Asteroid 2017 XY61 passed by the Earth at a distance of about 964 300 km (2.51 times the average distance between the Earth and the Moon, or 0.64% of the distance between the Earth and the Sun), slightly after 9.40 am GMT on Tuesday 19 December 2017. There was no danger of the asteroid hitting us, though were it to do so it would not have presented a significant threat. 2017 XY61 has an estimated equivalent diameter of 11-34 m (i.e. it is estimated that a spherical object with the same volume would be 11-34 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 30 and 10 km above the ground, with only fragmentary material reaching the Earth's surface.
2017 XY61 was discovered on 15 December 2017 (four days before its closest approach to the Earth) by the University of Arizona's Mt. Lemmon Survey at the Steward Observatory on Mount Lemmon in the Catalina Mountains north of Tucson. The designation 2017 XY61 implies that the asteroid was the 1549th object (object Y61) discovered in the first half of December 2017 (period 2017 X).
2017 XY61 has a 462 day orbital period and an eccentric orbit tilted at an angle of 0.75° to the plane of the Solar System, which takes it from 0.64 AU from the Sun (i.e. 64% of he average distance at which the Earth orbits the Sun, inside the orbit of the planet Venus) to 1.69 AU from the Sun (i.e. 1.69% of the average distance at which the Earth orbits the Sun, more than the distance at which the planet 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 2017 XY61 has occasional close encounters with the Earth, which it last came close to in July 2014 and is next predicted to approach in July 2019. The asteroid also has occasional close encounters with the planet Mars, which is last passed in March 2015.
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