A meteor that exploded over Russia in February was 20 metres in diameter and caused a blast equivalent to 600,000 tons of TNT, according to scientists studying the event.
The space rock blew apart almost 30 kilometres above the city of Chelyabinsk, briefly outshining the sun and inflicting severe burns on a number of observers below.
It was the largest object to hit the earth since the Tunguska event of 1908, when an exploding comet or asteroid destroyed 2,000 square kilometres of Siberian forest.
Analysis showed that the rock was a common type known as a chondrite – the kind most likely to cause a major extinction event in the future.
Professor Qing-Zhu Yin, from the Department of Earth and Planetary Sciences at the University of California at Davis, US, said the meteor strike was a “wake-up call”.
He added: “If humanity does not want to go the way of the dinosaurs, we need to study an event like this in detail.”
The Chelyabinsk object entered Earth’s atmosphere at just over 19 kilometres per second, slightly faster than had previously been reported, said the team.
Three-quarters of the rock evaporated in the explosion, said the researchers, whose findings are reported in the journal Science. Most of the rest of the object became a glowing orange dust cloud and only a small fraction – still weighing 4,000kg to 6,000kg – fell to the ground.
The largest single fragment, weighing about 650kg, was recovered from the bed of Lake Chebarkul in October.
Shock waves from the airburst smashed windows, rattled buildings, and knocked people off their feet, more than 1,200 of whom attended hospital.
Researchers visiting villages in the area found a region of shock-wave damage extending some 80 kilometres on either side of the meteor’s trajectory path.
The enormous stress of entering the atmosphere at high speed caused the object to heat up and explode. The break-up was facilitated by numerous “shock veins” in the rock left by an earlier impact in space.
After studying samples of the rock, scientists estimated it would be more than four billion years old. It went through a significant shock event about 115 million years after the formation of the Solar System 4.5 billion years ago.
The object may have come from the Flora asteroid family in the asteroid belt between the orbits of Mars and Jupiter. But the chunk that exploded over Chelyabinsk is not thought to have originated in the asteroid belt itself, the experts believe.
One possibility is that it came from a much larger “rubble pile” asteroid that broke apart 1.2 million years ago in an earlier close encounter with Earth.
Major impact events such as Tunguska or Chelyabinsk occur more frequently than is commonly thought, said Yin. He pointed out that four tons of material was recovered from a meteor shower in Jilin, China, in 1970.
“Chelyabinsk serves as a unique calibration point for high-energy meteorite impact events for our future studies,” Yin added.