Asteroid 2007 AG passed by the Earth at a distance of about 8 747 000 km (22.7 times the average distance between the Earth and the Moon, or 5.85% of the distance between the Earth and the Sun), slightly before 11.45 pm GMT on Tuesday 26 December 2017. There was no danger of the asteroid hitting us, though were it to do so it would have presented a significant threat. 2007 AG has an estimated equivalent diameter of 180-570 m (i.e. it is estimated that a spherical object with the same volume would be 180-570 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 1 175 000 to 8 825 000 times as powerful as the Hiroshima bomb. Such an impact would result in an impact crater 2.5-8.0 km in diameter and devastation on a global scale, as well as climatic effects that would last decades or even centuries.
The calculated orbit of 2007 AG. Minor Planet Center.
2007 AG was discovered on 8 January 2007 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 2007 AG implies that the asteroid was the seventh object (object G) discovered in the first half of January 2007 (period 2007 G).
2007 AG has a 223 day orbital period, with an elliptical orbit tilted at an angle of 11.9° to the plain of the Solar System which takes in to 0.45 AU from the Sun (45% of the distance at which the Earth orbits the Sun; slightly outside the orbit of the planet Mercury) and out to 0.99 AU (99% of the distance at which the Earth orbits the Sun). This means that close encounters between the asteroid and Earth are fairly common, with the last thought to have happened in January 2015 and the next predicted in December 2020. Although it does cross the Earth's orbit and is briefly further from the Sun on each cycle, 2007 AG 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, 2007 AG4 is also classified as a Potentially Hazardous Asteroid.
2007 AG also has frequent close encounters with the planets Mercury, which it is thought to have last passed in February this year, and is next predicted to pass in January 2025, and Venus, which it is next predicted to pass in June 2059. 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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