The asteroid 1997 WQ23 passed the Earth at a distance of 11 780 000 km (a little under thirty-one times the distance between the Earth and the Moon), slightly before 10.00 pm GMT on Monday 11 November 2013. There was no danger of the asteroid hitting us, though were this not the case it would have presented a considerable threat. 1997 WQ23 is estimated to be between 160 and 520 m in diameter, large enough to punch straight through our atmosphere and impact directly into the Earth's surface. Such an event would be expected to result in a crater 2.6 to 8 km in diameter, and devastation over a much wider area, as well as global climatic effects that could last for decades.
1997 WQ23 was discovered on 29 November 1997 by the the Lincoln Near-Earth Asteroid Research project at the Massachusetts Institute of Technology. The designation 1997 WEQ23 implies that it was the 591st asteroid discovered in the second half of November 1997 (period 1997 W).
1997 WQ23 has a 836 day orbital period and an eccentric orbit that takes it from 0.87 AU from the Sun (i.e. 87% of the average distance at which the Earth orbits the Sun) to 2.6 AU from the Sun (i.e. 260% of the average distance at which the Earth orbits the Sun, considerably outside 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), and due to its large size and Earth-orbit crossing trajectory it is also considered to be a Potentially Hazardous Asteroid.
See also Asteroid 2013 VD17 discovered after passing the Earth, Asteroid 6063 Jason (1984 KB) passes the Earth, Asteroid 138852 (2000 WN10) passes the Earth, Asteroid 2013 VN5 passes the Earth - then gets discovered and Asteroid 2013 UJ9 passes the Earth.
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