2I/Borisov
2I/Borisov, originally designated C/2019 Q4 (Borisov), is the first observed rogue comet and the second observed interstellar interloper after ʻOumuamua. It was discovered by the Crimean amateur astronomer and telescope maker Gennadiy Borisov on 29 August 2019 UTC (30 August local time).
Borisov in October 2019 | |
| Discovery | |
|---|---|
| Discovered by | Gennadiy Borisov |
| Discovery site | MARGO observatory, Nauchnyy, Crimea |
| Discovery date | 30 August 2019 Earliest precovery: 13 December 2018 |
| Orbital characteristics | |
| Epoch | 5 January 2020 |
| Observation arc | 389 days |
| Number of observations | 1191 |
| Perihelion | 2.00662±0.00002 AU |
| Semi-major axis | −0.85132±0.00007 AU |
| Eccentricity | 3.3570±0.0002 (JPL) 3.357 (MPC) |
| Inclination | 44.0535°±0.0001° |
| Last perihelion | 8 December 2019 |
| Next perihelion | interstellar object |
| Earth MOID | 1.09302 AU |
| Jupiter MOID | 2.388 AU |
| Physical characteristics | |
| Dimensions | ≤ 0.5 km(for an albedo of 0.04) ≤ 0.4 km |
2I/Borisov has a heliocentric orbital eccentricity of 3.36 and is not bound to the Sun. The comet passed through the ecliptic of the Solar System at the end of October 2019, and made its closest approach to the Sun at just over 2 AU on 8 December 2019. The comet passed closest to Earth on 28 December 2019. In November 2019, astronomers from Yale University said that the comet's tail was 14 times the size of Earth, and stated, "It's humbling to realize how small Earth is next to this visitor from another solar system." In the middle of March, 2020, the comet was observed to fragment; and later, in April, even more evidence of fragmentation was reported.