Asteroid 99942 (2004 MN4) Apophis passes the Earth.

Asteroid 99942 (2004 MN4) Apophis passed by the Earth at a distance of about 16 860 000 km (43.9 times the average distance between the Earth and the Moon, or 11.3% of the distance between the Earth and the Sun), at about 1.15 am GMT on Saturday 6 March 2021. There was no danger of the asteroid hitting us, though were it to do so it would have presented a considerable threat. 99942 (2004 MN4) Apophis has an estimated equivalent diameter of 370 m (i.e. it is estimated that a spherical object with the same volume would be 370 m in diameter), and an object of this size would be predicted to be capable of passing through the Earth's atmosphere relatively intact, impacting the ground directly with an explosion that would be 150 000 times as powerful as the Hiroshima bomb. Such an impact would result in an impact crater 5 km in diameter and devastation on a global scale, as well as climatic effects that would last decades or even centuries.

 

Asteroid 99942 (2004 MN4) Apophis imaged on 29 January 2013 with the CanariCam in imaging mode at the 10.4 m Gran Telescopio CANARIAS. Licandro et al. (2018).

99942 (2004 MN4) Apophis was discovered on 19 June 2004 by the University of Arizona's Kitt Peak-Spacewatch Project at the Steward Observatory in the Catalina Mountains north of Tucson. The designation 2004 MN4 implies that it was the 113th asteroid (asteroid N4 - 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 N4 = (4 x 25) + 13 = 113) discovered in the second half of July 2004 (period 2004 M), while the  longer designation, (99942), indicates that the asteroid was the 99 942nd asteroid ever discovered. Asteroids are not given this longer designation immediately, to avoid duplicate or false sightings. The name 'Apophis' was chosen by the asteroid's discoverers, Roy Tucker, David Tholen, and Fabrizio Bernardi, and is a Greek derivation of the name of the Egyptian God Apep, an enemy of the Sun God Ra. Apep took the form of a Serpent, and dwelt in eternal darkness, attempting to swallow Ra each night, but being held at bay by Set, the God of storms and the desert.

Viewing geometry during the two Herschel observing epochs at phase angles of roughly 60˚ angle before (left) and after opposition (right). Top: calculated observing geometry on basis of the nominal solution in Pravec et al. (2014). L is fixed vector of angular momentum, the Aries sign is the X axis of the elliptical frame, S is a direction to the Sun, and x, y, z are the axes of the asteroid co-rotating coordinate frame (corresponding to the smallest, intermediate and the largest moment of inertia of the body, respectively). Middle: The solar insolation in [W/m²]. Bottom: TPM temperature calculations assuming a Itokawa-like thermal inertia of 600 Jm⁻²s⁻⁰˙⁵K⁻¹. Müller et al (2014).

99942 (2004 MN4) Apophis has a 324 day (0.89 year) orbital period, with an elliptical orbit tilted at an angle of 3.34° to the plain of the Solar System which takes in to 0.75 AU from the Sun (75% of the distance at which the Earth orbits the Sun, and slightly outside the distance at which Venus orbits the Sun) and out to 1.10 AU (10% further away from the Sun than the Earth). Although it does cross the Earth's orbit and is briefly further from the Sun on each cycle, 99942 (2004 MN4) Apophis spends most of its time closer to the Sun than we are, and is therefore classified as an Aten Group Asteroid. As an asteroid probably larger than 150 m in diameter that occasionally comes within 0.05 AU of the Earth, 99942 (2004 MN4) Apophis is also classified as a Potentially Hazardous Asteroid.

Close encounters between the asteroid and Earth are fairly common, with the last thought to have happened in October 2020 and the next predicted in December 2026. 99942 (2004 MN4) Apophis also has occasional close encounters with the planet Venus, which it last came close to in April 2016 and is next predicted to pass in March 2024.

 

Animation of the orbit of 99942 (2004 MN4) Apophis. Wikimedia Commons.

Notably, 99942 (2004 MN4) Apophis is expected to make a particularly close pass of the Earth in April 2021, coming within 40 000 km of the Earth's surface, and having according some calculations, a 2.4% chance of impacting the Earth (this is not universally accepted, with many experts currently believing that there is no chance of 99942 (2004 MN4) Apophis impacting the Earth in 2029). A 2020 paper by a team led by Linda Dimare, of the Space Dynamics Services calculated a semilinear impact corridor for 99942 (2004 MN4) Apophis in 2029, suggesting that, while it is still very unlikely to impact the Earth, should it do so then it would present a serious threat to some ot the most populous areas of Europe and Asia.

 

Top panel. Google Earth 3D visualisation of the semilinear prediction of the 2029 impact regions of Apophis, using the observations available on 27 December 2004. Bottom panel. Monte Carlo prediction of the 2029 possible impact locations on ground of Apophis, using the same observational dataset of the above figure. Dimare et al. (2020).

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