2I/Borisov, originally designated C/2019 Q4 (Borisov), is the first observed interstellar comet and the 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 is formally named "2I/Borisov" by the International Astronomical Union (IAU), but is sometimes referred to as "Comet Borisov", especially in the popular press.[c] 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. Astronomers such as Davide Farnocchia, Bill Gray, and David Tholen were already confident that the comet was interstellar.
- On 24 September 2019 the IAU announced that the Working Group for Small Body Nomenclature kept the name Borisov giving the comet the interstellar designation of 2I/Borisov, formally announcing the comet was indeed interstellar.
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. 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. 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, while Guzik et al. estimated a size of about 0.5 km, assuming an albedo of 0.04. On 18 September, Siraj et al. argued that the nucleus was likely small, on the order of 2 km. 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 comet has a composition not very different from that found in typical Oort Cloud comets. Similarly, measurements taken at the Nordic Optical Telescope found the comet had color indexes resembling the Solar System's long period comets. The William Herschel Telescope, 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. This was the first detection of gas emissions from an interstellar object.
The comet will 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 will happen to 2I/Borisov. Jewitt and Luu compared 2I/Borisov to C/2019 J2, another comet of similar size that disintegrated in May 2019 at a distance of 1.9 au from the Sun. If 2I/Borisov disintegrates, it is possible that it leaves behind an inactive remnant, similar to 'Oumuamua.
|1I/2017 U1 (ʻOumuamua)||26.33 km/s|
|2I/2019 Q4 (Borisov)||32.2 km/s|
As seen from Earth, the comet is in the northern sky from September until mid-November. It will cross the ecliptic plane on 13 November 2019 entering the southern sky. 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.[e] In late December, it will be about 1.9 au from Earth and have a solar elongation of about 80°. Due to its 44° orbital inclination, 2I/Borisov does 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. In interstellar space, 2I/Borisov takes roughly 9000 years to travel a light-year relative to the Sun. It will leave the Solar System in the direction of Telescopium. 2I/Borisov has passed within 5.7 light-years of the binary star system Kruger 60 (13.18 light-years away, in Cepheus) at a low velocity of 3.4 km/s around a million years ago.
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, and even ʻOumuamua with an eccentricity of 1.2.[f] 2I/Borisov also has a hyperbolic excess velocity () 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. 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.
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 has been compared to the discovery of Pluto by Clyde Tombaugh. 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°. Borisov described his discovery thus:
|“||I observed it on August 29, but it was August 30 Greenwich Time.[g] I saw a moving object in the frame, it moved in a direction that was slightly different from that of main asteroids.[h] 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,[i] 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.[j] 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. Later using 151 observations over 12 days, NASA Jet Propulsion Laboratory's Scout gave an eccentricity range of 2.9–4.5. 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. 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. 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. 2I/Borisov's trajectory was better resolved with a larger observation arc collected from additional observatories. After a 13-day observation arc, a best-fit for the hyperbolic orbit had an eccentricity of about 3.5, with perihelion at 2 au.
Unless the comet fades or disintegrates unexpectedly, it should be observable until at least September 2020. Observation of 2I/Borisov is 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". Observations using the Hubble Space Telescope began on October 12, when the comet moved 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).[dubious ] 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.
- C/1980 E1 (Bowell) – the most eccentric comet known in the Solar System with an eccentricity of 1.057
- List of Solar System objects by greatest aphelion
- Photograph of Borisov's dust trail, taken on 12 October 2019 with the UVIS F350LP channel of Wide Field Camera 3, aboard the Hubble Space Telescope. At the time of this photograph, Borisov was 418,000,000 kilometres (260,000,000 mi) from Earth – similar to the average distance of the asteroid Ceres from the Sun – and traveling at a velocity of 177,000 kilometres per hour (110,000 mph).
- Objects with eccentricities greater than 1 will have a negative semimajor axis, giving them a positive orbital energy – the smaller the perihelion distance, and the larger above 1 the eccentricity is, the closer to 0 the semimajor axis value will be.
- This is the eighth comet discovered by Gennadiy Borisov, and thus the ambiguous term "Comet Borisov" is not formally used here.
- For objects gravitationally bound to the Sun, velocities when approaching the equivalent of an infinite distance from the Sun should be less than a few kilometres per second. At its furthest distance from the Sun, Halley's comet moves ≈1 km/s with respect to the Sun.
- 2I/Borisov will come to perihelion outside the orbit of Mars at a distance of 2.01 au, which is just inside the 4:1 Kirkwood gap located at 2.06 au.
- A comet with a parabolic trajectory (with an eccentricity of 1) would leave the Solar System in the direction it entered, having had its path altered by 180°. 2I/Borisov, with its higher eccentricity, has a more open trajectory and will have its path altered by only 34° as it passes through the Solar System.
- This is obviously a mistake: Crimea is 3 hours ahead of Greenwich, so it was August 30 in Crimea when Borisov first observed the comet.
- Of the 850,000 objects known to orbit the Sun, 756,000 (89%) are main-belt asteroids.
- NEO Rating for calculating the probability that a new object is a near-Earth candidate.
- The NEO Confirmation Page for confirming near-Earth objects and potentially hazardous objects.
- National Aeronautics and Space Administration; European Space Agency; Jewitt, David (16 October 2019). "Comet 2I/Borisov Compass Image". Hubblesite. Archived from the original on 17 October 2019. Retrieved 17 October 2019.
C/2019 Q4 (Borisov); HST WFC3/UVIS F350LP; Oct. 12, 2019
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Hubble took a series of snapshots as the comet streaked along at 110,000 miles per hour. [...] The comet was 260 million miles from Earth when Hubble took the photo. [...] This Hubble image, taken on October 12, 2019 [...] reveals a central concentration of dust around the nucleus (which is too small to be seen by Hubble).
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|Wikimedia Commons has media related to 2I/Borisov.|
- Image of 2I/Borisov from the Gemini Observatory, Hawaii
- Pictures of 2I/Borisov from Paris Observatory (LESIA)
- FAQ by Bill Gray at ProjectPluto
- Magnitude plot by Seiichi Yoshida @ aerith.net (with predicted brightness)
- 2I/Borisov at the JPL Small-Body Database
- Discovery animation
- gb00234 hyperbolic orbit simulation by Tony Dunn
- on YouTube
- on YouTube