Interstellar Comet 2I/Borisov will make its closest approach to the Earth on Saturday 28 December 2019, reaching a distance of 1.93 AU from the Earth (193% of the distance between the Earth and the Sun, or 289 753 000 km). At this distance the comet will be not naked eye visible, having a magnitude of slightly over 16.4, (roughly the same as Pluto's moon Charon), in the Constellation of Hydra, which is better observed from the Southern Hemisphere.
Hubble Space Telescope image of Interstellar Comet 2I/Borisov taken on 12 October 2019. David Jewitt/UCLA/ESA/NASA/Wikimedia Commons.
2I/Borisov was initially discovered on 30 August 2019 by telescope maker amateur astronomer Gennadiy Borisov at his private observatory in the Crimea, and given the designation C/2019 Q4 (Borisov), implying the 4th comet discovered in the second half of August 2019 (Period 2019 Q), and that it was discovered by Borisov. When it was subsequently realised that the object was a hyperbolic comet of interstellar origin it was renamed 2I/Borisov, implying that it is the second interstellar object ever discovered.
Interstellar Comet 2I/Borisov is a Parabolic Comet, which is to say a comet passing through the Inner Solar System on a parabolic orbit that will probably not bring it back again. This parabolic trajectory tilted at an angle of 44.1° to the plain of the Solar System, that brought it in to 2.01 AU from the Sun at perihelion (i.e. 2.01 times as far from the Sun as the planet Earth, or further from the Sun than the planet Mars). It is a surprisingly large object, with a coma, or planetary halo, fourteen times the size of the Earth, though this probably implies a nucleus less than 2 km in diameter.
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