2I/Borisov

The comet is formally called "2I/Borisov" by the International Astronomical Union (IAU), with "2I" or "2I/2019 Q4" being its designation and "Borisov" being its name, but is sometimes referred to as "Comet Borisov", especially in the popular press. As the second observed interstellar interloper after 1I/ʻOumuamua, it was given the "2I" designation, where "I" stands for interstellar. The name Borisov follows the tradition of naming comets after their discoverers. Before final designation as 2I/Borisov, the object was referred to by other names: • Early orbit solutions suggested that the comet could be a near-Earth object and was thus listed on IAU's Minor Planet Center's (MPC) Near-Earth Object Confirmation Page (NEOCP) as gb00234. • Further refinements after thirteen days of observation made clear the object was a hyperbolic comet, and it was given the designation C/2019 Q4 (Borisov) by the Minor Planet Center on 11 September 2019. A number of other astronomers including Davide Farnocchia, Bill Gray, and David Tholen concluded that the comet was interstellar. • On 24 September 2019 the IAU announced that the Working Group for Small Bodies Nomenclature kept the name Borisov giving the comet the interstellar designation of 2I/Borisov, formally announcing the comet was indeed interstellar. == Characteristics ==

Unlike ʻOumuamua, which had an asteroidal appearance, 2I/Borisov's nucleus was surrounded by a coma, a cloud of dust and gas.) in November 2019 Size and shape Early estimates of nucleus 2I/Borisov diameter have ranged from . 2I/Borisov has, unlike Solar System comets, noticeably shrunk during Solar System flyby, losing at least 0.4% of its mass before perihelion. Also, the amplitude of non-gravitational acceleration places an upper limit of 0.4 km on nucleus size, consistent with a previous Hubble Space Telescope upper limit of 0.5 km. A study with CSA's NEOSSat found a period of 13.2 ± 0.2 days, which is unlikely to be the nuclear spin. Monte Carlo simulations based on the available orbit determinations suggest that the equatorial obliquity of 2I/Borisov could be about 59 degrees or 90 degrees; the latter is favored for the latest orbit determination. Chemical makeup and nucleus structure David Jewitt and Jane Luu estimate from the size of its coma the comet is producing 2 kg/s of dust and is losing 60 kg/s of water. They extrapolate that it became active in June 2019 when it was between 4 and 5 AU from the Sun. 2I/Borisov's composition appears uncommon yet not unseen in Solar System comets, being relatively depleted in water and diatomic carbon (C2), but enriched in carbon monoxide and amines (R-NH2). The molar ratio of carbon monoxide to water in 2I/Borisov tail is 35–105%, resembling the unusual blue-tailed comet C/2016 R2 (PanSTARRS) in contrast to the average ratio of 4% for Solar System comets. 2I/Borisov has also produced a minor amount of neutral nickel emission attributed to an unknown volatile compound of nickel. The nickel to iron abundance ratio is similar to Solar System comets. Trajectory As seen from Earth, the comet was in the northern sky from September until mid-November. It crossed the ecliptic plane on 26 October near the star Regulus, and the celestial equator on 13 November 2019, entering the southern sky. On 8 December 2019, the comet reached perihelion (closest approach to the Sun) and was near the inner edge of the asteroid belt. In late December, it made its closest approach to Earth, 1.9 AU, and had a solar elongation of about 80°. Due to its 44° orbital inclination, 2I/Borisov did not make any notable close approaches to the planets. 2I/Borisov entered the Solar System from the direction of Cassiopeia near the border with Perseus. This direction indicates that it originates from the galactic plane, rather than from the galactic halo. It will leave the Solar System in the direction of Telescopium. In interstellar space, 2I/Borisov takes roughly years to travel a light-year relative to the Sun. 2I/Borisov's trajectory is extremely hyperbolic, having an orbital eccentricity of 3.36. This is much higher than the 300-plus known weakly hyperbolic comets, with heliocentric eccentricities just over 1, and even ʻOumuamua with an eccentricity of 1.2. 2I/Borisov also has a hyperbolic excess velocity (v_\infty) of , 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 . 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. == Observation ==

Discovery The comet was discovered on 30 August 2019 by amateur astronomer Gennadiy Borisov at his personal observatory MARGO in Nauchnij, Crimea, using a 0.65 meter telescope he designed and built himself. Comet chemistry A preliminary (low-resolution) visible spectrum of 2I/Borisov indicated that it was similar to typical Oort Cloud comets. This resembled carbon-chain depleted comets By the end of November 2019, C2 production had dramatically increased, and the C2 to CN ratio reached 0.61, coinciding with the appearance of bright amine (NH2) bands. Estimates for the outgassing of water, based on the detection of atomic oxygen, indicated a rate similar to Solar System comets. In September 2019, neither water nor hydroxyl (OH) lines were directly detected. The first unambiguous detection of OH lines was made on 1 November 2019, and OH production peaked in early December 2019. Suspected nucleus fragmentation The comet came within about 2 AU of the Sun, a distance at which many small comets have been found to disintegrate. The probability that a comet disintegrates strongly depends on the size of its nucleus; Guzik et al. estimated a probability of 10% that this would happen to 2I/Borisov. The Hubble Space Telescope can be used to study the fragmentation of small comet nuclei, The ejection is estimated to have begun around 7 March, and may have occurred during one of the outbursts that occurred near that time. A follow-up study, reported on 6 April 2020, observed only a single object, and noted that the fragment had vanished. Later analysis of the event showed that the ejected dust and fragments had a combined mass of only about 0.1% of the total mass of the nucleus, making it a large outburst rather than a fragmentation. == Exploration ==

The high hyperbolic excess velocity of 2I/Borisov of makes it hard for a spacecraft to reach the comet with existing technology: according to a team of the Initiative for Interstellar Studies, a spacecraft could theoretically have been sent in July 2018 to intercept 2I/Borisov using a Falcon Heavy-class launcher, or on a Space Launch System (SLS)-class booster, but only if the object had been discovered much earlier than it was to meet the optimal launch date. Launches after the actual discovery date would eliminate the possibility to use Falcon Heavy-class rockets, requiring Oberth maneuvres near Jupiter and near the Sun and a larger launch vehicle. Even an SLS-class launcher would only have been able to deliver a payload (such as a CubeSat) into a trajectory that would intercept 2I/Borisov in 2045 at a relative speed of . According to congressional testimony, NASA may need at least five years of preparation to launch such an intercepting mission. == See also ==