On Monday 28 May 2012, at about 3.00 pm, GMT, the newly discovered Asteroid 2012 KP24 passed the Earth at a distance of 51 000 km (seven times as close as the Moon ever gets). The asteroid, a 25 m rock had been discovered only three days previously on 25 May, by the Catalina Sky Survey. The asteroid has a calculated orbital period of 593 days, with an average distance from the Sun of 1.38 AU (i.e 1.38 times the average distance between the Earth and the Sun), though at its closest it is 0.98 AU from the Sun, so that it crosses the Earth's orbit twice every 593 days; it last passed close to the Earth itself in 1939, when it passed us at slightly over 1.6 million km. At its furthest it is 1.84 AU from the Sun, taking it outside the orbit of Mars; it thus crosses Mars's orbit twice every 593 days as well.
There has never (since its discovery) been any danger of a collision with 2012 KP24, though a collision with either Earth or Mars in the remote future is a possibility in the remote future. A more likely scenario is that is might pass close enough to one of the planets for the gravity of the larger body to through it onto a different path, possibly into the Sun or out of the Solar System altogether. Even if it did impact onto the Earth a body this size would be unlikely to do us serious harm; it would probably be fairly unpleasant for anyone standing directly underneath, but there would be no danger of serious global effects.
See also Fragments of 22 April meteor found in California, Fireball over Nevada and California, The Lyrid Meteors, Asteroid 2012 DA14 may pass within 21 000 km of the Earth and How the Moon got its strange magnetic field.
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