Asteroid 2018 CS1 passed by the Earth at a distance of about 344 000 km (0.90 times the average distance between the Earth and the Moon, or 0.23% of the distance between the Earth and the Sun), at about 2.50 pm GMT on Sunday 4 February 2018. There was no danger of the asteroid hitting us, though were it to do so it would not have presented a significant threat. 2018 CS1 has an estimated equivalent diameter of 4-14 m (i.e. it is estimated that a spherical object with the same volume would be 4-14 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 43 and 27 km above the ground, with only fragmentary material reaching the Earth's surface.
The calculated orbit of 2018 CS1. Minor Planet Center.
2018 CS1 was discovered on 7 February 2018 (three days 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 CT implies that it was the 43rd asteroid (asteroid S1) discovered in the first half of February 2018 (period 2018 C).
2018 CS1 has a 570 day orbital period and an eccentric orbit tilted at an angle of 5.03° to the plane of the Solar System, which takes it from 0.87 AU from the Sun (i.e. 87% of he average distance at which the Earth orbits the Sun) to 1.82 AU from the Sun (i.e. 182% 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 2018 CS1 has occasional close encounters with the Earth, with the last having happened in August 2017.
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