Asteroid (474231) 2001 HZ7 passed by the Earth at a distance of about 15 210 000 km (39.6 times the average distance between the Earth and the Moon, or 10.2% of the distance between the Earth and the Sun), slightly before 7.10 am GMT on Saturday 1 April 2017. There was no danger of the asteroid hitting us, though were it to do so it would have presented a considerable threat .(474231) 2001 HZ7 has an estimated equivalent diameter of 210-650 m (i.e. it is estimated that a spherical object with the same volume would be 210-650 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 17 500-760 000 000 times as powerful as the Hiroshima bomb. Such an impact would result in an impact crater 3-8 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 (474231) 2001 HZ7. Minor Planet Center.
(474231) 2001 HZ7 was discovered on 21 April 2001 by the Massachusetts Institute of Technology's Lincoln Near Earth Asteroid Research Laboratory in Socorro, New Mexico. The designation 2001 HZ7 implies that it was the 200th asteroid (asteroid Z7) discovered in the second half of April 2001 (period 2001 H), while the longer designation (474231) indicates that it was the 474 231st asteroid discovered overall (asteroids are not given this longer designation immediately, to ensure that numbered objects are genuine asteroids that have not been previously described).
(474231) 2001 HZ7 has a 650 day orbital period and an eccentric orbit tilted at an angle of 5.42° to the plane of the Solar System, which takes it from 0.73 AU from the Sun (i.e. 73% of he average distance at which the Earth orbits the Sun, slightly more than the distance at which the planet Venus orbits the Sun) to 2.20 AU from the Sun (i.e. 220% of the average distance at which the Earth orbits the Sun, and considerably outside the orbit of 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 December 2009 and the next predicted in October 2018 As an asteroid probably larger than 150 m in diameter that occasionally comes within 0.05 AU of the Earth, (474231) 2001 HZ7 is also classified as a Potentially Hazardous Asteroid.
(474231) 2001 HZ7 also has frequent close encounters with the planets Venus, which it is thought to have last passed in February this year, and is next predicted to pass in November 2034, and Mars which it last came close to in May 1915 and is next predicted to pass in November 2048. 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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