Asteroid 2017 YO3 passed by the Earth at a distance of about 799 300 km (2.08 times the average distance between the Earth and the Moon, or 0.53% of the distance between the Earth and the Sun), at about 2.35 pm GMT on Wednesday 20 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 YO3 has an estimated equivalent diameter of 4-14 m (i.e. it is estimated that a spherical object with the same volume would be 4-14 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 43 and 27 km above the ground, with only fragmentary material reaching the Earth's surface.
The calculated orbit of 2017 YO3. Minor Planet Center.
2017 YO3 was discovered on 12 December 2017 (eight 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 YO3 implies that the asteroid was the 89th object (object O3) discovered in the first half of December 2017 (period 2017 Y).
2017 TO3 has a 240 day orbital period, with an elliptical orbit tilted at an angle of 8.73° to the plain of the Solar System which takes in to 0.49 AU from the Sun (49% of the distance at which the Earth orbits the Sun; slightly outside the orbit of the planet Mercury) and out to 1.02 AU (2% further away from the Sun than the Earth). This means that close encounters between the asteroid and Earth are fairly common, with the last thought to have happened in April 2016 and the next predicted in January 2020. Although it does cross the Earth's orbit and is briefly further from the Sun on each cycle, 2008 TC4 spends most of its time closer to the Sun than we are, and is therefore classified as an Aten Group Asteroid.
2017 YO3 also has frequent close encounters with the planets Mercury, which it is thought to have last passed in September 2013, and is next predicted to pass in January 2032, and Venus, which it last came close to in December 2013 and is next predicted to pass in May 2024. Asteroids which make close passes to multiple planets are considered to be in unstable orbits, and are often eventually knocked out of these orbits by these encounters, either being knocked onto a new, more stable orbit, dropped into the Sun, knocked out of the Solar System or occasionally colliding with a planet.
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