Asteroid 2015 BX509 passed by the Earth at a distance of 13 610 000 km (35.48 times the average distance between the Earth and the Moon, or 9.1% of the average distance between the Earth and the Sun), slightly before 3.25 pm GMT on Thursday 5 February 2015. There was no danger of the asteroid hitting us, though had it done so it would have presented a minor threat. 2015 BX509 has an estimated equivalent diameter of 47-150 m (i.e. it is estimated that a spherical object with the same volume would be 47-150 m in diameter), and an object towards the upper end of this range would be expected to be capable of passing through the atmosphere reasonably intact, impacting the ground in an explosion equivalent to about 71.5 megatonnes of TNT (roughly 4200 times the energy of the Hiroshima bomb) and creating a crater about 2.4 km in diameter. Such an event would cause devastation over a wide area, and could cause climatic problems for decades.
The calculated orbit of 2015 BX509. JPL Small Body Database.
2015 BX509 was discovered on 23 January 2015 (twelve days before its closest approach to the Earth) by the University of Hawaii's PANSTARRS telescope on Mount Haleakala on Maui. The designation 2015 BX509 implies that it was the 12 748th asteroid (asteroid B509) discovered in the second half of January 2015 (period 2015 B).
2015 BX509 has a 1002 day year orbital period and an eccentric orbit tilted at an angle of 7.4° to the plane of the Solar System, which takes it from 0.92 AU from the Sun (i.e. 92% of the average distance at which the Earth orbits the Sun) to 3.00 AU from the Sun (i.e. 300% of the average distance at which the Earth orbits the Sun, about twice the distance at which the planet Mars orbits the Sun). It is therefore classed as an Apollo Group Asteroid (an asteroid that is on average further from the Sun than the Earth, but which does get closer).
See also...
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Asteroid 2013 BZ45 passes the Earth. Asteroid 2013 BZ45 passed by the Earth at a distance of 9 670 000 km (25.16 times the average distance between the Earth and the Moon, or 6.5 % of the average distance between the Earth and the Sun), slightly before 8.30 pm GMT on Tuesday 3 February...
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