Asteroid 2013 RT73 flew past the Earth earlier this week, reaching a minimum distance of 3.178 million km (8.3 times as distant as the Moon) at one minute past midnight GMT on Monday 16 September 2013. The asteroid, which was only discovered last week (2013 RT73 means the 1844th such body discovered in the first two weeks of September 2013), measures 49 m across, and would be potentially dangerous to a localized are if it hit the Earth. It is estimated that a body this size entering the Earth's atmosphere would release roughly the same amount of energy as 5.2 megatons of conventional explosive, slightly over 3000 times the size of the blast caused by the Hiroshima bomb, in an airburst at an altitude of 8.7 km. This would be unpleasant to be underneath, but would not cause global devastation, and only fragments of the asteroid would reach the surface. Fortunately such impacts are thought to only happen roughly once every 750 years.
2013 RT73 is calculated to have a 3.58 year orbital period, spending most of its time outside the orbit of Mars, and only occasionally coming close to the Earth; the innermost point in its orbit (the perihelion) actually being 1.3 million km outside of Earth's orbit, making it highly unlikely it will ever hit us (in theory interaction with another body could deflect its orbit slightly so that it does intersect with the Earth's but such an interaction would be far more likely to deflect it further away.
See also Asteroid 2013 RS43 misses the Earth, The dust tail of 3200 Phaethon, The Perseid Meteors, The perihelion of Comet 103P/Hartley 2 and The ejecta of Main-Belt Comet P/2012 T1 (PANSTARRS).
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