Asteroid 2018 UL passed by the Earth at a distance of about 220 960 km (0.57 times the average distance between the Earth and the Moon, or 0.14% of the distance between the Earth and the Sun), slightly before 11.35 am GMT on Wednesday 17 October 2018. There was no danger of the asteroid hitting us, though were it to do so it would have presented a significant threat. 2018 UL has an estimated equivalent diameter of 2-8 m (i.e. it is estimated that a spherical object with the same volume would be 2-8 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 35 km above the ground, with only fragmentary material reaching the Earth's surface.
2018 UL was discovered on 18 October 2018 (the day after its closest approach to the Earth) by the University of Arizona's Catalina Sky Survey, which is located in the Catalina Mountains north of Tucson. The designation 2018 UL implies that it was the eleventh asteroid (asteroid L) discovered in the second half of October 2018 (period 2018 U).
2018 UL has a 569 day orbital period and an eccentric orbit tilted at an angle of 0.76° to the plane of the Solar System, which takes it from 0.97 AU from the Sun (i.e. 97% of he average distance at which the Earth orbits the Sun) to 1.72 AU from the Sun (i.e. 172% of the average distance at which the Earth orbits the Sun, and further from the Sun than 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 extremely common, with the last having occurred in January 2006 and the next predicted in January 2020. 2018 UL also has occasional close encounters with the planet Mars, with the last having occurred in December 1952 and the next predicted for November 2036.
Follow Sciency Thoughts on Facebook.