Asteroid 2018 CZ2 passed by the Earth at a distance of about 11 951 000 km (31.1 times the average distance between the Earth and the Moon, or 7.99% of the distance between the Earth and the Sun), abut 2.00 pm GMT on Friday 2 March 2018. There was no danger of the asteroid hitting us, though were it to do so it would have presented a significant threat. 2018 CZ2 has an estimated equivalent diameter of 100-330 m (i.e. it is estimated that a spherical object with the same volume would be 100-330 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 225 to 90 000 times as powerful as the Hiroshima bomb. Such an impact would result in an impact crater 1.2-5 km in diameter and devastation on a global scale, as well as climatic effects that would last years or even decades.
The calculated orbit of 2018 CZ2. Minor Planet Center.
2018 CZ2 was discovered on 9 February 2018 (21 days before its closest approach to the Earth) by the Palomar Transient Factory at Palomar Observatory. The designation 2018 CZ2 implies that it was the 75th asteroid (asteroid Z2) discovered in the first half of February 2018 (period 2018 C).
2018 CZ2 has a 1175 day orbital period and an eccentric orbit tilted at an angle of 5.41° to the plane of the Solar System, which takes it from 0.72 AU from the Sun (i.e. 72% of he average distance at which the Earth orbits the Sun, roughly the distance at which the planet Venus orbits the Sun) to 3.64 AU from the Sun (i.e. 364% of the average distance at which the Earth orbits the Sun, and more than twice as distant from the Sun than the planet Mars). 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 close encounters between the asteroid and Earth are extremely common, with the last having occurred in July 2005 and the next predicted in August 2021. As an asteroid probably larger than 150 m in diameter that occasionally comes within 0.05 AU of the Earth, 2018 CZ2 is also classified as a Potentially Hazardous Asteroid.
2018 CZ2 also has frequent close encounters with the planets Venus, which it is next predicted to pass in October 2024, and Jupiter which it last came close to in August 2000 and is next predicted to pass in January 2026). Asteroids which make close passes to multiple planets are considered to be in unstable orbits, and are often eventually knocked out of these orbits by these encounters, either being knocked onto a new, more stable orbit, dropped into the Sun, knocked out of the Solar System or occasionally colliding with a planet.
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