Canada Day and U.S. Independence Day fireworks this week in honour of the countries’ 111th 141st* and 232nd birthdays, respectively, can’t hold a Roman candle to the natural fireworks that erupted in western Siberia exactly 100 years ago.
On June 30, 1908, at around 7:17 a.m. local time, natives and settlers near the Podkammennaya Tunguska river saw a column of blue light, nearly as bright as the sun, moving across the sky.
About ten minutes later, there was a flash, and a powerful shock wave ripped through the forest, flattening 80 million trees in a butterfly-shaped pattern over a total area of more than 2,000 square kilometers.
People were knocked off their feet and animals blown into the air. Windows broke in towns hundreds of kilometers away. Across Europe and Asia, seismographs registered an earthquake.
For days thereafter, so much dust hung in the upper atmosphere that nighttime skies glowed all over the world–so brightly, Londoners could read their newspapers outside at midnight.
Over the decades since, numerous expeditions have trekked to the remote region. It’s now estimated that the explosion was a thousand times more powerful than that of the atomic bomb that destroyed Hiroshima. And since no impact crater has been found, and trees at ground zero were left upright (though stripped of all bark and branches), it’s thought the mostly likely cause was an object from outer space hurtling into the atmosphere and exploding six to ten kilometers above the ground.
(Why did it explode so far up? Meteroids hit the atmosphere at more than 10 kilometres a second. Air simply can’t get out of the way of something moving that fast, so it compresses in front of the object, creating enormous heat and pressure. When the heat and pressure reach a critical point, the object violently disintegrates.)
The puzzling thing is that no one has ever found any fragments of the Tunguska object. That may mean that, rather than being a stony or metallic fragment of an asteroid, it may have been a piece of a comet. Not only do comets move at much higher speeds than a typical asteroid, so that a smaller object could pack a greater punch, they are essentially dirty snowballs. A fragment of ice and dust would have been completely vaporized in the explosion, leaving nothing behind to be found on the ground.
Since nobody knows exactly what the object was, nobody knows how big it was, either. Estimates range from three to 70 metres in diameter.
Researchers continue to explore the region. Three scientists from Italy think that the oval-shaped Lake Cheko, located about 10 kilometres from ground zero, may actually be a crater left behind by a metre-wide fragment of the object. Buried ten metres beneath its cone-shaped bottom is a dense object about that size. They hope to retrieve it on a future expedition.
A common claim (originally made in the Guinness Book of World Records in 1966) is that if the Tunguska object had struck 4 hours, 47 minutes later it would have obliterated St. Petersburg. That’s not true, because that only figures in the Earth’s rotation, not the planet’s movement through space: if the object hadn’t arrived just when it did, it would have missed Earth completely.
Still, it’s sobering to think what a Tunguska-sized explosion would do to a modern city. And given enough time, something that big will undoubtedly hit the planet again…or something even larger and nastier will.
That’s why it’s important to try to identify as many potential threats as we can. Canadians are helping: after its 2010 launch, the Canadian-made NEOSSat (Near Earth Orbit Surveillance Satellite) will give Earth a new eye in the sky, a small but incredibly sensitive space telescope, that scientist can use to spot asteroids that cross Earth’s orbit and may therefore someday pose a threat.
It’s currently estimated there are 100,000 asteroids greater than 140 metres in diameter in near-Earth space. Fortunately, space is incredibly big…but an asteroid only has to get lucky once for us to have a really bad day, as bad as that day at Tunguska a hundred years ago…
…or, just possibly, inconceivably worse.
*UPDATE: For some reason I wrote this and even sent it ot the newspaper without noticing that I somehow misplaced 30 years in the first sentence and said this was the 111th birthday of Canada, when of course it’s the 141st.
Unless it’s still 1978. Which apparently in my head it is. Far out, man.
