Asteroid 2019 RT3 passed by the Earth at a distance of about 555 800 km (1.45 times the average distance between the Earth and the Moon, or 0.37% of the distance between the Earth and the Sun), slightly after 4.10 am GMT on Monday 9 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 RT3 has an estimated equivalent diameter of 18-57 m (i.e. it is estimated that a spherical object with the same volume would be 18-57 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 25 and 10 km above the ground, with only fragmentary material reaching the Earth's surface.
2019 RT3 was discovered on 11 September 2019 (two days after its closest approach to the Earth) by the Atlas MLO Telescope at Mauna Loa Observatory in Hawaii. The designation 2019 RT3 implies that the asteroid was the 91st object (bject T3 - in numbering asteroids the letters A-Z, excluding I, are assigned numbers from 1 to 25, so that T3 = (24 x 3) + 19 = 91) discovered in the first half of September 2019 (period 2019 R).
2019 RT3 is calculated to have an 904 day orbital period and an eccentric orbit tilted at an angle of 4.35° to the plane of the Solar System, which takes it from 0.99 AU from the Sun (i.e.99% of the the average distance at which the Earth orbits the Sun) to 2.67 AU from the Sun (i.e. 267% of the average distance at which the Earth orbits the Sun, outside 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). 2019 RT3 is thought to have had one previous close encounter with the Earth, on 13 October 2014.
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