Asteroid 2015 SO2 passed by the Earth at a distance of about 9 911 000 km (25.8 times the average distance between the Earth and the Moon, or 6.63% of the distance between the Earth and the Sun), slightly before 3.20 am GMT on Saturday 30 September 2017. There was no danger of the asteroid hitting us, though were it to do so it would have presented a genuine threat. 2015 SO2 has an estimated equivalent diameter of 31-99 m (i.e. it is estimated that a spherical object with the same volume would be 31-99 m in diameter), and an object at the upper end of this size range 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 225 times as powerful as the Hiroshima bomb. Such an impact would result in an impact crater over a kilometre in diameter and devastation on a wide scale, as well as climatic effects that would last years or even decades.
Image of 2015 SO2 (center of field, at intersection of lines), taken with the 60-cm, f/3.3 Cichocki telescope at Črni Vrh Observatory in western Slovenia. Blaž Mikuž/Črni Vrh Observatory.
2015 SO2 was discovered on 21 September 2015 by the Črni Vrh Observatory in western Slovenia. The designation 2015 SO2 implies that it was the 64th asteroid (asteroid O2) discovered in the second half of September 2015 (period 2015 S).
The calculated orbit of 2015 SO2. Minor Planet Center.
2015 SO2 has a 364 day orbital period, with an elliptical orbit tilted at an angle of 9.17° to the plain of the Solar System which takes in to 0.89 AU from the Sun (89% of the distance at which the Earth orbits the Sun) and out to 1.11 AU (11% 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 September 2016 and the next predicted in September 2018. Although it does cross the Earth's orbit and is briefly further from the Sun on each cycle, 2015 SO2 spends most of its time closer to the Sun than we are, and is therefore classified as an Aten Group Asteroid.