Tuesday 4 April 2017

Asteroid 2015 TC25 passes the Earth.

Asteroid 2015 TC25 passed by the Earth at a distance of  2 935 000 km (7.63 times the average distance between the Earth and the Moon, 1.96% of the average distance between the Earth and the Sun), slightly before 2.55 pm GMT on Wednesday 26 March 2017. There was no danger of the asteroid hitting us, though had it done so it would have presented no threat. 2015 TC25 has an estimated equivalent diameter of 2 m (i.e. it is estimated that a spherical object with the same volume would be 2 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 more than 45 km above the ground, with only fragmentary material reaching the Earth's surface.

 Image of 2015 TC25 made from Happy Jack, Arizona in 2015. Discovery Channel Telescope/Lowell Observatory/North Arizona University.

2015 TC25 was discovered on 11 October 2015 by the University of Arizona's Catalina Sky Survey, which is located in the Catalina Mountains north of Tucson. The designation 2015 TC25 implies that it was the 628th asteroid (asteroid C25) discovered in the first half of October 2015 (period 2015 T).

2015 TC25 has a 382 day orbital period and an eccentric orbit tilted at an angle of 3.63° to the plane of the Solar System, which takes it from 0.91 AU from the Sun (i.e. 91% of he average distance at which the Earth orbits the Sun) to 1.15 AU from the Sun (i.e. 115% of the average distance at which the Earth 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 the asteroid and Earth are common, with the last having occurred in July 2015 and the next predicted in April 2018.   

The calculated orbit of 2015 TC25. Minor Planet Center.

Unusually for such a small body, 2015 TC25 was observed in detail by a number of instruments during a close flyby in October 2015, when it came within the orbitt of the Moon. On this occasion observations made by the 4.3-m Lowell Discovery Channel Telescope, 3-m NASA Infrared Telescope Facility and 2.4m Magdalena Ridge Observatory Telescope were able to determine a number of features of the asteroid.

2015 TC25 was found to have an irregular shape and a  very high rotational speed, completing one rotation every 133.8 seconds. An object this small with such a high rotation rate will almost certainly be a single boulder without any covering of regolith, since the centrefugal force generated by its spinning would overcome its week gravity and remove any such debris on its surface. 

 It also has a very high albedo (i.e. it reflects a lot of light), consistent with its belonging to the E-type Asteroid class. E-type Asteroids have the highest albedoes of any asteroid class, and have been linked to enstatite achondrite meteorites (aubrites), meteorites comprised largely of the light-coloured  mineral orthopyroxine (which is otherwise associated with volcanic rocks), with a dark fusion crust. Most known E-type Asteroids are found in the Hungaria region (about 1.9 AU from the Sun), though there is also a scattering of these objects in the Inner Main Belt, between 2.1 and 2.7 AU from the Sun.

Based upon their spectra E-type Asteroids are divided into Nysa-like asteroids, Angelina-like asteroids and Hungaria-like asteroids, after the asteroids (44) Nysa, (64) Angelina and (434) Hungaria. Comparison of the spectra of 2015 TC25 to these three bodies shows that it is closest to (44) Nysa, though it is distinct from that body. In a paper published on the online arXiv database at Cornell University Library, on 1 December 2016, a team of scientists led by Vishnu Reddy of the Lunar and Planetary Laboratory at the University of Arizona, suggest that 2015 TC25 may in fact be a detached boulder from (44) Nysa, and that the difference in the specra of the two bodies may reflect the lack of regolith on 2015 TC25.

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