Asteroid 2017 YH1 passed by the Earth at a distance of about 916 400 km (2.38 times the average distance between the Earth and the Moon, or 0.61% of the distance between the Earth and the Sun), slightly before 0.30 am GMT on Monday 18 December 2017. There was no danger of the asteroid hitting us, though were it to do so it would have presented a significant threat. 2017 YH1 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 at the upper end of this size range 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 600-120 000 times as powerful as the Hiroshima bomb. Such an impact would result in an impact crater of 1.5-5.0 km in diameter and devastation on a global scale, as well as climatic effects that would last years or even decades.
The calculated orbit of 2017 YH1. Minor Planet Center.
2017 YH1 was discovered on 20 December 2017 (two days after its closest approach to the Earth) by the University of Arizona's Catalina Sky Survey, which is located in the Catalina Mountains north of Tucson. The designation 2017 YH1 implies that it was the 33rd asteroid (asteroid H1) discovered in the second half of December 2017 (period 2017 Y).
2017 YH1 has a 892 day orbital period and an eccentric orbit tilted at an angle of 15.7° to the plane of the Solar System, which takes it from 0.86 AU from the Sun (i.e. 86% of he average distance at which the Earth orbits the Sun) to 2.76 AU from the Sun (i.e. 276% of the average distance at which the Earth orbits the Sun, and considerably more distant from the Sun than the planet Mars). 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 the asteroid has occasional close encounters with the Earth, with the next predicted in January 2052. As an asteroid probably larger than 150 m in diameter that occasionally comes within 0.05 AU of the Earth, 2017 YH1 is also classified as a Potentially Hazardous Asteroid.
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