Asteroid 2015 OQ21 passed by the Earth at a distance of 567 900 km (1.48 times the average distance between the Earth and the Moon, or 0.38% of the average distance between the Earth and the Sun), at about midnight GMT between Thursday 23 and Friday 24 July 2015. There was no danger of the asteroid hitting us, though had it done so it would have presented only a minor threat. 2015 OQ21 has an estimated equivalent diameter of 4-16 m (i.e. it is estimated that a spherical object with the same volume would be 4-16 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 43 and 25 km above the ground, with only fragmentary material reaching the Earth's surface.
The calculated orbit of 2015 OQ21. JPL Small Body Database.
2015 OQ21 was discovered on 19 July 2015 (four days before its closest approach to the Earth) by the University of Hawaii's PANSTARRS telescope on Mount Haleakala on Maui. The designation 2015 OC21 implies that it was the 528th asteroid (asteroid C21) discovered in the second half of July 2015 (period 2015 O).
2015 OQ21 has a 254 day orbital period, with an elliptical orbit tilted at an angle of 1.33° to the plain of the Solar System which takes in to 0.50 AU from the Sun (50% of the distance at which the Earth orbits the Sun, considerably inside the orbit of Venus) and out to 1.07 AU (7% further away from the Sun than the Earth). This means that close encounters between the asteroid and Earth are fairly common, with the last thought to have happened in April this year and the next predicted in May 2017. Although it does cross the Earth's orbit and is briefly further from the Sun on each cycle, 2015 OQ21 spends most of its time closer to the Sun than we are, and is therefore classified as an Aten Group Asteroid. This also means that close encounters between 2015 OQ21 and Venus are quite common, with the last having occurred in February 2010 and the next predicted for August 2016.
See also...
Asteroid (242191) 2003 NZ6 passed by the Earth at a distance of 12 470 000 km (32.4 times the average distance between the Earth and the Moon, or 8.33% of the average distance between the Earth and the...
Asteroid (385186) 1994 AW1 passed by the Earth at a distance of 9 725 000 km (25.3 times the average distance between the Earth and the Moon, or 6.50% of the average distance between the Earth and the...
Asteroid 2011 YC29 passed by the Earth at a distance of 9 438 000 km (24.5 times the average distance between the Earth and the Moon, or 6.31% of the average distance between the Earth and the Sun), at about 10.10 am GMT on Wednesday 15 July...
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