Sunday 15 January 2017

Asteroid 2017 AS4 passes the Earth.

Asteroid 2017 AS4 passed by the Earth at a distance of 561 900 km (1.46 times the average distance between the Earth and the Moon, 0.38% of the average distance between the Earth and the Sun), at about 5.05 pm GMT on Sunday 8 January 2017. There was no danger of the asteroid hitting us, though had it done so it would have presented no threat. 2017 AS4 has an estimated equivalent diameter of 8-26 m (i.e. it is estimated that a spherical object with the same volume would be 8-26 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 37 and 20 km above the ground, with only fragmentary material reaching the Earth's surface. 

The calculated orbit of 2017 AS4. Minor Planet Center.

2017 AS4 was discovered on 5 January 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 AS4 implies that the asteroid was the 118th object (object S4) discovered in the first half of January 2017 (period 2017 A).

2017 AS4 is calculated to have a 1184 day orbital period and an elliptical orbit tilted at an angle of 1.95° to the plain of the Solar System that takes it from 0.69 AU from the Sun (i.e. 90% of the average distance at which the Earth orbits the Sun, slightly inside the orbit if the planet Venus) to 3.68 AU from the Sun (i.e. 368% of the average distance at which the Earth orbits the Sun, considerably more than twice the distance at which the planet Mars orbits the Sun). 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 do happen occasionally, with the next predicted in January 2068.

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