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Asteroid 2014 RC will be making a close-approach to Earth, at 1/10th the distance of the Moon on September 7th. The asteroid is about the size of the Chelyabinsk meteor, and was discovered only 7 days before it will make the close approach. It worries me how sensitive asteroid surveys are, yet of the 1000+ asteroids that impacted Earth in the last century (about 500 of them were only a couple of feet across, mind you) only 2 of them were discovered before they impacted. Even the Chelyabinsk meteorite itself wasn't discovered before it came down as a huge fireball on Earth. If its orbit had been less oblique, it could have even created a crater at least 50 feet in diameter. It's possible that there's an asteroid as large as 500 meters heading straight towards us and we wouldn't know until it's too late.