Asteroid 2017 DF passed by the Earth at a distance of 802 400 km (2.09 times the average distance between the Earth and the Moon, 0.54% of the average distance between the Earth and the Sun),slightly before 9.30 am GMT on Sunday 19 February 2017. There was no danger of the asteroid hitting us, though had it done so it would have presented no threat. 2017 DF has an estimated equivalent diameter of 9-28 m (i.e. it is estimated that a spherical object with the same volume would be 9.28 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 17 km above the ground, with only fragmentary material reaching the Earth's surface.
The calculated orbit of 2017 DF. Minor Planet Center.
2017 DF was discovered on 17 Februaryy 2017 (two 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 DF implies that the asteroid was the sixth object (object F) discovered in the second half of February 2017 (period 2017 D).
2017 DF has a 779 day orbital period and an eccentric orbit tilted at an angle of 0.86° to the plane of the Solar System, which takes it from 0.99 AU from the Sun (i.e. 99% of he average distance at which the Earth orbits the Sun) to 2.31 AU from the Sun (i.e. 2.31% of the average distance at which the Earth orbits the Sun, considerably outside the orbit of 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 close encounters between the asteroid and Earth are common, with the last having occurred in April 2000 and the next predicted in January 2036. 2017 DF also has occasional close encounters with the planet Mars, with the next predicted in November 2037.
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