leading journals Nature and Science.
For example, video taken by dashboard cameras – commonly used by Russians to counter car crime – revealed the speed of the incoming fireball to be almost 60 times the speed of sound.
Examination of security camera footage of glass being blown out of windows allowed researchers to estimate the energy in the hypersonic shock-wave created by the fireball.
Taken together, this revealed that the chunk of cosmic debris was the size of a six-storey office block and started out with a mass of about 12,500 tonnes.
Torn apart by aerodynamic stresses, the interplanetary rock exploded with ferocious energy. Analysis of the dashboard camera videos of the searing light from the fireball pointed to a release of energy equivalent to about half a million tonnes of TNT, far more even than the atom bombs dropped on Japan.
The resulting fragments left a trail of debris for 90 kilometres on either side of the fireball’s trajectory, including one 650-kilogram chunk dumped into a lake and recovered last month.
So what was it that crashed into the Earth that day? Chemical analysis of the fragments has shown that the meteor was an “ordinary chondrite”, a fragment of one of the myriad asteroids that mostly orbit the sun between Mars and Jupiter.
Again, all those YouTube images allowed scientists to find out much more. By tracking the trajectory back into space, they have been able to reconstruct the meteor’s entire orbit around the sun.
This has turned out to be strikingly similar in size, shape and orientation to that of an asteroid discovered 14 years ago, code-named 1999NC43. Astronomers now believe that what we saw in February was the final act in a drama that began literally millions of years ago, when a fragment was smashed off the 2km-wide parent object, and then slowly but inexorably made its way towards an impact with our planet.
It is an impressive piece of detective work, and one that shows how valuable the observations of ordinary people can be.
Scientists have long been wary about taking the claims of the public seriously, however. The study of meteors in particular was held back for centuries by the snooty attitude of scholars towards the claims of their less-educated contemporaries.
That changed in 1803 when a French scientist, Jean-Baptiste Biot, was dispatched to the village of l’Aigle, near Alençon in northern France, to investigate stories that thousands of stones had fallen out of the skies in April that year.
A few months later, Biot informed his fellow scholars at the French Academy of Sciences that the claims were well-founded.
He based his assertion partly on the fact that the stones from the sky appeared to be quite unlike anything from the locality. Chemical analysis did reveal, on the other hand, a striking resemblance to stones at the centre of similar claims from elsewhere in France.
Perhaps the most compelling evidence, however, was Biot’s demonstration that the eyewitnesses could not all be dismissed as simple peasants. He showed that there was corroboration of their stories among members of the professions, including the clergy.
When combined with previous anecdotes from other times and place, the conclusion was ineluctable: stones really do fall from the heavens, and can land anywhere, any time.
Proof of this has sometimes only emerged very recently, however – the first meteorites were found in the UAE just eight years ago.
Scientists are hardly alone in being reluctant to accept the apparently outrageous. The phenomenon is well-known to social psychologists as “cognitive dissonance”, the tendency to reject ideas that do too much violence to our current worldview.
Cognitive dissonance has continued to afflict the study of meteor impacts. Over the years, scientists have found many impact craters scarring the Earth and Moon, plus whole families of asteroids on near collision-course with our planet. Even so, acceptance of the risk they pose to life on Earth has been slow in coming.
The discovery of evidence that a meteor impact pushed the dinosaurs into extinction 65 million years ago created a flurry of interest in the 1980s, as did the impact of a small comet, Shoemaker-Levy 9, with Jupiter in July 1994. Public concern did not last long.
Among scientists, however, the Chelyabinsk event has rung alarm bells. Most obviously, it has highlighted the inadequacies of current methods for spotting these objects before they strike.
The chances of a direct hit on a major conurbation are low, but the devastating consequences are surely enough to justify a more determined global effort.
More worrying is the fact that we have now had two warning shots across our cosmic bows almost within living memory.
In June 1908, a huge meteor exploded over the Tunguska River in Siberia with a blast far more powerful even than the Chelyabinsk event.
Astronomers thought such events were much less common; now they think they may have radically underestimated their frequency.
So far we’ve been lucky, but unless we take the lessons of Chelyabinsk more seriously, we may yet share the dismal fate of the dinosaurs.
Robert Matthews is visiting reader in science at Aston University, Birmingham, England