Asteroid (242191) 2003 NZ6 passed by the Earth at a distance of 12 470 000 km (32.4 times the average distance between the Earth and the Moon, or 8.33% of the average distance between the Earth and the Sun), at about 5.55 pm GMT on Saturday 18 July 2015. There was no danger of the asteroid hitting us, though had it done so it would have presented a considerable threat. (242191) 2003 NZ6 has an estimated equivalent diameter of 300-940 m (i.e. it is estimated that a spherical object with the same volume would be 300-940 m in diameter), and an object of this size would pass through the atmosphere and directly impact the ground with a force of about 1110-1 45 000 megatons (roughly 65 300 to 2 650 000 times the explosive energy of the Hiroshima bomb), causing devastation over a wide area and creating a crater about 4.5-13 kilometers across, and resulting in global climatic problems that could last for decades or even centuries.
The calculated orbit of (242191) 2003 NZ6. JPL Small Body Database.
(242191) 2003 NZ6 was discovered on 9 July 2003 by the Massachusetts Institute of Technology's Lincoln Near Earth Asteroid Research Laboratory in Socorro, New Mexico. The designation 2003 NZ6 implies that it was the 175th asteroid (asteroid Z6) discovered in the first half of July 2003 (period 2003 Z), while the designation 242191 implies that it was 242 191st asteroid ever discovered (asteroids are not given this longer designation immediately to avoid naming double or false sightings).
(242191) 2003 NZ6 has a 258 day orbital period, with an elliptical orbit tilted at an angle of 18.2° to the plain of the Solar System which takes in to 0.43 AU from the Sun (43% of the distance at which the Earth orbits the Sun, slightly outside the orbit of Mecury) and out to 1.18 AU (18% 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 May 2012 and the next predicted in April 2017. Although it does cross the Earth's orbit and is briefly further from the Sun on each cycle, (242191) 2003 NZ6 spends most of its time closer to the Sun than we are, and is therefore classified as an Aten Group Asteroid. This also means that close encounters between (242191) 2003 NZ6 and Venus are also quite common, with the last having occurred in April 2012 and the next predicted for August this year.
See also...
Asteroid (385186) 1994 AW1 passed by the Earth at a distance of 9 725 000 km (25.3 times the average distance between the Earth and the Moon, or 6.50% of the average distance between the Earth and the...
Asteroid 2011 YC29 passed by the Earth at a distance of 9 438 000 km (24.5 times the average distance between the Earth and the Moon, or 6.31% of the average distance between the Earth and the Sun), at about 10.10 am GMT on Wednesday 15 July...
Asteroid 2015 JH2 passed by the Earth at a distance of 11 830 000 km (30.8 times the average distance between the Earth and the Moon, or 7.91 % of the average distance between the Earth and the Sun), slightly before 2.40 am GMT on Thursday 9 July...
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