Asteroid 2011 WK15 passed by the Earth at a distance of 17 210 000 km (44.78 times the average distance between the Earth and the Moon, or 11.5 % of the average distance between the Earth and the Sun), slightly after 1.30 am GMT on Thursday 12 February 2015. There was no danger of the asteroid hitting us, though had it done so it would have presented a considerable threat. 2011 WK15 has an estimated equivalent diameter of 220-680 m (i.e. it is estimated that a spherical object with the same volume would be 220-680 m in diameter), and an object of this size would pass through the atmosphere and directly impact the ground with a force of about 500-15 000 megatons (about 30 000-880 000 times the explosive energy of the Hiroshima bomb), causing devastation over a wide area and creating a crater almost 3-10 kilometers across.
The calculated orbit of 2011 WK15. JPL Small Body Database.
2011 WK15 was discovered on 22 November 2011 by the University of Hawaii's PANSTARRS telescope on Mount Haleakala on Maui. The designation 2011 WK15 implies that it was the 385th asteroid (asteroid K15) discovered in the second half of November 2011 (period 2011 W).
While 2011 WK15 occasionally comes near to the Earth, it does not actually cross our orbital path. It has an elliptical 1027 day orbit, at an angle of 7° to the plane of the Solar System, that takes it from 1.05 AU from the Sun (1.05 times the average distance at which the Earth orbits the Sun), slightly outside our orbit, to 2.93 AU from the Sun, (2.93 times the distance at which the Earth orbits the Sun, almost twice the distance at which the planet Mars orbits the Sun). As a Near Earth Object that remains strictly outside the orbit of the Earth it is classed as an Amor Family Asteroid. This orbit also means that close encounters between 20111 WK15 and the Earth are extremely common, with the last having occured in January 2001 and the next predicted for April 2029.
See also...
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