Asteroid (418849) 2008 WM64 passed by the Earth at a distance of about 10 760 000 km (28.0 times the average distance between the Earth and the Moon, or 7.19% of the distance between the Earth and the Sun), slightly before 9.30 am GMT on Wednesday 21 December 2016. There was no danger of the asteroid hitting us, though were it to do so it would have presented a considerable threat. (418849) 2008 WM64 has an estimated equivalent diameter of 140-450 m (i.e. it is estimated that a spherical object with the same volume would be 140-450 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 300-176 000 times as powerful as the Hiroshima bomb. Such an impact would result in an impact crater 2-7 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 (418849) 2008 WM64. Minor Planet Center.
(418849) 2008 WM64 was discovered on 24 November 2008 by the University of Arizona's Mt. Lemmon Survey at the Steward Observatory on Mount Lemmon in the Catalina Mountains north of Tucson. The designation 2008 WM64 implies that the asteroid was the 1612th object (object M64) discovered in the second half of November 2008 (period 2008 WM64), while the longer designation 418849indicates that it was the 418 849th 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).
(418849) 2008 WM64 is calculated to have a 368 day orbital period and an elliptical orbit tilted at an angle of 33.5° to the plain of the Solar System that takes it from 0.90 AU from the Sun (i.e. 90% of the average distance at which the Earth orbits the Sun) to 1.11 AU from the Sun (i.e. 111% of the average distance at which the Earth orbits the Sun). 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 July this year and the next predicted in July next year. As an asteroid probably larger than 150 m in diameter that occasionally comes within 0.05 AU of the Earth, (418849) 2008 WM64 is also classified as a Potentially Hazardous Asteroid.