In a rare double whammy event, the earth was hit 458 million years ago by a pair of asteroids that left behind two craters 16km apart in Sweden.

The crash happened 12 million years after a mighty collision in the asteroid belt which caused a 200km wide asteroid to break up with its pieces landing in the earth's orbit, says a study published in Scientific Reports.

Two of these crashed into shallow seas that covered modern-day Scandinavia, according to the study.

Many other smaller fragments would have burnt up in the atmosphere in a shower of meteors.

The two craters - a 7.5-km Lockne crater, located around 20 kms south of the city of Oestersund, and a 700-metre crater at nearby Malingen - have been shown to be results of that collision caused by 600-metre and 150-metre long objects.

The team, led by Jens Ormoe of the Centre for Astrobiology in Madrid, carried out drilling into the craters, looking for traces of sediment altered by the impact.

They also mapped the debris thrown as far away as tens of kilometres.

While modelling of asteroids that come close to Earth suggests that about 16% of these objects travel in pairs, only 10 of the 188 craters on earth are believed to be caused by "doublets".

The potential for danger from asteroid bombardments cannot be underestimated even though most of the smaller objects burn out in the atmosphere. As recently as last year, an 18-metre wide object exploded in the skies above Russia causing much damage and injury.

Nasa has been on the look-out for such objects but has failed to classify beyond 10 % of the potentially dangerous near-earth objects.

Various means of tackling such objects have been suggested in Armageddon-like scenarios but deflection rather than bombardment has been touted as the ideal solution.

In fact, we have to thank asteroids for chiselling and making the earth what it is today, says a study that finds evidence of asteroids shaping our planet a million years ago.

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