Asteroid 2019 RB2 passed by the Earth at a distance of about 424 700 km (1.11 times the average distance between the Earth and the Moon, or 0.28% of the distance between the Earth and the Sun), slightly before 6.14 am GMT on Wednesday 4 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 RB2 has an estimated equivalent diameter of 5-17 m (i.e. it is estimated that a spherical object with the same volume would be 5-17 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 40 and 25 km above the ground, with only fragmentary material reaching the Earth's surface.
2019 RB2 was discovered on 5 September 2019 (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 2019 RB2 implies that it was the 50th asteroid (asteroid B2 - in numbering asteroids the letters A-Y, excluding I, are assigned numbers from 1 to 24, with a number added to the end each time the alphabet is ended, so that A = 1, A1 = 25, A2 = 49, etc., which means that B2 = 2 + (24 X 2) = 50) discovered in the first half of September 2019 (period 2019 R).
2019 RB2 is calculated to have an 904 day orbital period and an eccentric orbit tilted at an angle of 2.70° to the plane of the Solar System, which takes it from 1.00 AU from the Sun (i.e. the the average distance at which the Earth orbits the Sun) to 2.65 AU from the Sun (i.e. 265% 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). This means that close encounters between the asteroid and the Earth are quite common, with the next predicted for June 2024.
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