Asteroid 2017 TA passed by the Earth at a distance of about 1 021 000 km (2.66 times the average distance between the Earth and the Moon, or 0.68% of the distance between the Earth and the Sun), slightly after 9.30 pm GMT on Friday 6 October 2017. There was no danger of the asteroid hitting us, though were it to do so it would have presented a real threat. 2017 TA has an estimated equivalent diameter of 21-85 m (i.e. it is estimated that a spherical object with the same volume would be 21-85 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, and 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), less than a kilometre above the ground, in an explosion that would be 1650 times as powerful as the Hiroshima bomb. This would not be large enough to cause global effects, but would be pretty unpleasant for anyone directly underneath.
The calculated orbit of 2017 TA. Minor Planet Center.
2017 TA was discovered on 3 October 2017 (three 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 TA implies that the asteroid was the first object (object A) discovered in the first half of October 2017 (period 2017 T).
2017 TA has a 540 day orbital period and an eccentric orbit tilted at an angle of 19.8° 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, slightly inside the orbit of Venus) to 1.60 AU from the Sun (i.e. 160% of the average distance at which the Earth orbits the Sun, slightly 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). Close encounters between the 2017 TA and Earth are common, with the last thought to have happened in October 2014 next predicted in September 2020.
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