Asteroid 2015 TF passed by the Earth at a distance of 11 550 000 km (30.0 times the average distance between the Earth and the Moon, or 7.72% of the average distance between the Earth and the Sun), slightly after 4.45 am GMT on Tuesday 27 October 2015. There was no danger of the asteroid hitting us, though had it done so it would have presented a genuine threat. 2015 TF has an estimated equivalent diameter of 68-220 m (i.e. a spherical body with the same mass would be 68-220 m in diameter), and an object towards the upper end of this range would pass through the atmosphere and directly impact the ground with a force of about 300 megatons (about 17 650 times the explosive energy of the Hiroshima bomb), causing devastation over a wide area and creating a crater over 3 km across, and resulting in global climatic problems that could last for years or even decades.
The calculated orbit of 2015 TF. JPL Small Body Database.
2015 TF was discovered on 2 October 2015 (25 days before its closest approach to the Earth) by the University of Arizona's Kitt Peak-Spacewatch Project at the Steward Observatory in the Catalina Mountains north of Tucson. The designation 2015 TF implies that it was the sixth asteroid (asteroid F) discovered in the first half of October 2015 (period 2015 T).
2015 TF has a 421 day orbital period and an eccentric orbit tilted at an angle of 10.8° to the plane of the Solar System, that takes it from 0.55 AU from the Sun (i.e. 55% of the average distance at which the Earth orbits the Sun, and between the orbits of Venus and Mercury) to 1.65 AU from the Sun (i.e. 165% of the average distance at which the Earth orbits the Sun, slightly over 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 also means that 2015 TF has occasional close encounters with the planets Mars and Venus. Its most recent close approach to Venus is calculated to have occurred in May 1943, with the next predicted for February 2054, and the most recent close encounter with Mars happened in November 1986 with the next predicted for August 2037. Asteroid orbits that have close encounters with multiple planets are considered to be quite unstable, as any perturbations can quickly become magnified, throwing the astroid onto a new orbital path.
See also...
Eyewitnesses across much of northern Europe reported seeing a bright fireball in the sky moving southwest to northeast at about 6.05 pm GMT on Saturday 31 October 2015. The event was seen from the Netherlands, Germany, Denmark, southern Sweden...
Asteroid 1998 XN2 passed by the Earth at a distance of 11 610 000 km (30.2 times the average distance between the Earth and the Moon, or 7.76% of the average distance between the Earth and the Sun), slightly before 2.55 pm GMT on Sunday 25 October...
Asteroid 2015 TA206 passed by the Earth at a distance of 18 560 000 km (48.3 times the average distance between the Earth and the Moon, or 12.4% of the average distance between the Earth and the Sun), slightly after 4.20 pm GMT on Friday 23 October...
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