Asteroid 2015 KM18 passed by the Earth at a distance of 8 760 000 km (22.8 times the average distance between the Earth and the Moon, or 5.86% of the average distance between the Earth and the Sun), slightly before 7.55 am GMT on Sunday 17 May 2015. There was no danger of the asteroid hitting us, though had it done so it would have presented only a minor threat. 2015 KM18 has an estimated equivalent diameter of 24-75 m (i.e. it is estimated that a spherical object with the same volume would be 24-75 m in diameter), and an object of this size would be expected to explode in an airburst (an explosion caused by superheating from friction with the Earth's atmosphere, which is greater than that caused by simply falling, due to the orbital momentum of the asteroid) in the atmosphere between 20 and 2 km above the ground, with only fragmentary material reaching the Earth's surface, although since an object at the upper end of this range would be expected to release an amount of energy equivalent to about 20 megatons of TNT (roughly 1200 times the energy released by the Hiroshima bomb), then being directly underneath it might be fairly unpleasant.
The calculated orbit of 2015 KM18. JPL Small Body Database.
2015 KM18 was discovered on 19 May 2015 (two days after its closest approach to the Earth) by the University of Arizona's Catalina Sky Survey, which is located in the Catalina Mountains north of Tucson. The designation 2015 KM18 implies that it was the 462nd asteroid (asteroid M18) discovered in the second half of May 2015 (period 2015 K).
While 2015 KM18 occasionally comes near to the Earth, it does not actually cross our orbital path. It has an elliptical 635 day orbit, at an angle of 18.0° to the plane of the Solar System, that takes it from 1.08 AU from the Sun (1.06 times the average distance at which the Earth orbits the Sun), slightly outside our orbit, to 1.83 AU from the Sun, (1.83 times the distance at which the Earth orbits the Sun and considerably more than 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.
See also...
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Asteroid 2015 HQ171 passed by the Earth at a distance of 446 400 km (1.16 times the average distance between the Earth and the Moon, or 2.98% of the average distance between the Earth and the Sun), slightly after 3.50 am GMT on Saturday 2 May 2015. There was no danger of the asteroid hitting us...
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