Asteroid 2019 UC passed by the Earth at a distance of about 1 120 000 km (2.92 times the average distance between the Earth and the Moon, or 0.75% of the distance between the Earth and the Sun), slightly before 1.30 am GMT on Tuesday 29 October 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 UC has an estimated equivalent diameter of 27-87m (i.e. it is estimated that a spherical object with the same volume would be 27-87 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 18 and 0.5 km above the ground, with only fragmentary material reaching the Earth's surface, although, since an object at the upper end of the range would be expected to explode with a force equivalent to 30 megatons of TNT, being directly underneath it would probably be quite unpleasant.
2019 UC was discovered on 18 October 2019 (eleven days after its closest approach to the Earth) by the Atlas MLO Telescope at Mauna Loa Observatory in Hawaii. The designation 2019 UC implies that the asteroid was the third object (object C - in numbering asteroids the letters A-Z, excluding I, are assigned numbers from 1 to 25, so that E4 = (24 x 0) + 3 = 3) discovered in the second half of October 2019 (period 2019 U).
2019 UC has a 666 day orbital period and an eccentric orbit tilted at an angle of 2.82° to the plane of the Solar System, which takes it from 0.88 AU from the Sun (i.e. 80% of he average distance at which the Earth orbits the Sun) to 2.10 AU from the Sun (i.e. 210% of the average distance at which the Earth orbits the Sun, beyond 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 2019 UC occasionally comes close to the Earth, with the last such encounter having happened in February 2009, and the next predicted for March 2020. 2019 UC also has occasional close encounters with the planet Mars, which it last came close to in August 1988 and is predicted to pass again in April 2031.
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