Asteroid (494999) 2010 JU39 passed by the Earth at a distance of about 9 006 000 km (23.5 times the average distance between the Earth and the Moon, or 6.02% of the distance between the Earth and the Sun), slightly after 3.40 pm GMT on Friday 28 June 2019. There was no danger of the asteroid hitting us, though were it to do so it would have presented a significant threat. (494999) 2010 JU39 has an estimated equivalent diameter of 230-720 m (i.e. it is estimated that a spherical object with the same volume would be 230-720 m in diameter), and an object of this size would be predicted to be capable of passing through the Earth's atmosphere relatively intact, impacting the ground directly with an explosion that would be between about 17 500 and 1 100 000 times as powerful as the Hiroshima bomb. Such an impact would result in an impact crater between 3 and 11 km in diameter and devastation on a global scale, as well as climatic effects that would last decades or even centuries.
(494999) 2010 JU39 was discovered on 9 May 2010 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 2010 JU39 implies that the asteroid was the 956th object (asteroid U39 - in numbering asteroids the letters A-Y, excluding I, are assigned numbers from 1 to 24, with a number added to the end each time the alphabet is ended, so that A = 1, A1 = 25, A2 = 49, etc., which means that U39 = 20 + (24 X 39) = 956) discovered in the first half of May 2010 (period 2010 J).
(494999) 2010 JU39 has a 322 day orbital period, with an elliptical orbit tilted at an angle of 36.2° to the plain of the Solar System which takes in to 0.42 AU from the Sun (42% of the distance at which the Earth orbits the Sun, and slightly more than the distance at which Merculry orbits the Sun) and out to 1.42 AU (42% further away from the Sun than the Earth, and slightly inside the orbit of the planet Mars). This means that close encounters between the asteroid and Earth are fairly common, with the last thought to have happened in February 2017 and the next predicted in January 2025. Although it does cross the Earth's orbit and is briefly further from the Sun on each cycle, (494999) 2010 JU39 spends most of its time closer to the Sun than we are, and is therefore classified as an Aten Group Asteroid. As an asteroid probably larger than 150 m in diameter that occasionally comes within 0.05 AU of the Earth, (494999) 2010 JU39 is also classified as a Potentially Hazardous Asteroid.
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