Doom from above: the threat of meteorites

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."

Russian roulette
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.

The dinosaurs didn’t have a space programme, however.

“This is an international problem and requires an international solution,” says Yeomans of Nasa. “Who would be charged with making a deflection attempt? Those guidelines have yet to be worked out.”

Fitzsimmons and others are considering three solutions. One is to punch the asteroid off course using a speeding spacecraft. Another is a gravity tractor, where a small craft hovers above and uses gravitational attraction to tug the asteroid.

The nuclear option
For something more dramatic, high- energy and controversial, a nuclear explosion could evaporate the asteroid's outer layer and push it away, says Fitzsimmons. Nukes are banned in space by UN treaty, but conventional explosives could also be used. A first step, however, is to test options on a non-Earth-bound asteroid. We should have time.

“We are expecting to have an impact warning time of years [for large objects],” adds Fitzsimmons, “generally 10 or 20 years.” But there are risks. “We don’t want to rest on our laurels,” says Fitzsimmons. “It’s like saying we know buses arrive on average at this time – but sometimes one comes along unannounced.”

After the Russian air blast, a UN team called for international co-operation to face the threat of near-space objects.

Asteroids, which lie in a band beyond Mars, break out of their orbit due to the pull of Jupiter or Saturn, and can take on elliptical paths that cross our own. But it is not only asteroids we need to worry about. There are also the comets – icy bodies that come from the outer solar system from beyond the planet Neptune.

"In near-Earth space, asteroids outnumber comets by 100 to 1, that is the good news," says Yeomans. "But if one is coming from the outer solar system, it woudn't be obvious until it reached the orbit of Jupiter. And it takes nine months to travel from Jupiter to Earth. So comets are a problem. They are far less likely to hit us, but more difficult to deal with."

Skyfall: A meteorite stolen from Trinity
Last February the geology department in Trinity had a meteorite piece stolen from its collections. This was the main part of a meteorite that fell on Dundrum, Tipperary, on August 12th, 1865, a regular rocky meteor known as a chondrite. The meteorite has since been recovered, but Trinity's geology department no longer has its meteorite collection on display.

Meteorites have become highly collectable in the past decade and Ian Sanders of Trinity says this has made it tough for scientists to afford them. He gets a call on average once a week from people who think they have found a meteorite. “Usually it is a piece of dark-coloured rock that people pick up” that is magnetic, he says. But invariably the hopefuls are disappointed. Some 99 per cent of “finds” are not the real thing. “Genuine meteorites have never been recovered in Ireland except in the few cases where they were actually seen to fall,” Sanders warns would-be meteorite hunters.

Most meteorites falling to Earth are a pale grey-green colour, and somewhat speckled with tiny, sub-millimetre flecks of shiny metal. These are denser than typical Earth rocks; if freshly fallen, they are coated by a thin black or dark brown glassy skin that is smooth or slightly dimpled. The rest are mostly either dense iron meteorites or meteorites of volcanic-type rock.