Saturday 24 April 2021

Asteroid 2021 HN passes the Earth.

Asteroid 2021 HN passed by the Earth at a distance of about 254 200 km (0.66 times the average distance between the Earth and the Moon, or 0.17% of the distance between the Earth and the Sun), slightly before 7.55 am GMT on Monday 19 April 2021. There was no danger of the asteroid hitting us, though were it to do so it would not have presented a significant threat. 2021 HN has an estimated equivalent diameter of 8-25 m (i.e. it is estimated that a spherical object with the same volume would be 8-25 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) betwenn 35 and 18 km above the ground, with only fragmentary material reaching the Earth's surface.


Asteroid 2021 HN imaged on 17 April 2021 from London, England. Image is a single three minute exposure. Asteroid is the point indicated by the red lines, which has moved only slightly over the course of the image gathering, while the longer lines are stars that have moved considerably in the same time. Northolt Branch Observatories/Twitter.

2021 HN was discovered on 17 April 2021 (twi days before its closest approach to the Earth) 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 2021 HN implies that the asteroid was the 13th object (asteroid Z - in numbering asteroids the letters A-Z, excluding I, are assigned numbers from 1 to 25, with a number added to the end each time the alphabet is ended, so that A = 1, A1 = 26, A2 = 51, etc., which means that N = 13) discovered in the second half of April 2021 (period 2021 H - the year being split into 24 half-months represented by the letters A-Y, with I being excluded).

The relative positions of 2021 HN and the Earth on 9 March 2021. JPL Small Body Database.

2021 HN has a 1157 day (3.17 year) orbital period, with an elliptical orbit tilted at an angle of 16.0° to the plain of the Solar System which takes in to 0.79 AU from the Sun (79% of the distance at which the Earth orbits the Sun) and out to 3.52 AU (352% of the distance at which the Earth orbits the Sun, and motre than the distance at which the planet Mars 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). This means that 2021 HN has occasional close encounters with the Earth, with the last thought to have happened in September 1932 and the next predicted in September 2024. 2021 HN also has occasional close encounters with the planet Jupiter, which it last came close to in April 2013.

The orbit and current position of 2021 HN. The Sky Live 3D Solar System Simulator.

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