Asteroid 2015 HQ171 passed by the Earth at a distance of 446 400 km (1.16 times the average distance between the Earth and the Moon, or 2.98% of the average distance between the Earth and the Sun), slightly after 3.50 am GMT on Saturday 2 May 2015. There was no danger of the asteroid hitting us, though had it done so it would have presented only a minor threat. 2015 HQ171 has an estimated equivalent diameter of 6-21 m (i.e. it is estimated that a spherical object with the same volume would be 7-25 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 36 and 20 km above the ground, with only fragmentary material reaching the Earth's surface.
Image of 2015 HQ171 taken on 30 April 2015 from Ceccano in Italy. The asteroid is the point indicated by the two lines at right angles in the center of the picture. The longer lines are stars, their elongation being caused by the telescope traking the asteroid over the length of the exposure, in this case 120 seconds. Gianluca Masi/Virtual Telescope.
2015 HQ171 was discovered on 29 April 2015 (two days before its closest approach to the Earth) by the University of Arizona's Catalina Sky Survey, which is located in the Catalina Mountains north of Tucson. The designation 2015 HQ171 implies that it was the 4291st asteroid (asteroid Q171) discovered in the secondt half of April 2015 (period 2015 H).
The calculated orbit of 2015 HQ171. JPL Small Body Database.
2015 HQ171 has an 465 day orbital period and an eccentric orbit tilted at an angle of 8.80° to the plane of the Solar System, which takes it from 0.86 AU from the Sun (i.e. 86% of the average distance at which the Earth orbits the Sun) to 1.49 AU from the Sun (i.e. 1.49% of the average distance at which the Earth orbits the Sun, slightly less than the average distance at which the planet Mars orbits the Sun, but still further from the Sun than Mars at its closest). 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 also means that close encounters between 2015 HQ171 and the Earth are quite common, with the last having occurred in October 2006 next predicted for December this year.
See also...
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