Doom from above: the threat of meteorites
An asteroid 10km in diameter wiped out the dinosaurs and extinguished life on Earth. The scientific community is working to avoid a repeat
A meteorite trail is seen above a residential apartment block in the Urals city of Chelyabinsk, on February 15th. A heavy meteor shower rained down on central Russia, sowing panic as the hurtling space debris smashed windows and injured dozens of stunned locals. Photograph: Oleg Kargopolov/AFP/Getty Images
Astronomer Alan Fitzsimmons has a lot on his mind. He is interested in our solar system, but also in how to save planet Earth. The Queen’s University Belfast scientist studies how to deflect space rocks on a collision course with our planet, as part of NeoShield, a European-funded project. One option is to nuke an incoming asteroid, another to ram it.
Extra impetus arrived on February 15th when an asteroid exploded over central Russia, producing a blast 30 times as powerful as the Hiroshima bomb. The Chelyabinsk meteor had travelled 18km per second, before disintegrating at high altitude. About 1,600 people were injured in the sparsely populated area and thousands of buildings damaged.
It was a small asteroid, Fitzsimmons says, about the size of a bus. “An object that small [20m wide] will not make it to Earth’s surface, but will explode at altitude,” he says. By coincidence, hours later a 20m by 40m asteroid whizzed by Earth, closer to us than our communication satellites, but it was predicted. Chelyabinsk arrived unannounced.
“We still miss these small asteroids all the time, even with our sophisticated telescopes,” says Fitzsimmons, who uses a telescope in Hawaii to survey for danger in another project, Pan-Starrs. Once a minute, a 1.4 billion-pixel image of part of the sky is recorded and then analysed for comets and asteroids that may be winging our way. It detected the 10,000th “near-Earth object” early this June. Cataloguing these is a priority for Nasa too.
Don Yeomans of Nasa’s near-Earth object programme says 90 per cent of objects over a kilometre across have been identified, and Nasa is now focusing on those more than 130m. After following them for a few years, scientists can work out their orbit and do impact probability calculations, often for a hundred years into the future.
“A one-kilometre object gives you a global catastrophe,” Fitzsimmons warns. “An object 40m to 70m across would cause major damage locally, with the severity depending on the exact velocity, size and composition.”
Anything greater than 30m across will likely strike Earth’s surface, but damage potential is still under investigation.
“To better predict the hazards of future impacts, we need to know how much damage objects of different sizes and types of material produced,” says David Kring, an impact expert at the Lunar and Planetary Institute in Houston. “Chelyabinsk is providing the first high-precision calibration point on the curve that relates impact energy to surface damage.”
The Russian fireball was a weak stony asteroid called a chondrite; one fragment knocked an eight-metre hole in the ice of Chebarkul Lake. Experts say a 20m object enters Earth’s atmosphere about once a century on average, so it was a rare event. Chances are it wouldn’t hit a city.
Impressive impact craters in Canada and South Africa stretch almost 200km across, but the most famous lies under the Yucatan Peninsula, Mexico. “The Chicxulub impact event, which wiped out the dinosaurs and extinguished life on Earth, was produced by an asteroid about 10km in diameter,” says Kring. “It is much more difficult for such a large object to go undetected, but not impossible. The scientific community is working hard to detect all objects that size, to minimise that risk.”
A rock this size equals catastrophe. The Chicxulub blast laid down a tell-tale layer of iridium-rich dust spread across the globe, a clue to its force. “That asteroid would have come in at around 20km a second, and something that size would have no chance of being slowed down by the atmosphere. It would have penetrated way down into the crust, where all that kinetic energy was converted instantly to heat. The rock would have simply exploded and vaporised, giving a hot dusty atmosphere,” says Ian Sanders, a meteorite geologist in Trinity College.