Saturday 27 November 2021

Asteroid 2021 WP passes the Earth.

Asteroid 2021 WP passed by the Earth at a distance of about 155 600 km (0.48 times the average distance between the Earth and the Moon, or 0.10% of the distance between the Earth and the Sun), slightly after 2.00 pm GMT on Sunday 21 November 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 WP has an estimated equivalent diameter of 5-15 m (i.e. it is estimated that a spherical object with the same volume would be 5-15 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) between 40 and 26 km above the ground, with only fragmentary material reaching the Earth's surface.

The relative positions of 2021 WP and the Earth on 21 November 2021. JPL Small Body Database.

2021 WP was discovered on 24 November 2021 (three days after its closest approach to the Earth) by the Asteroid Terrestrial-impact Last Alert System (ATLAS) search program's HKO Telescope on Mount Haleakala, Hawai'i.  The designation 2021 WP implies it was the fifteenth asteroid (asteroid P; 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 P = 15) discovered in the second half of November 2021 (period 2021 W; the year being split into 24 half-months represented by the letters A-Y, with I being excluded).

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

2021 WP has a 424 day (1.26 year) orbital period, with an elliptical orbit tilted at an angle of 3.81° to the plain of the Solar System which takes in to 0.93 AU from the Sun (93% of the distance at which the Earth orbits the Sun, and slightly inside the orbit of the planet Venus) and out to 1.27 AU (127% 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). Asteroid 2021 WP is calculated to have last passed the Earth in June this year, is predicted to make another close pass of the Earth in January 2029.

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