The 1.1-kilometre (0.7-mile) diameter rock wacked into southeastern Bavaria, leaving an "exceptional field" of meteorites and impact craters that stretch from the town of Altoetting to an area around Lake Chiemsee, the scientists said in an article in the latest issue of US magazine Astronomy.

Colliding with the Earth's atmosphere at more than 43,000 kmsmiles per hour, the space rock probably broke up at an altitude of 70 kmsmiles), they believe.

The biggest chunk smashed into the ground with a force equivalent to 106 million tonnes of TNT, or 8,500 Hiroshima bombs.

"The forest beneath the blast would have ignited suddenly, burning until the impact's blast wave shut down the conflagration," the investigators said.

"Dust may have been blown into the stratosphere, where it would have been transported around the globe easily... The region must have been devastated for decades."

The biggest crater is now a circular lake called Tuettensee, measuring 370 metres (1,200 feet) across. Scores of smaller craters and other meteorite impacts can be spotted in an elliptical field, inflicted by other debris.

The study was carried out by the Chiemgau Impact Research Team, whose five members included a mineralogist, a geologist and an astronomer.

It was sparked by a find in 2000 by amateur archaeologists who were digging in the area around Lake Chiemsee and found pieces of metal containing minerals not previously seen in the region.

Aerial infrared photography established that the distinctive holes in the local countryside had the characteristic round form and "clear uplifted rim" of an impact crater, the Astronomy report said.

Minerals ejected around the crater were found by geological analysis to be gupeiite and xifengite, iron-silicon alloys that were also found in meteorites recovered in China and Antarctica.

Additional evidence comes from local discoveries of Celtic artefacts, which appear to have been scorched on one side.

That helped to establish an approximate date for the impact of between 480 and 30 BC.

The figure may be fine-tuned to around 200 BC, thanks to tree-ring evidence from preserved Irish oaks, which show a slowing in growth around 207 BC.

This may have been caused by a veil of dust kicked up the impact, which filtered out sunlight.

In addition, Roman authors at about the same time wrote about showers of stones falling from the skies and terrifying the populace.

The object is more likely to have been a comet than an asteroid, given the length of the ellipse and scattered debris, the report says.

Comets, which race in long orbits around the solar system, are believed to be loose assemblies of rubble held together with an ice rich in methane, ammonia and water.

Asteroids are believed to be denser, more structured rocks. They mainly orbit in a band between Jupiter and Mars, but they can be deflected off course and put on the same trajectory as Earth, an event that is extremely rare but has the potential for a catastrophe.

The long reign of the dinosaurs was put to an end by climate change some 65 million years ago, inflicted by a massive space rock impact in what is now modern-day Mexico.

In 1908, a comet or asteroid exploided over Tunguska, Siberia, flattening the forest for hundreds of square kilometres around.

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