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The most spectacular cosmic event so far this century was the fireball that streaked across Russian skies last February. Burning 30 times brighter than the sun, it exploded high above the city of Chelyabinsk in an airburst that caused more than 1,200 injuries.
The world’s largest meteorite strike since the 1908 Tunguska explosion in Siberia, the incident provided a huge volume of evidence from video footage and human witnesses, as well as some fragments that reached the ground. The largest surviving piece was a 650kg rock recovered from Lake Chebarkul.
Now astronomers have published three scientific papers (two in Nature and one in Science) and come to some interesting conclusions about the event – and the frequency with which small asteroids are likely to hit Earth. They warn that we may be more vulnerable to impacts than experts had realised.
The Chelyabinsk meteorite entered the upper atmosphere above the southern Urals. At that point, it was a rock about 20m wide, weighing 12,500 tonnes and travelling at 19km per second. As it sped through the air, friction caused unsustainable heating and pressure.
At an altitude of 30km it exploded with an energy equivalent to 500 kilotonnes of TNT, the power of a medium-sized atomic bomb. The brightness of the blast caused eye damage and sunburn in some witnesses but the main injuries were the result of broken glass as thousands of windows shattered.
More serious damage was avoided because the asteroid was intrinsically weak so it broke up at high altitude. “Luckily most of the kinetic energy was absorbed by the atmosphere,” says Jiri Borovicka of the Czech Academy of Sciences Astronomical Institute. “A more solid rock that might have blasted closer to the ground would have caused considerably more damage.”
The scientists estimate that the blast vaporised three-quarters of its original mass. Most of the rest turned to dust.
The break-up in the atmosphere was facilitated by “shock veins”, weaknesses resulting from one or more impacts earlier in the asteroid’s history. Analysis at the University of California Davis suggests that it went through a “shock event”, or impact, 4.45 billion years ago. That was 115 million years after the formation of the solar system when collisions between asteroids and planets were common.
The Chelyabinsk object probably splintered off another, much larger asteroid. Peter Jenniskens of Nasa’s Ames Research Center believes it was part of a “rubble pile” asteroid that broke up in a close encounter with Earth 1.2 million years ago as a result of the planet’s gravitational pull. Calculations by Borovicka and his colleagues suggest that it broke off near-Earth asteroid 86039, which is now 2.2km in diameter.
Astronomers are currently undertaking follow-up calculations. “Chelyabinsk serves as a unique calibration point for high energy meteorite impact events for our future studies,” says Qing-Zhu Yin of UC Davis. “If humanity does not want to go the way of the dinosaurs, we need to study an event like this in detail.”
Before this year, astronomers believed that objects the size of the Chelyabinsk asteroid (or bigger) hit Earth once every 100 to 150 years. They are now coming to see that as a considerable underestimate.
A team headed by Peter Brown of the University of Western Ontario analysed all the meteorite airbursts detected over the past few decades by instruments designed to detect nuclear weapons testing. By comparing these with the Chelyabinsk airburst, the researchers conclude that Chelyabinsk-sized impacts could occur every 15 or 20 years.
Astronomers are calling for the sky surveys that map potentially dangerous asteroids to be extended to smaller bodies. About 95 per cent of big asteroids more than 1km across in near-Earth orbits – which could end human civilisation – have been mapped and none is due to collide with us.
Although telescopes could never map all the smaller asteroids, astronomers say it would be useful to set up a warning system. Most of the Chelyabinsk casualties could have been avoided by advising people to stay away from windows.
Artists join forces with researchers
Faced with the vast divide between the Wellcome Trust’s well-resourced scientific research unit in Kenya and the poverty of the surrounding community, many might throw up their hands in despair. Artists Miriam Syowia Kyambi and James Muriuki instead threw their surgical gloves up in the air.
Conjure, their series of photographs, shows those gloves billowing surreally in the sky, framed by a local building, and hovering over a ploughed field, bright white against the richly coloured African backdrops.
The works are among the fruits of a collaboration between Wellcome’s researchers around the world and locally hired artists in residence. The project, “Foreign Bodies, Common Ground”, is currently being showcased at the Trust’s headquarters in London.
“By bringing in artists, we wanted to explore the interaction of scientists with the local community,” says Danielle Olsen, the curator. The work highlights variations and occasional tensions between attitudes, ethics and practices – and the links between art and science.
The Kenyans, among six groups chosen, focused on photography to connect to the local community in Kilifi. They also invited residents to pose in a laboratory, dressed in white coats with local fabrics weaved in.
In Ho Chi Minh City, Lena Bui was inspired by the swarming of bacteria and the transmission of animal infections to humans. Her resulting abstract, Invisible Currents, builds beautifully on traditional Asian drawing techniques, while videos of speeding motorbikes magnify and translate microscopic biological patterns into everyday life. Bui also gained access to remote communities harvesting bird feathers – putting them at risk of flu – making videos where the Trust’s researchers were not welcome.
Just as striking in a very different medium, at Wellcome’s Sanger Institute in the UK, Katie Paterson created Fossil Necklace, a single piece that reflects her work tracking down, buying, carving and stringing together 170 beads made from fossils, representing the evolution of life over billions of years.
Some of the artists concede that their work did not necessarily improve local attitudes to Wellcome’s research. But it has at least triggered greater reflection by all those involved. Andrew Jack
‘Foreign Bodies, Common Ground’ is at the Wellcome Collection, 183 Euston Road, London NW1 2BE, until February 9