Why a Mysterious Explosion Hints at Future Disaster
A century ago we almost lost a major city. Why don’t we act to stop it happening again?
First the sky ripped in two, bursting into flames as it tore. Then the Earth shook, and a column of fire roared upwards from the forest. Moments later a shock wave echoed out, ripping a hundred million trees from the soil and twice encircling the globe. Seismographs in Armenia, Germany, Java and Washington picked up the tremors — a moment that shook the Earth, literally.
For days skies glowed around the world, a consequence of ice crystals lingering in the upper atmosphere. Londoners delighted in the strange events, taking photographs in the eerily lit nights. Despite all this, the explosion was ignored for almost twenty years, a mystery that few seemed to care about.
Perhaps the world can be forgiven for this lack of interest. The year was 1908, and the scene was Tunguska, a remote area of Siberia. Russia, scarcely an easy country to traverse at the best of times, was in the grip of revolution. The decade that followed was consumed by war, and few had time to care about mysterious happenings in Siberia.
But when they did get around to visiting, scientists found a scene of utter devastation. Over an area of more than two thousand square kilometres (eight hundred square miles), the forest had been felled. At the centre lay no crater, as the investigators had expected to find, but instead a region where trees stood upright. They were scorched, and branchless, as though swept by a terrible wind of fire.
Decades later a similar pattern of destruction would be seen in Hiroshima, moments after the deadly blast of an atom bomb. The characteristics of that explosion — a detonation in the air, a highly concentrated surge of energy — probably resemble the events in Tunguska.
Researchers concluded that the explosion likely happened in the atmosphere, a few miles above the surface. The initial force of it would have been directed vertically down, stripping and burning the trees, but leaving them standing. Those further away would have been swept by fast moving winds, destroying anything that stood in their path.
Calculations show it must have been of incredible power. Tens of millions of trees over an area twice the size of New York City were flattened. That is orders of magnitude worse than Hiroshima — perhaps a thousand times worse — placing the explosion on a par with the most devastating hydrogen bombs ever tested.
Once scientists saw the extent of the destruction, the obvious conclusion was a meteorite impact. But explorers struggled to find any evidence that one had actually struck the Earth. There was no big crater, no space debris, nothing at all save a huge area of devastated forest.
Some speculated that the meteorite had been destroyed in the explosion, or that it had been a comet, one vaporised by the heat. Or perhaps it was something even stranger — a black hole, or a fragment of antimatter smashing into the Siberian taiga.
Though the question has never quite been settled, astronomers now think something — probably a fragment of a comet or asteroid — struck the atmosphere at a shallow angle. After penetrating to within ten miles of the surface, it bounced off, rather like a stone skipping across a lake. In its wake it left a powerful explosion, rippling across the planet.
Whatever the cause, the event was concerning. If a comet could smash into the remote Siberian wilderness, another could hit somewhere more populated — over central Europe say, or along the East Coast of America. Worries became even more pronounced in 1994, when astronomers watched a comet crash into Jupiter. That impact left a string of angry dark spots across the giant planet, each larger than the Earth.
Statisticians were able to provide some comfort. Events the size of Tunguska happen rarely, perhaps once every thousand years or so. Smaller events are more common — but fortunately much of the Earth is still empty. Even if an asteroid or comet hits the planet, the chances that it falls over an urban area are low.
Low, but not zero. These are black swan events — rare, but incredibly disruptive. They can happen without warning — one could strike tonight, anywhere — and without mercy. Despite all our technology, all our weaponry, there is little we can do about the cold logic of orbital mechanics.
In the 1990s, with public anxiety skyrocketing, NASA set about trying to spot every threatening asteroid in the Solar System. Each was catalogued, its orbit carefully tracked, and its threat to civilization monitored. Thousands of asteroids have been found, but none, so far, are considered to be high risk.
There is, however, another danger, one we’ve done little so far to address. That is the risk from comets, icy visitors from deep space. Their very nature makes them more dangerous than asteroids — they travel faster, thus creating bigger explosions, and they come from much further away, making them extraordinarily hard to spot.
For most of their lives, comets hide in the far reaches of the Solar System. Out there, in the frozen darkness, they are next to invisible, drifting silently through space. Only when they happen to swing by the inner Solar System — a journey that can take half a million years or more — do they become visible, melting in the glare of the Sun and forming long, sometimes dramatic, tails.
Why these comets decide to make the long journey earthward is still a matter of debate. Astronomers believe — though no hard evidence has been found- that a shell of icy comets surrounds the Solar System, stretching light years from the Sun. Though mostly stable, over long periods of times — millions of years — regular disturbances sweep across this shell.
That can come from passing stars: every now and then a foreign star travels close to our own, perhaps knocking a few comets inwards to the planets. Seventy thousand years ago a red dwarf likely passed within a light year of our Sun, almost certainly sending a cascade of comets heading our way.
But they also can come from the ebb and flow of the galactic tide, a force akin to that which creates the daily tides on Earth. The constant push and pull of the galaxy stretches and contracts the shell of comets around our Sun, occasionally pushing one out of its comfortable orbit.
Whatever the cause, the fact that comets sometimes come racing towards us is indisputable. They can come from any direction — up or down, east or west — at any time of year, with almost no warning. They travel fast, and can pack a deadly punch — as the events in Siberia a century ago show.
What can we do about this threat? NASA has estimated that we could get up to two years notice, giving us a small window to act, if necessary. Even so, that is probably not long enough to do anything meaningful. We’d need to design and build a spacecraft, and then hope that whatever action it takes is enough to prevent a collision.
That short window has made some think we should start preparing now. One solution could be found in the Comet Interceptor, a mission that will launch at the end of this decade. Once in space, mission planners will park the probe in orbit, waiting until a suitable comet appears. When it does, the Interceptor will travel out to meet the comet, investigating its surface and characteristics.
That same strategy could be deployed to deflect a comet, should that ever become necessary. Rather than waiting for the inevitable day we see one coming towards us, we could have a fleet of spacecraft ready and waiting to intercept it and deflect it from its deadly course.
A century ago we were lucky. The Tunguska blast hit a remote region, one inhabited by few people. Had it struck four hours earlier, St Petersburg would have been obliterated, wiping out a million lives in an instant. As it was, not a single person died, and the world could spin on in blissful ignorance for another few decades.
Despite that, another impact will come, perhaps tomorrow, perhaps in a century. Our luck will not last forever — we should start preparing now.