Asteroid 2017 HJ passed by the Earth at a distance of 135 700 km (0.35 times the average distance between the Earth and the Moon, 0.09% of the average distance between the Earth and the Sun), slightly before 5.45 am GMT on Wednesday 16 April 2017. There was no danger of the asteroid hitting us, though had it done so it would have presented no threat. 2017 HJ has an estimated equivalent diameter of 6-20 m (i.e. it is estimated that a spherical object with the same volume would be 6-20 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 35 and 22 km above the ground, with only fragmentary material reaching the Earth's surface.
The calculated orbit of 2017 HJ. Minor Planet Center.
2017 HJ was discovered on 17 April 2017 (the day of 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 HJ implies that the asteroid was the ninth object (object J) discovered in the second second of April 2017 (period 2017 H).
2017 HJ has a 520 day orbital period and an eccentric orbit tilted at an angle of 0.62° to the plane of the Solar System, which takes it from 0.63 AU from the Sun (i.e. 63% of he average distance at which the Earth orbits the Sun, inside the orbit of the planet Venus) to 1.90 AU from the Sun (i.e. 1.90% 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 September 2012 and the next predicted in February 2020.
2017 HJ also has frequent close encounters with the planets Venus, which it is thought to have last passed in March this year, and is next predicted to pass in October 2031, and Mars which it last came close to in November 2016 and is next predicted to pass in July 2061. 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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