Asteroid 2018 SD2 passed by the Earth at a distance of about 87 860 km (0.30 times the average distance between the Earth and the Moon, or 0.006% of the distance between the Earth and the Sun), slightly after 6.05 am GMT on Tuesday 25 September 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 SD2 has an estimated equivalent diameter of 3-11 m (i.e. it is estimated that a spherical object with the same volume would be 3-11 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 50 and 30 km above the ground, with only fragmentary material reaching the Earth's surface.
2018 SD2 was discovered on 21 September 2018 (four days before its closest approach to the Earth) by the Atlas MLO Telescope at Mauna Loa Observatory in Hawaii. The designation 2018 SD2 implies that the asteroid was the 54th object (object D2) discovered in the second half of September 2018 (period 2018 S).
2018 SD2 has a 318 day orbital period, with an elliptical orbit tilted at an angle of 0.25° to the plain of the Solar System which takes in to 0.78 AU from the Sun (78% of the distance at which the Earth orbits the Sun, and slightly more than the distance at which Venus orbits the Sun) and out to 1.04 AU (4% further away from the Sun than the Earth). This means that close encounters between the asteroid and Earth are fairly common, with the last thought to have happened in 18 May this year and the next predicted in July 2026. Although it does cross the Earth's orbit and is briefly further from the Sun on each cycle, 2018 SD2 spends most of its time closer to the Sun than we are, and is therefore classified as an Aten Group Asteroid. This also means that the asteroid has occasional close encounters with the planet Venus, with the last calculated to have occurred in September 2014, and the next predicted for November 2020.
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