Asteroid 1998 XN2 passed by the Earth at a distance of 11 610 000 km (30.2 times the average distance between the Earth and the Moon, or 7.76% of the average distance between the Earth and the Sun), slightly before 2.55 pm GMT on Sunday 25 October 2015. There was no danger of the asteroid hitting us, though had it done so it would have presented a considerable threat. 1998 XN2 has an estimated equivalent diameter of 230-710 m (i.e. it is estimated that a spherical object with the same volume would be 230-710 m in diameter), and an object of this size would pass through the atmosphere and directly impact the ground with a force of about 400-1600 megatons (roughly 23 500 to 94 000 times the explosive energy of the Hiroshima bomb), causing devastation over a wide area and creating a crater about 3.5-12 kilometers across, and resulting in global climatic problems that could last for decades or even centuries.
The calculated orbit of 1996 XN2. JPL Small Body Database.
1998 XN2 was discovered on 9 December 1998 by the Massachusetts Institute of Technology's Lincoln Near Earth Asteroid Research Laboratory in Socorro, New Mexico. The designation 1998 XN2 implies that it was the 2nd asteroid (asteroid 2) discovered in the first half of December 1998 (period 1998 X).
1998 XN2 has a 1033 day orbital period and an eccentric orbit tilted at an angle of 1.78° to the plane of the Solar System, which takes it from 0.92 AU from the Sun (i.e. 92% of the average distance at which the Earth orbits the Sun) to 3.08 AU from the Sun (i.e. 308% of the average distance at which the Earth orbits the Sun, slightly outside twice the orbit of 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 1998 and the next predicted in August 2032. As an asteroid probably larger than 150 m in diameter that occasionally comes within 0.05 AU of the Earth, 1998 XN2 is also classified as a Potentially Hazardous Asteroid.
See also...
Asteroid (445830) 2012 CL19 passed by the Earth at a distance of 6 687 000 km (17.4 times the average distance between the Earth and the Moon, or 4.47% of the average distance between the Earth and the...
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 (1566) 1949 MA Icarus passed by the Earth at a distance of 8 054 000 km (20.9 times the average distance between the Earth and the Moon, or 5.38 % of the average distance between the Earth and the...
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