2I/Borisov

Early estimates of nucleus 2I/Borisov diameter have ranged from 1.4 to 16 km.[27][28] 2I/Borisov has, unlike Solar System comets, noticeably shrunk during Solar System flyby, losing at least 0.4% of its mass before perihelion.[7] Also, the amplitude of non-gravitational acceleration place an upper limit of 0.4 km on nucleus size, consistent with a previous Hubble Space Telescope upper limit of 0.5 km.[6] The comet did not come much closer to Earth than 300 million km, which prevents using radar to directly determine its size and shape. This could be done using the occultation of a star by 2I/Borisov but an occultation would be difficult to predict, requiring a precise determination of its orbit, and the detection would necessitate a network of small telescopes.[29]

A study using observations from Hubble could not find a variation in the light curve. According to this study the rotational period must be larger than 10 hours.[30] A study with CSA's NEOSSat found a period of 13.2 ± 0.2 day, which is unlikely to be the nuclear spin.[31] 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.[32]

Dave 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.[6] A search of image archives found precovery observations of 2I/Borisov as early as 13 December 2018, but not on 21 November 2018, indicating it became active between these dates.[33]

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).[34] 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.[35]

The 2I/Borisov has also produced a minor amount of neutral nickel emission attributed to an unknown volatile compound of nickel.[36] The nickel to iron abundance ratio is similar to Solar System comets.[37]

Trajectory across sky in cylindrical coordinates, with 30 August 2019 discovery position along with precovery positions marked back to 13 December 2018 which help narrow down the incoming trajectory

Interstellar velocity inbound (${\displaystyle v_{\infty }}$)[lower-alpha 4]

Object	Velocity
1I/2017 U1 (ʻOumuamua)	26.33 km/s 5.55 AU/yr[38]
2I/2019 Q4 (Borisov)	32.2 km/s 6.79 AU/yr[39]

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,[40] 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)[3] and was near the inner edge of the asteroid belt. In late December, it made its closest approach to Earth,[11][14] 1.9 AU, and had a solar elongation of about 80°.[5] Due to its 44° orbital inclination, 2I/Borisov did not make any notable close approaches to the planets.[3] 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.[39][41] It will leave the Solar System in the direction of Telescopium.[39][41] In interstellar space, 2I/Borisov takes roughly 9000 years to travel a light-year relative to the Sun.[lower-alpha 5]

2I/Borisov's trajectory is extremely hyperbolic, having an orbital eccentricity of 3.36.[3][4] This is much higher than the 300+ known weakly hyperbolic comets, with heliocentric eccentricities just over 1, and even ʻOumuamua with an eccentricity of 1.2.[42][lower-alpha 6] 2I/Borisov also has a hyperbolic excess velocity (${\displaystyle v_{\infty }}$) of 32 km/s, 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.[43] These two parameters are important indicators of 2I/Borisov's interstellar origin.[39][44] For comparison, the Voyager 1 spacecraft, which is leaving the Solar System, is traveling at 16.9 km/s (3.57 AU/a).[45] 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.[39]

2I/Borisov — orbit diagrams

Trajectory of Borisov (yellow) crossing the ecliptic plane; 'Oumuamua (red) shown for comparison

Borisov approaches the ecliptic plane between the orbits of Jupiter (pink) and Mars (orange)

Borisov's trajectory and position (white) as of 13 October 2019 (top view)

The comet was discovered on 30 August 2019 by amateur astronomer Gennadiy Borisov at his personal observatory MARGO in Nauchnyy, Crimea, using a 0.65 meter telescope he designed and built himself.[46] The discovery has been compared to the discovery of Pluto by Clyde Tombaugh.[47] Tombaugh was also an amateur astronomer who was building his own telescopes, although he discovered Pluto using Lowell Observatory's astrograph. At discovery, it was inbound 3 AU from the Sun, 3.7 AU from Earth, and had a solar elongation of 38°.[48] Borisov described his discovery thus:[49]

I observed it on August 29, but it was August 30 Greenwich Time.[lower-alpha 7] I saw a moving object in the frame, it moved in a direction that was slightly different from that of main asteroids.[lower-alpha 8] I measured its coordinates and consulted the Minor Planet Center database. Turned out, it was a new object. Then I measured the near-Earth object rating,[lower-alpha 9] it is calculated from various parameters, and it turned out to be 100% – in other words, dangerous. In such cases I must immediately post the parameters to the world webpage for confirmation of dangerous asteroids.[lower-alpha 10] I posted it and wrote that the object was diffuse and that it was not an asteroid, but a comet.

