Asteroid 2017 SM2 passed by the Earth at a distance of about 309 800 km (0.81 times the average distance between the Earth and the Moon, or 0.21% of the distance between the Earth and the Sun), slightly before 7.35 am GMT on Wednesday 20 September 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 SM2 has an estimated equivalent diameter of 6-21 m (i.e. it is estimated that a spherical object with the same volume would be 6-21 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 between 35 and 20 km above the ground, with only fragmentary material reaching the Earth's surface.
The calculated orbit of 2017 RJ2 Minor Planet Center.
2017 SM2 was discovered on 17 September 2017 (three days before its closest approach to the Earth) by the Atlas MLO Telescope at Mauna Loa Observatory in Hawaii. The designation 2017 SM2 implies that the asteroid was the 62nd object (object M2) discovered in the second half of September 2017 (period 2017 S).
2017 SM2 has a 535 day orbital period and an eccentric orbit tilted at an angle of 6.08° to the plane of the Solar System, which takes it from 0.87 AU from the Sun (i.e. 87% of he average distance at which the Earth orbits the Sun) to 1.70 AU from the Sun (i.e. 170% of the average distance at which the Earth orbits the Sun, and slightly more distant from the Sun than 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). 2017 SM2 also has frequent close encounters with the planet Mars, which it is thought to have last passed in December 1940, and is next predicted to pass in June 2027.
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