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.
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).
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.
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.
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