Each June, a giant comet stalks the Earth
Last week the Earth came close to being hit by a chunk of cosmic debris packing the punch of a Hiroshima-type atomic bomb.
The house-sized asteroid 2011 MN shot past the Earth at more than 25,000kph last Monday, coming closer than some communications satellites before flying off back into the void of space.
So why were we not warned? One reason was that although it was a close-run thing, astronomers had pinned down the path of 2011 MN well enough to be pretty confident it would miss us.
Another was that even if it had struck, it would have lost most of its energy as it entered our atmosphere, most likely burning up to give a brief but impressive shower of "shooting stars".
Even so, the incident serves as a timely reminder that the cosmos can still spring nasty surprises on us - and especially at this time of the year. At the end of June our planet cuts across a part of space littered with debris that some astronomers believe has caused devastating impacts on Earth during recorded history.
Most of this debris is nothing more than specks of ice and dust, some of which enters our atmosphere and burns up in a flash.
But every so often the Earth encounters something much larger lurking in the debris trail: tower block-sized chunks of a comet that entered our solar system thousands of years ago.
Travelling at more than 100,000kph, these are big enough to survive entry through the atmosphere, and they could strike with the explosive violence of dozens of H-bombs.
This may sound like another of those terrifying yet incredibly rare events scientists seem so keen to scare us with these days.
But this is no once-in-a-million-years scenario. There is now impressive evidence that just such a cosmic impact happened barely a century ago.
Known as the Tunguska Event, it took place in the morning of June 30, 1908, over a remote part of Siberia. Eyewitnesses reported seeing a colossal fireball and trail of black smoke crossing the morning sky; some even claimed to feel the heat on their bodies. As it approached the ground, an explosion triggered a blast-wave that knocked people off their feet. A huge forest fire then broke out, sending vast plumes of smoke up into the air.
It took scientists almost 20 years to reach the remote site of the incident and begin gathering evidence. Leonid Kulik, a Soviet mineralogist at the Soviet Academy of Sciences in St Petersburg, confirmed the reports that something catastrophic had taken place.
He found a scene of complete devastation as far as the eye could see, with charred remnants of trees lying like matchsticks, all pointing outward from an epicentre around 75 kilometres north of the Tunguska river.
Kulik was convinced he was looking at the effects of a giant meteor impact, and set about looking for fragments of the cosmic visitor. Yet he failed to find anything: no chunks of meteor fragments, not even an impact crater.
Inevitably, this prompted more speculative explanations, ranging from the impact of a small black hole with the Earth to some form of UFO incident.
Among astronomers, however, there is now little doubt that Kulik was right, and the cause was a 50m-diameter meteor - which left no debris because it broke apart under atmospheric stresses about 10km above the ground.
Many scientists are content to regard the Tunguska Event as just one of those freakish phenomena that can be consigned to history. But some astronomers have pointed to a number of curious coincidences about the timing of the event.
In 1975, seismometers placed on the Moon by the Apollo missions detected a colossal meteor storm blasting the moon over five days - around the end of June.
Shortly afterwards, the American meteorite expert Jack Hartung pointed out that a medieval report of what appeared to be a huge explosion on the Moon could be dated to the end of June in the year 1178.
Perhaps most telling of all was the discovery in 1947 of a trail of cosmic debris which the Earth cuts across every year - around the end of June.
The impact risks posed by this so-called Taurid meteor stream were first highlighted almost 30 years ago by two British astronomers, Dr Victor Clube and Dr Bill Napier.
At the time, the scientific community was struggling to accept claims that the extinction of the dinosaurs was linked to a cosmic impact near today's Gulf of Mexico around 65 million years ago. As a result, the authors' claim of a threat to present-day Earth fell largely on deaf ears.
Now catastrophic meteor impacts no longer seem so outlandish, and mounting evidence backs the idea that a giant comet entered our solar system 25,000 years ago, leaving debris in the Taurid stream that still threatens the Earth.
Last year, the Royal Astronomical Society published a study by Dr Napier linking the Taurid meteor stream with evidence for cosmic impacts around 12,900 years ago, ranging from massive fires across North America to the extinction of many animal species in the region.
Despite this, efforts to hunt down the giant disintegrating comet remain low-key. In the mid-1990s, the US space agency Nasa set up Spaceguard, an international network of telescopes designed to discover and track potential Earth-impacting objects.
But astronomers admit they do not have the technology needed to detect more than a few per cent of any Tunguska-like objects that might be out there.
Until such technology becomes available, the location of the giant comet that stalks the Earth will remain a mystery - one that we may regret failing to solve one June day in the not-too-distant future.
Robert Matthews is visiting reader in science at Aston University, Birmingham, England
Updated: July 3, 2011 04:00 AM