<<2I/Borisov is the first observed interstellar comet and second observed interstellar interloper, after ʻOumuamua. 2I/Borisov has a heliocentric orbital eccentricity of 3.3 and is not bound to the Sun. The comet will pass through the ecliptic of the Solar System in December 2019, with the closest approach to the Sun at just under 2 au on 8 December 2019. The comet was discovered on 30 August 2019 by amateur astronomer Gennadiy Borisov
at his personal observatory MARGO using a 0.65-meter telescope he designed and built himself. The discovery of 2I/Borisov by Borisov has been compared to the discovery of Pluto by Clyde Tombaugh.
Unlike ʻOumuamua which had an asteroidal appearance, Borisov's initial observations and subsequent third-party validation affirmed the presence of a coma around the body, indicating a cloud of dust and gas that would classify the body as a comet. Initial estimates for the size of 2I/Borisov's nucleus, published on 12 September 2019, ranged from 2 to 16 km, based on observations made by Karen Meech at the University of Hawaii. An improved size estimate, based on the production rate of certain molecules in the comet's coma, was published by Alan Fitzsimmons, Karen Meech and others on 26 September. They estimated that the nucleus is between 1.4 and 6.6 km in diameter. On 13 September 2019, the Gran Telescopio Canarias obtained a preliminary (low-resolution) visible spectrum of 2I/Borisov that revealed that this object has a surface composition not very different from that found in typical Oort Cloud comets. Also, the William Herschel Telescope [WHT] located at the island of La Palma, reported the detection of cyanide (formula: CN) emission at 388 nm (this type of emission has been detected in many other comets, including comet Halley)
and put constraints on the production rate of other molecules like diatomic carbon (formula C2
2I/Borisov's trajectory is extremely hyperbolic, having an orbital eccentricity of 3.3 to 3.4. This is much higher than the 300+ known weakly hyperbolic comets, with heliocentric eccentricities just over 1.0, and even ʻOumuamua with an eccentricity of 1.2. 2I/Borisov also has a hyperbolic excess velocity of 32 km/s (6.77 au/yr), much higher than what could be explained by perturbations, which could produce velocities when approaching an infinite distance from the Sun of less than a few km/s. These two parameters are important indicators of 2I/Borisov's interstellar origin. For comparison, the Voyager 1 spacecraft, which is leaving the Solar System, is traveling at 16.9 km/s (3.57 au/yr). 2I/Borisov has a much larger eccentricity than ʻOumuamua due to its higher excess velocity and its significantly higher perihelion distance. At this larger distance, the Sun's gravity is less able to alter its path as it passes through the Solar System.
2I/Borisov entered the Solar System from the direction of Cassiopeia near the border with Perseus and very close to the galactic plane. From September until mid-November the comet is in the northern sky and will cross the celestial equator on 13 November 2019 entering the southern sky. Due to the 44 degree orbital inclination, 2I/Borisov does not make any notable close approaches to the planets. On 6 December 2019, the comet will be an equal distance of 2 au from the Sun and Earth. On 8 December 2019, the comet will come to perihelion (closest approach to the Sun) and will be near the edge of the inner asteroid belt. In late December, it will be about 1.9 au from Earth and have a solar elongation of about 80°. It will leave the Solar System in the direction of Telescopium.
2I/Borisov's velocity (32 km/s relative to the Sun) and the direction from which it entered the Solar System indicates that it originates from the galactic disk of the Milky Way, rather than from the galactic halo. One theoretical possibility is that 2I/Borisov had passed within 1.7 pc of the star Kruger 60
[radial velocity = –33.1/–31.9 km/s] at a low velocity of 3.4 km/s around a million years ago. In interstellar space, 2I/Borisov takes roughly 9000 years to travel a light-year relative to the Sun.
Observations using the Hubble Space Telescope are planned to begin on October 8, when the comet moves far enough from the Sun to be safely observed by the telescope. Hubble is less affected by the confounding effects of the coma than ground-based telescopes, which will allow it to study the rotational light curve of 2I/Borisov's nucleus. This should facilitate an estimate of its size and shape. The observations will serve as a baseline for possible further observations, as the comet approaches perihelion and then leaves the Solar System. In the event that the nucleus disintegrates, as is sometimes seen with small comets, Hubble can be used to study the evolution of the disintegration process.
The higher hyperbolic excess velocity of 2I/Borisov of 32 km/s makes it even harder to reach for a spacecraft than 1I/'Oumuamua (26 km/s). According to a team of the Initiative for Interstellar Studies, a two-ton spacecraft could theoretically have been sent in July 2018 to intercept 2I/Borisov using a Falcon Heavy-class launcher, but only if the object had been discovered much earlier than it was. Launches after the actual discovery date would require a significantly larger launcher such as the Space Launch System (SLS) and Oberth manoeuvres near Jupiter and near the Sun. By September 2019, even an SLS-class launcher would only be able to deliver a 3 kg (6.6 lb) payload (such as a CubeSat) into a trajectory that could intercept 2I/Borisov in 2045 at a relative speed of 34 km/s. According to congressional testimony, NASA may need at least five years of preparation to launch such an intercepting mission.>>