Submerged clue: Is Siberia's Lake Cheko the impact crater from a metre-sized fragment that survived the explosion?

Credit: University of Bologna

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PARIS: A hundred years ago last week, a gigantic explosion ripped open the dawn sky above the swampy taiga forest of western Siberia, leaving a scientific riddle that endures to this day.

A dazzling light pierced the heavens, preceding a shock wave with the power of a thousand atomic bombs which flattened 80 million trees over a swathe of more than 2,000 km². Local Evenki nomads recounted how the blast tossed homes and animals into the air. In Irkutsk, 1,500 km away, seismic sensors registered what was initially deemed to be an earthquake.

What caused the so-called Tunguska Event, named after the Podkamennaya Tunguska river near where it happened, has spawned at least a half a dozen theories.

Finger of blame

The biggest finger of blame points at a rogue rock whose destiny – after travelling in space for millions of years – was to intersect with Earth at exactly 7:17 am on 30 June 1908.

Even the most ardent defenders of the sudden impact theory acknowledge that there are many gaps, however. They strive to find answers, believing this will strengthen defences against future Tunguska-type threats, which experts say occur at a frequency ranging from one-in-200 years to one-in-1,000 years.

"Imagine an unspotted asteroid laying waste to a significant chunk of land and imagine if that area, unlike Tunguska and a surprising amount of the globe today, were populated," the British science journal Nature commented last week.

If a rock was the culprit, the choices lie between an asteroid – the rubble that can be jostled out of its orbital belt between Mars and Jupiter and set on collision course with Earth – and a comet, one of the "icy dirtballs" of frozen, primeval material that loop around the Solar System.

Why no fragments?

Comets move at far greater speeds than asteroids, which means they release more kinetic energy pound-for-pound upon impact. A small comet would deliver the same punch as a larger asteroid. But no fragments of the Tunguska villain have ever been found, despite many searches.

Finding a piece is important, for it will boost our knowledge about the degrees of risk from dangerous Near Earth Objects (NEOs), said Italian researchers Luca Gasperini, Enrico Bonatti and Giuseppe Longo based at the Marine Science Institute in Bologna.

When a new asteroid is detected, its orbit can be plotted for scores of years in the future. Comets are far less numerous than asteroids but are rather more worrying, as they are a largely unknown entity.

Most comets have yet to be spotted because they take decades or even hundreds of years to go around the Sun and pass our home. As a result, any comet on a collision course with Earth could quite literally come out of the dark, leaving us negligible time to respond.