Asteroid 2017 SF20 passed by the Earth at a distance of about 691 000 km (1.80 times the average distance between the Earth and the Moon, or 0.46% of the distance between the Earth and the Sun), slightly after 2.45 pm GMT on Tuesday 26 September 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 SF20 has an estimated equivalent diameter of 4-13 m (i.e. it is estimated that a spherical object with the same volume would be 4-13 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 20 km above the ground, with only fragmentary material reaching the Earth's surface.
The calculated orbit of 2017 SF20 Minor Planet Center.
2017 SF20 was discovered on 28 September 2017 (two days after 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 SF20 implies that the asteroid was the 506th object (object F20) discovered in the second half of September 2017 (period 2017 S).
2017 SF20 has a 887 day orbital period and an eccentric orbit tilted at an angle of 3.07° to the plane of the Solar System, which takes it from 0.92 AU from the Sun (i.e. 92% of he average distance at which the Earth orbits the Sun, slightly inside the orbit of Venus) to 2.69 AU from the Sun (i.e. 269% of the average distance at which the Earth orbits the Sun, considerably 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).
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