Asteroid 2016 WJ1 passed the Earth at a distance of 8 050 000 km (about 20.9 times the distance between the Earth and the Moon or 5.38% of the distance between the Earth and the Sun), slightly after 5.35 pm GMT on Friday 16 December 2016. There was no danger of the asteroid hitting us, though if were to do so it would present a significant threat. 2016 WJ1 is estimated to be between 100 and 300 m in diameter, and an asteroid of this size would be expected to pass directly through the atmosphere, striking the Earth's surface and creating a crater between 1.2 and 13.6 km in diameter, as well as causing devastation over a wide area and global climatic effects that could last for years or even decades.
The calculated orbit of 2016 WJ1. Minor Planet Center.
2016 WJ1 was discovered on 19 November 2016 (27 Days before its closest approach to the Earth) 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 2016 WJ1 implies that the asteroid was the 34th object (object J1) discovered in the second half of November 2016 (period 2016 WJ1).
2016 WJ1 has a 566 day orbital period and an eccentric orbit tilted at an angle of 2.89° to the plane of the Solar System, which takes it from 0.67 AU from the Sun (i.e. 67% of he average distance at which the Earth orbits the Sun, inside the orbit of the planet Venus) to 2.01 AU from the Sun (i.e. 201% 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 July 2009 this year and the next predicted in March 2020. As an asteroid probably larger than 150 m in diameter that occasionally comes within 0.05 AU of the Earth, 2016 WJ1 is also classified as a Potentially Hazardous Asteroid.
2016 WJ1 also has frequent close encounters with the planets Venus, which it is thought to have last passed in December 1993, and is next predicted to pass in May 2026, and Mars which it last came close to in March 2015 and is next predicted to pass in October 2029). 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.