Asteroid 2011 YG6 passed by the Earth at a distance of about 17 940 000 km (47.0 times the average distance between the Earth and the Moon, or 12.0% of the distance between the Earth and the Sun), slightly after 5.10 am GMT on Thursday 22 December 2016. There was no danger of the asteroid hitting us, though were it to do so it would have presented a considerable threat. 2011 YG6 has an estimated equivalent diameter of 110-360 m (i.e. it is estimated that a spherical object with the same volume would be 110-360 m in diameter), and an object of this size would be predicted to be capable of passing through the Earth's atmosphere relatively intact, impacting the ground directly with an explosion that would be 600-118 000 times as powerful as the Hiroshima bomb. Such an impact would result in an impact crater 1.5-5 km in diameter and devastation on a global scale, as well as climatic effects that would last decades or even centuries.
The calculated orbit of 2011 YG6. Minor Planet Center.
2011 YG6 was discovered on 24 December 2011 by the University of Arizona's Catalina Sky Survey, which is located in the Catalina Mountains north of Tucson. The designation 2011 YG6 implies that it was the 157th asteroid (asteroid G6) discovered in the second half of December 2011 (period 2011 Y).
2011 YG6 is calculated to have a 459 day orbital period and an elliptical orbit tilted at an angle of 28.1° to the plain of the Solar System that takes it from 0.82 AU from the Sun (i.e. 82% of the average distance at which the Earth orbits the Sun) to 1.51 AU from the Sun (i.e. 151% of the average distance at which the Earth orbits the Sun, slightly more than 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). This means that close encounters between the asteroid and Earth are extremely common, with the last having occurred in December 2011 and the next predicted in December 2021. As an asteroid possibly larger than 150 m in diameter that occasionally comes within 0.05 AU of the Earth, 2011 YG6 is also classified as a Potentially Hazardous Asteroid.