Saturday 28 January 2017

Asteroid 2017 BR5 passes the Earth.

Asteroid 2017 BR5 passed by the Earth at a distance of 811 000 km (2.11 times the average distance between the Earth and the Moon, 0.54% of the average distance between the Earth and the Sun), slightly before 5.30 am GMT on Sunday 22 January 2017. There was no danger of the asteroid hitting us, though had it done so it would have presented no threat. 2017 BR5 has an estimated equivalent diameter of 5-18 m (i.e. it is estimated that a spherical object with the same volume would be 5-18 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 40 and 25 km above the ground, with only fragmentary material reaching the Earth's surface.

The calculated orbit of 2017 BR5. Minor Planet Center.
 three days after
2017 BR5 was discovered on 25 January 2017 (eight days before its closest approach to the Earth) by the Defense Advanced Research Projects Agency's Space Surveillance Telescope. The designation 2017 BR5 implies that it was the 142nd asteroid (asteroid R5) discovered in the second half of January 2017 (period 2017 B).

2017 BR5 is calculated to have a 833 day orbital period and an elliptical orbit tilted at an angle of 5.39° to the plain of the Solar System that takes it from 0.89 AU from the Sun (i.e. 89% of the average distance at which the Earth orbits the Sun) to 2.57 AU from the Sun (i.e. 257% of the average distance at which the Earth orbits the Sun, considerably more than 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 the asteroid has occasional close encounters with the Earth, with the last having occurred in February 1949.

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