Asteroid 2019 TT1 passed by the Earth at a distance of about 1 115 000 km (2.90 times the average distance between the Earth and the Moon, or 0.75% of the distance between the Earth and the Sun), slightly before 6.35 pm GMT on Sunday 13 October 2019. There was no danger of the asteroid hitting us, though were it to do so it would not have presented a significant threat. 2019 TT1 has an estimated equivalent diameter of 11-36 m (i.e. it is estimated that a spherical object with the same volume would be 11-36 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 30 and 20 km above the ground, with only fragmentary material reaching the Earth's surface.
2019 TT1 was discovered on 3 October 2019 (ten days before its closest encounter with the Earth) by the University of Arizona's Mt. Lemmon Survey at the Steward Observatory on Mount Lemmon in the Catalina Mountains north of Tucson. The designation 2019 TT1 implies that the asteroid was the 43rd object (asteroid T1 - in numbering asteroids the letters A-Y, excluding I, are assigned numbers from 1 to 24, with a number added to the end each time the alphabet is ended, so that A = 1, A1 = 25, A2 = 49, etc., which means that T1 = 19 + (24 X 1) = 46) discovered in the first half of October 2019 (period 2010 T).
2019 TT1 has a 652 day orbital period and an eccentric orbit tilted at an angle of 9.19° to the plane of the Solar System, which takes it from 0.78 AU from the Sun (i.e. 78% of he average distance at which the Earth orbits the Sun) to 2.16 AU from the Sun (i.e. 216% of the average distance at which the Earth orbits the Sun, and further from the Sun as 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). This means that 2019 TT1 occasionally comes close to the Earth, with the last such encounter having happened in September 2002, and the next predicted for March 2027. 2019 TT1 also has occasional close encounters with the planet Venus, with the last having happened in October 2012, and Mars, which it last came close to in October 2001.
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