Asteroid 2015 CH13 passed by the Earth at a distance of 282 400 km (0.73 times the average distance between the Earth and the Moon, which is to say inside the orbit of the Moon, though the Moon was on the opposite side of the Earth at the time, or 0.19 % of the average distance between the Earth and the Sun), slightly after 6.15 am GMT on Wednesday 11 February 2015. There was no danger of the asteroid hitting us, though had it done so it would have presented only a minor threat. 2015 CH13 has an estimated equivalent diameter of 3-11 m (i.e. it is estimated that a spherical object with the same volume would be 3-11 m in diameter), and an object of this size would be expected to break up in the atmosphere between 45 and 30 km above the ground, with only fragmentary material reaching the Earth's surface.
The calculated orbit of 2015 CH13. JPL Small Body Database.
2015 CH13 was discovered on 12 February 2015 (the day after its closest approach to the Earth) by the University of Hawaii's PANSTARRS telescope on Mount Haleakala on Maui. The designation 2015 CH13 implies that it was the 333rd asteroid (asteroid H13) discovered in the first half of February 2015 (period 2015 C).
2015 CH13 has an 729 day orbital period and an eccentric orbit tilted at an angle of 2.98° to the plane of the Solar System, which takes it from 0.70 AU from the Sun (i.e. 70% of the average distance at which the Earth orbits the Sun, slightly inside the orbit of Venus) to 2.46 AU from the Sun (i.e. 246% of the average distance at which the Earth orbits the Sun, considerably more than the disctance 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). This means that close encounters between the asteroid and Earth are extremely common, with the last having occurred in February 2013 and the next predicted in March 2017. The asteroid also has fairly frequent encounters with the planets Venus, which it last made a close fly-by off in December 2008, and Mars, which it last made a close approach to in June 1995. Orbits which take asteroids close to multiple planets are considered to be highly unstable, and often result in the asteroid eventually being knocked onto a new orbital trajectory which often results in it either colliding with a planet or the Sun or being ejected from the Solar System altogether.
See also...
Asteroid 2015 BP4 passed by the Earth at a distance of 15 770 000 km (40.96 times the average distance between the Earth and the Moon, or 10.5 % of the average distance between the Earth and the Sun), at about 3.15 am GMT on Wednesday 11 February...
Asteroid 2015 BM510 passed by the Earth at a distance of 5 311 000 km (13.82 times the average distance between the Earth and the Moon, or 3.5 % of the average distance between the Earth and the Sun), slightly after 9.00 pm GMT on Tuesday 10 February...
Asteroid 2013 BS45 passed by the Earth at a distance of 13 380 000 km (34.8 times the average distance between the Earth and the Moon, or 8.9 % of the average distance between the Earth and the Sun), slightly after 11.15 am GMT on Monday 9 February...
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