2I/Borisov's interstellar origin required a couple of weeks to confirm. Early orbital solutions based on initial observations included the possibility that the comet could be a near-Earth object 1.4 AU from the Sun in an elliptical orbit with an orbital period of less than 1 year.[22] Later using 151 observations over 12 days, NASA Jet Propulsion Laboratory's Scout gave an eccentricity range of 2.9–4.5 .[55] But with an observation arc of only 12 days, there was still some doubt that it was interstellar because the observations were at a low solar elongation, which could introduce biases in the data such as differential refraction. Using large non-gravitational forces on the highly eccentric orbit, a solution could be generated with an eccentricity of about 1, an Earth minimum orbit intersection distance (MOID) of 0.34 AU (51 million km), and a perihelion at 0.90 AU around 30 December 2019.[56] However, based on available observations, the orbit could only be parabolic if non-gravitational forces (thrust due to outgassing) affected its orbit more than any previous comets.[57] Eventually with more observations the orbit converged to the hyperbolic solution that indicated an interstellar origin and non-gravitational forces could not explain the motion.[23]

The last observations were in July 2020, seven months after perihelion.[3] Observation of 2I/Borisov was aided by the fact that the comet was detected while inbound towards the Solar System. ʻOumuamua had been discovered as it was leaving the system, and thus could only be observed for 80 days before it was out of range. Because of its closest approach occurring near traditional year-end holidays, and the capability to have extended observations, some astronomers have called 2I/Borisov a "Christmas comet".[13][58] Observations using the Hubble Space Telescope began on 12 October, when the comet moved far enough from the Sun to be safely observed by the telescope.[59] 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.

A preliminary (low-resolution) visible spectrum of 2I/Borisov was similar to typical Oort Cloud comets.[60][61] Its color indexes also resemble the Solar System's long period comets.[6] Emissions at 388 nm indicated the presence of cyanide (formula CN), which is typically the first detected in Solar System comets including comet Halley.[62] This was the first detection of gas emissions from an interstellar object.[63] The non-detection of diatomic carbon had also been reported in October 2019, with the ratio C₂ to CN being less than either 0.095[64] or 0.3 .[65] The diatomic carbon was positively detected in November 2019, with measured C₂ to CN ratio of 0.2±0.1 .[66] This resembles a carbon-chain depleted group of comets, which are either Jupiter family comets[64] or rare blue-colored carbon monoxide comets exemplified by C/2016 R2.[67] By the end of November 2019, C₂ production had dramatically increased, and C₂ to CN ratio reached 0.61, along with appearance of bright amine (NH₂) bands.[68] Atomic oxygen has also been detected, from this observers estimated an outgassing of water at a rate similar to Solar System comets.[69][70] Initially, neither water nor OH lines were directly detected[71] in September 2019. First unambiguous detection of OH lines was done 1 November 2019, and OH production peaked in early December 2019.[72]

The comet did come 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.[15] Jewitt and Luu compared 2I/Borisov to C/2019 J2 (Palomar), another comet of similar size that disintegrated in May 2019 at a distance of 1.9 AU from the Sun.[73][6] 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.[74][6]

The severe outburst in February-March 2020, led to suspected "ongoing nucleus fragmentation" from the comet by 12 March.[17] Indeed, images from the Hubble Space Telescope taken on 30 March 2020 show a non-stellar core indicating that Comet 2I/Borisov has ejected sunward a large fragment.[18][19][75] The ejection is estimated to have begun around 7 March,[76] and may have occurred during one of the outbursts that occurred near that time.[77] The ejected fragment appeared to have vanished by 6 April 2020.[78]

A followup study, reported on 6 April 2020, observed only a single object, and noted that the fragment component had now disappeared.[78][20] Later analysis of the event showed the ejected dust and fragments have a combined mass of about 0.1% of total mass of nucleus, making the event a large outburst rather than fragmentation.[79]