We've all seen the films where an asteroid hurtles towards our planet, threatening civilisation.

What's less well known is that menacing space rocks sometimes come in twos.

Researchers have outlined some of the best evidence yet for a double space impact, where an asteroid and its moon apparently struck Earth in tandem.

Using tiny, plankton-like fossils, they established that neighbouring craters in Sweden are the same age - 458 million years old.

Details of the work were presented at the 45th Lunar and Planetary Science Conference in The Woodlands, Texas, and the findings are to be published in the Meteoritics and Planetary Science journal.

However, other scientists cautioned that seemingly contemporary craters could have landed weeks, months or even years apart.

A handful of possible double impacts (or doublets) are already known on Earth, but Dr Jens Ormo says there are disputes over the precision of dates assigned to these craters.

"Double impact craters must be of the same age, otherwise they could just be two craters right next to each other," the researcher from the Centre for Astrobiology in Madrid, Spain, told BBC News.

Dr Ormo and his colleagues studied two craters called Lockne and Malingen, which lie about 16km apart in northern Sweden. Measuring about 7.5km wide, Lockne is the bigger of the two structures; Malingen, which lies to the south-west, is about 10 times smaller.

Binary asteroids are thought to form when a so-called "rubble pile" asteroid begins to spin so fast under the influence of sunlight that loose rock is thrown out from the object's equator to form a small moon.

Telescope observations suggest that about 15% of near-Earth asteroids are binaries, but the percentage of impact craters on Earth is likely to be smaller.

Only a fraction of the binaries that strike the Earth will have the necessary separation between the asteroid and its moon to produce separate craters (those that are very close together will carve out overlapping structures).

Calculations suggest around 3% of impact craters on Earth should be doublets - a figure that agrees with the number of candidates already identified by researchers.

The unusual geological characteristics of both Lockne and Malingen have been recognised since the first half of the 20th Century. But it took until the mid-1990s for Lockne to be formalised as a terrestrial impact crater.

In the last few years, Dr Ormo has drilled about 145m down into the Malingen structure, through the sediment that fills it, down to crushed rocks known as breccias and deeper, reaching the intact basement rock.

Lab analysis of the breccias revealed the presence of shocked quartz, a form of the quartz mineral that is created under intense pressures and is associated with asteroid strikes.

This area was covered by a shallow sea at the time of the Lockne impact, so marine sediments would have begun to fill in any impact craters immediately after they were created.

(BBC)