Asteroid 2007 YQ56 passed by the Earth at a distance of about 11 441 000 km (29.8 times the average distance between the Earth and the Moon, or 7.65% of the distance between the Earth and the Sun), slightly after 10.05 pm GMT on Tuesday 1 January 2019. There was no danger of the asteroid hitting us, though were it to do so it would have presented a significant threat. 2007 YQ56 has an estimated equivalent diameter of 200-620 m (i.e. it is estimated that a spherical object with the same volume would be 200-620 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 between 15 000 and 5 900 000 times as powerful as the Hiroshima bomb. Such an impact would result in an impact crater between 3 and 8 km in diameter and devastation on a global scale, as well as climatic effects that would last decades or even centuries.
2007 YQ56 was discovered on 31 December 2007 by the Lowell Observatory Near-Earth-Object Search, at Anderson Mesa Station, near Flagstaff, Arizona. The designation 2007 YQ56 implies that it was the 1360th asteroid (asteroid Q56 - in numbering asteroids the letters A-Y, excluding I, are assigned numbers from 1 to 24, with a number added to the end each time the alphabet is ended, so that A = 1, A1 = 25, A2 = 49, etc., so that Q56 = 16 + (56 X 24) = 1360) discovered in the second half of December 2007 (period 2007 Y - the year being split into 24 half-months represented by letters),
2007 YQ56 has an 394 day orbital period and an eccentric orbit tilted at an angle of 37.3° to the plane of the Solar System, which takes it from 0.81 AU from the Sun (i.e. 81% of the the average distance at which the Earth orbits the Sun) to 1.47 AU from the Sun (i.e. 147% of the average distance at which the Earth orbits the Sun, slightly less than the distance at which Mars orbits). 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 July 2014 and the next predicted in June 2025. As an asteroid probably larger than 150 m in diameter that occasionally comes within 0.05 AU of the Earth, 2007 YQ56 is also classified as a Potentially Hazardous Asteroid.
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