Asteroid attack: the end of the world?
The Earth is constantly being bombarded by rocks from space. How worried should we be?
Are dangerous asteroids common?
On Monday, a hefty space rock about 492 feet wide – officially designated by Nasa as a “near-Earth object” – passed relatively close to Earth, travelling at some 38,500mph. There was, in fact, nothing unusual or threatening about this: the asteroid came no closer than three million miles away, about 13 times further away than the Moon, and it was merely one of 17 that Nasa had predicted would pass close to Earth between this week and the end of the year.
What else is out there?
So far, Nasa has classified more than 21,000 asteroids and more than 100 comets as near-Earth objects. Of that group, about 2,000 are considered “potentially hazardous”, meaning they have orbits within 4.5 million miles of Earth’s and are big enough to cause massive devastation on impact. The US Congress has directed Nasa to find and track at least 90% of the objects measuring 459 feet in diameter or larger that pass within 30 million miles of Earth’s orbit. So far, nothing has been discovered that poses a threat within the next 100 years. But scientists have identified only an estimated 40% of near-Earth objects. Researchers were stunned in July when a previously undetected “city killer” asteroid that was up to 427 feet wide came within 45,000 miles of Earth – less than one-fifth the distance to the Moon. If the asteroid had struck the Earth, “it would have gone off like a very large nuclear weapon,” said Michael Brown, an astronomer at Australia’s Monash University.
Why did scientists miss it?
Asteroids are tough to spot in the void of space. Astronomers continually take pictures of the sky, using computers to look for movement across the background of stars. Smaller asteroids shine less brightly and need to come relatively close to Earth at some point in their orbits to be detected by ground-based telescopes. The city killer that zipped by this summer had an “eccentric orbit” that made it detectable for only brief periods of time. It also came close to the nearly full Moon, the brightness of which made it tougher to spot. But don’t panic: your odds of being killed by an asteroid are exceedingly slim at up to 1 in 250,000, according to odds calculated by Professor Stephen A. Nelson of Tulane University, New Orleans. You’re far more likely to die in an airplane crash (1 in 30,000) or an earthquake (1 in 130,000).
How often is the Earth hit?
The planet is under constant bombardment. About 100 tonnes of dust and gravel-size particles enter the planet’s atmosphere daily, burning up from friction as they crash through air molecules at more than 45,000mph. These tiny collisions generate so much heat that they are visible as shooting stars. But though they can create spectacular fireballs, space rocks – as long as they’re no bigger than about 82 feet – generally cause little or no damage. But not always. In 2013, an undetected 66-foot asteroid exploded 14 miles above Chelyabinsk in Russia, releasing 20 to 30 times more energy than the nuclear bomb dropped on Hiroshima. The resulting shock wave shattered windows throughout the city, injuring more than 1,100 people. Scientists expect events like the one at Chelyabinsk about once every 60 years.
What about bigger asteroids?
In 1908, a space rock estimated to be 160 to 260 feet wide exploded above an uninhabited part of Siberia, levelling 80 million trees and leaving hundreds of blackened reindeer carcasses across an area of some 1,000 square miles. If the asteroid had arrived just four hours later, it could have hit and destroyed
St. Petersburg. A rock that big strikes Earth once or twice every 1,000 years. The asteroid thought to have wiped out the dinosaurs 65 million years ago was even bigger, measuring six to nine miles in diameter. It smashed into what is now the Gulf of Mexico, triggering massive tsunamis and sending up toxic plumes of sulfur (see box). Collisions like that happen about once every 100 million years.
Could we stop an asteroid from hitting?
In theory. But it would be pointless to try to blow up the approaching rock, as in the movies. As hardy remnants of the formation of our solar system, asteroids have survived countless collisions in space. Some scientists think that if you blew apart an asteroid, the gravitational pull of its core would force the rock back together. By one estimate, it would take a nuclear weapon 4,000 times more powerful than the biggest nuke ever created to destroy an asteroid double the size of the one that killed the dinosaurs. It would be far better to try to nudge it off course.
How could you knock it off course?
A nuclear explosion about 1,000 feet from the asteroid would probably do the trick. A custom spacecraft would deliver the payload, but Nasa won’t be testing the concept any time soon, because deploying nuclear weapons in space is both risky and banned by international law. Another option is to use a spacecraft as a battering ram, a method Nasa will try out in 2021 when it launches a probe against a non-threatening asteroid. We could also try to subtly redirect an asteroid using the gravity of a spaceship flying close by for an extended period, a so-called gravity tractor – a tiny nudge could alter an asteroid’s trajectory by thousands of miles over time. But it would take at least a year to prepare the spacecraft and its trajectory for such a journey. With only a few months’ lead time, the best we could manage when faced with a city killer would be mass evacuations, a scenario Nasa and, to a lesser extent, the European Space Agency, actively prepare for. Over the long term “these events are not rare; they happen”, Nasa’s Jim Bridenstine told a Planetary Defence Conference earlier this year. “This is not about movies. This is about ultimately protecting the only planet we know right now to host life.”