On 15 February, 2013, astronomers were expecting an asteroid called 367943 Duende to scoot past planet Earth. The impending close encounter hardly garnered any press attention at all. Unfortunately the asteroid chose the wrong day for its fly-by as the world's attention was distracted by a huge explosion over the city of Chelyabinsk in Russia. This was a close encounter of a different kind, and it was serendipitously captured for posterity thanks to Russian drivers' dashcams¹.

Red Letter Day

What began as an otherwise-normal day for Chelyabinsk's million-plus citizens soon turned into an unforgettable one as a once-in-a-lifetime event unfolded. The sky was cloudy at 9.20am local time when there was a sudden glare which became a blinding flash. It arced across the sky at 18km/s from eastern horizon to west as people, who thought they were under attack², ran for cover. When the spectacular flare faded, smoke trails could be seen. As people stared and wondered what they had just witnessed, a shockwave hit their city, blowing out over 100,000 windows. One factory's roof caved in and a wall collapsed. About 1,000 people were injured, mostly by glass cuts, but most astonishingly amidst all the chaos, no-one was killed. What could have caused this shocking event?

Superbolide

The Earth resides in a cosmic shooting gallery. Our planet is being bombarded by space rocks all the time, we just don't notice most of them. Occasionally a comet will cause a fuss and we have regular meteor showers to look forward to. Once in a blue moon an event will occur like the Tunguska Incident of 1908, but thankfully they are rare. The space rock which shocked the citizens of Chelyabinsk was an Apollo-class³ asteroid which became a bolide (an especially-bright exploding fireball). It was so spectacular that it earned the name superbolide, as it surpassed magnitude  −17. What was originally just a lump of rock was heated up by friction with the Earth's atmosphere to the point that it exploded while it was still 23,300m above the Earth. The explosion was measured at roughly 90 kilotons; that's six times the power of the atomic bomb which exploded above the city of Hiroshima, Japan, during WWII.

Caught on Cams

Thanks to most of the drivers in Russia having dashcams to record traffic incidents for later insurance claims, the superbolide was recorded from the moment it appeared above their city. Those with the technical savvy uploaded what they had captured onto the Internet; within minutes the images had spread around the world. Round-the-clock news programmes showed video footage of the entire breathtaking event on television, a journalistic dream. But near-Earth object scientists must have been the happiest people on the planet as for the first time in history they were able to study video to reconstruct the angle and path of the superbolide, and estimate its size (over 17m) and mass (10,000 tons).

Close but no Cigar

Sixteen hours after the Chelyabinsk impact, a 30m lump of rock with a mass of 40,000 tons, which astronomers had been expecting, shot past the Earth within the orbit of our communication satellites. Those who knew about this event breathed a huge sigh of relief. We dodged a bullet this time, as asteroid 367943 Duende is much bigger than the Chelyabinsk superbolide, and would do correspondingly more damage. Astronomers knew that it would not collide with the Earth this time round, but it was still good to see it receding into the distance. Remarkably, although both of these rare asteroid events occurred on the same day, they weren't related to each other. It was just a cosmic coincidence.

Meteorite Hunt

A meteorite is a piece of space rock that makes it as far as the ground. When the superbolide exploded, much of it was vaporised, but hopes were high that large chunks of it might be found scattered around Chelyabinsk. In the right market meteorites are more precious than gold, so there were plenty of prospectors out looking for fragments. Luckily it was winter so not only was snow on the ground, there were frozen lakes too. One such, Chebarkul Lake, had a 6m-wide hole in it; scientists were hopeful that a large chunk of meteorite could be located. The lake was dredged and a 1.5m-long portion of the Chelyabinsk impactor, weighing more than 570kg (that's when the scales broke) was recovered.

Meteorite Medals

Recovered fragments of the Chelyabinsk meteorite were crafted into special commemorative medals, seven of which were presented to Gold Medal winners on the first anniversary of the impact event, 15 February, 2014, at the Winter Olympics in Sochi, Russia. One of the lucky recipients was Great Britain's Lizzy Yarnold, who won the women's Skeleton event.

Too Close for Comfort

Monitoring systems are in place to alert us to near-Earth objects. People can keep a watch for themselves on the daily-updated Spaceweather.com, which displays the Potentially Hazardous Asteroids (space rocks that come close to the Earth). At the time of writing they have logged over 1,500 PHAs, the smallest being 20m in length, the largest, 1.8km. These are the ones they know about, and none of them are on a collision course with the Earth, they're just in the vicinity.

So how much damage could one of these rocks do? The Chelyabinsk superbolide broke 100,000 windows. The Tunguska incident of 1908 knocked down 80 million trees over an area of 2,000 square kilometres. At the moment there is no technology which would enable us to protect our planet from an asteroid hit, even if we knew about it in advance. All we could do is evacuate the projected strike area, if the authorities had enough warning. Unfortunately, we might not get much notice. On New Year's Day 2014, an asteroid, 2014 AA, was discovered by the Mount Lemmon Survey (whose job it is to detect NEOs). The following day it entered the Earth's atmosphere and what remained plunged into the ocean somewhere between West Africa and Central America. Thankfully the asteroid was only 3-4m in diameter.

Many people believe that the extinction of the dinosaurs might have been caused by a strike by a really big asteroid at Yucatan, Mexico, although we can't know for sure. The effect of such a collision would be a catastrophe affecting the whole world. There are many Near Earth Objects out there. We can only hope one of the biggies doesn't have our planet's name on it.