Asteroid 2017 YS1 passed by the Earth at a distance of about 743 100 km (1.93 times the average distance between the Earth and the Moon, or 0.50% of the distance between the Earth and the Sun), at about 4.45 pm GMT on Sunday 24 December 2017. There was no danger of the asteroid hitting us, though were it to do so it would not have presented a significant threat. 2017 YS1 has an estimated equivalent diameter of 3-9 m (i.e. it is estimated that a spherical object with the same volume would be 3-9 m in diameter), and an object of this size would be expected to explode in an airburst (an explosion caused by superheating from friction with the Earth's atmosphere, which is greater than that caused by simply falling, due to the orbital momentum of the asteroid) in the atmosphere more than 30 km above the ground, with only fragmentary material reaching the Earth's surface.
The calculated orbit of 2017 YS1. Minor Planet Center.
2017 YS1 was discovered on 22 December 2017 (two 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 2017 YS1 implies that the asteroid was the 43rd object (object S1) discovered in the second half of December 2017 (period 2017 Y).
2017 YS1 has a 354 day orbital period, with an elliptical orbit tilted at an angle of 5.67° to the plain of the Solar System which takes in to 0.93 AU from the Sun (93% of the distance at which the Earth orbits the Sun) and out to 1.02 AU (2% 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 June this year and the next predicted in December 2018. Although it does cross the Earth's orbit and is briefly further from the Sun on each cycle, 2017 YS1 spends most of its time closer to the Sun than we are, and is therefore classified as an Aten Group Asteroid.
Follow Sciency Thoughts on Facebook.