Asteroid 2015 FF passed by the Earth at a distance of 1 596 000 km (4.15 times the average distance between the Earth and the Moon, or 10.7% of the average distance between the Earth and the Sun), slightly before 8.30 pm GMT on Friday 20 March 2015. There was no danger of the asteroid hitting us, though had it done so it would have presented only a minor threat. 2015 FF has an estimated equivalent diameter of 9-28 m (i.e. it is estimated that a spherical object with the same volume would be 9-28 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 34 and 20 km above the ground, with only fragmentary material reaching the Earth's surface.
The calculated orbit of 2015 FF. JPL Small Body Database.
2015 FF was discovered on 16 March 2015 (four days before its closest approach to the Earth) by the University of Hawaii's PANSTARRS telescope on Mount Haleakala on Maui. The designation 2015 FF implies that it was the sixth asteroid (asteroid F) discovered in the second half of March 2015 (period 2015 F).
2015 FF has an 522 day orbital period and an eccentric orbit tilted at an angle of 2.03° to the plane of the Solar System, which takes it from 0.84 AU from the Sun (i.e. 84% of the 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, greater than the distance at which the planet Mars orbits the Sun). 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 2015 FF and the Earth are quite common, with the last calculated to have happened in August 2012 and the next predicted for August 2022. Close encounters between 2015 FF and other planets are also thought to be fairly frequent, with the asteroid having last encountered Mars in September 2005 and it being predicted to make a close pass of Venus in January 2021. 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.
See also...
Asteroid 2015 FE passed by the Earth at a distance of 12 010 000 km (31.3 times the average distance between the Earth and the Moon, or 8.03% of the average distance between the Earth and the Sun), at about 4.45 pm GMT on Thursday 19 March 2015...
Asteroid 2015 DG198 passed by the Earth at a distance of 12 300 000 km (32.0 times the average distance between the Earth and the Moon, or 8.22% of the average distance between the Earth and the Sun), slightly before 9.10 pm GMT on Sunday 15...
Asteroid 2015 FS passed by the Earth at a distance of 16 620 000 km (43.4 times the average distance between the Earth and the Moon, or 11.1% of the average distance between the Earth and the Sun), slightly after 11.40 pm GMT on Saturday 14...
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