Last night an asteroid skimmed the Earth at a distance of around 40,000 km with only a few days warning.
Asteroids_passing_Earth
Image: NASA
Asteroid 2014RC was only detected on August 31, just a few days before it whizzed past us, getting uncomfortably close to many of the satellites we rely on for telecommunications. So are we at risk from other asteroids of a similar size?
As astronomer Alan Duffy from Swinburne University of Technology in Melbourneexplained on Australia’s ABC News Breakfast this morning, it’s likely we’ll see more of these asteroids appear “out of nowhere”. 
Asteroid 2014RC is around the size of a school bus, with an initially estimated width of 20 metres and weight of 8,300 tonnes. Updated measurements from NASA on 9 September puts the width at 12 metres and 1,800 tonnes. This is a similar speed and slightly smaller size to the asteroid that exploded over Chelyabinsk in Russia last year, releasing around 20 to 40 times more energy than the Hiroshima atom bomb blast and injuring 1,000 people.
If something roughly this size hit the Earth all in one spot, it could potentially destroy a city, says Duffy. But it would take something significantly larger, around 1 kilometre in width, to cause mass devastation of the type that wiped out the dinosaurs. Perhaps what is most worrying is that it’s very likely we wouldn’t know about it until a few days before.
“While NASA believes it has found over 90 percent of the dinosaur-killing sized asteroids of 1 kilometre and larger, these smaller 'city'-killers are far harder to find and we've seen only a few thousand of the expected 1 million objects that pass the Earth. On average we find a new asteroid each month, so there's a long way to go,” Duffy told ScienceAlert over email. The reason these asteroids are so hard to spot is because they’re often coal black, so they reflect very little light and are billions of times fainter than the Moon.
But that doesn’t mean we need to panic about them hitting us all the time. Duffy cites a 2006 article published in Meteoritics by fireball experts Philip Bland and Natalya Artemieva, which calculates the risk of something like asteroid 2014RC hitting Earth's surface once every 20 to 30 years.
“As Chelyabinsk was hit by just such an object last year I wouldn't expect something that big again until the 2030s,” said Duffy.
Larger impacts are far rarer. Asteroids that are at least 200 metres across, which could cause explosions on land and devastating tsunamis, predicted to hit us once every 100,000 years on average, and dinosaur killing kilometre-sized objects occurring only once ever hundreds of millions of years.
And with asteroids that size, it’s very likely we’d see them coming long before they arrived. Which is good news, because the only chance we have of avoiding impact requires at least a decade of lead time, Duffy explained. Our best bet in this scenario is simply to park a spacecraft beside an asteroid and let gravity take over, he adds.
“The asteroid will pull the satellite towards it, but so too by a tiny amount, will the satellite pull the asteroid towards it,” said Duffy. “Slowly over time the asteroid will move off course and sail by the Earth just like 2014RC did this morning.” 
If nothing else, last night’s near miss was yet another reason that we need to invest in the monitoring of asteroids and the telescopes and spacecraft that detect them. 
Watch the full ABC interview here.

**UPDATE 9 September 2014: The article originally stated the asteroid was 70,000 tonnes in mass due to calculations being based on a radius of 20 metres, rather than a width of 20 metres. NASA has now updated its measurements of the asteroid and so the new estimated mass is 1,800 tonnes, and the new width is 12 metres. This makes the asteroid school-bus-sized rather than house-sized.**
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