Asteroid 2019 SU2 passed by the Earth at a distance of about 72 000 km (0.19 times the average distance between the Earth and the Moon, or 0.05% of the distance between the Earth and the Sun), slightly before 2.50 am GMT on Saturday 21 September 2019. There was no danger of the asteroid hitting us, though were it to do so it would not have presented a significant threat. 2019 SU2 has an estimated equivalent diameter of 2-6 m (i.e. it is estimated that a spherical object with the same volume would be 2-6 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 more than 38 km above the ground, with only fragmentary material reaching the Earth's surface.
2019 SU2 was discovered on 22 September 2019 (the day after its closest encounter with 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 2019 SU2 implies that the asteroid was the 68th object (asteroid U2 - 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 U2 = 20 + (24 X 2) = 68) discovered in the second half of September 2019 (period 2010 S).
2019 SU2 has a 1409 day orbital period and an eccentric orbit tilted at an angle of 0.45° to the plane of the Solar System, which takes it from 0.93 AU from the Sun (i.e. 93% of he average distance at which the Earth orbits the Sun) to 3.98 AU from the Sun (i.e. 3.98% of the average distance at which the Earth orbits the Sun, and more that twice as far from the Sun as 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).
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