Asteroid 2008 TC4 passed by the Earth at a distance of about 12 911 000 km (33.6 times the average distance between the Earth and the Moon, or 8.63% of the distance between the Earth and the Sun), slightly before 1.45 am GMT on Tuesday 24 October 2017. There was no danger of the asteroid hitting us, though were it to do so it would have presented a significant threat. 2008 TC4 has an estimated equivalent diameter of 94-300 m (i.e. it is estimated that a spherical object with the same volume would be 94-300 m in diameter), and an object at the upper end of this size range 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 65 000 times as powerful as the Hiroshima bomb. Such an impact would result in an impact crater over 4.6 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 2008 TC4. Minor Planet Center.
2008 TC4 was discovered on 7 October 2007 by the Massachusetts Institute of Technology's Lincoln Near Earth Asteroid Research Laboratory in Socorro, New Mexico. The designation 2008 TC4 implies that it was the 103rd asteroid (asteroid C4) discovered in the first half of October 2008 (period 2008 T).
2008 TC4 has a 252 day orbital period, with an elliptical orbit tilted at an angle of 10.6° to the plain of the Solar System which takes in to 0.35 AU from the Sun (35% of the distance at which the Earth orbits the Sun; inside the orbit of the planet Mercury) and out to 1.21 AU (21% further away from the Sun than the Earth). This means that close encounters between the asteroid and Earth are fairly common, with the last thought to have happened in October 2015 and the next predicted in October 2019. Although it does cross the Earth's orbit and is briefly further from the Sun on each cycle, 2008 TC4 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, 2008 TC4 is also classified as a Potentially Hazardous Asteroid.
2008 TC4 also has frequent close encounters with the planets Mercury, which it is thought to have last passed in April this year, and is next predicted to pass in February 2022, and Venus, which it last came close to in February 2004 and is next predicted to pass in April 2021. 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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