Asteroid 2011 ES4 passed by the Earth at a distance of about 121 300 km (0.32 times the average distance between the Earth and the Moon, or 0.08% of the distance between the Earth and the Sun, which is 3.4 times the height of satellites in geostationary orbits), slightly after 4.10 pm GMT on Tuesday 1 September 2020. There was no danger of the asteroid hitting us, though were it to do so it would not have presented a significant threat. 2011 ES4 has an estimated equivalent diameter of 16-49 m (i.e. it is estimated that a spherical object with the same volume would be 16-49 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.
The orbit and current position of 2020 QN4. The Sky Live 3D Solar System Simulator.
2011 ES4 was discovered on 2 March 2011 by the University of Arizona's Mt. Lemmon Survey at the Steward Observatory on Mount Lemmon in the Catalina Mountains north of Tucson. The designation 2011 ES4 implies that the asteroid was the 114th object (asteroid S4 - 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 S4 = (24 x 4) + 18 = 114) discovered in the first half of March 2011 (period 2011 E).
2011 ES4 has a 415 day (1.14 year) orbital period, with an elliptical orbit tilted at an angle of 3.37° to the plain of the Solar System which takes in to 0.83 AU from the Sun (0.83% of the distance at which the Earth orbits the Sun) and out to 1.35 AU (1.35% of the distance at which the Earth orbits the sun). 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). 2011 ES4 has had several previous close encounters with the Earth, which it last came close to in December last year (2019); it also came close to the planet Venus in September 2013.
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