사용자:이강철/번역1

2I/Borisov, originally designated C/2019 Q4 (Borisov),^[7][12] is the first observed interstellar comet and the second observed interstellar interloper after ʻOumuamua.^[13][14][15] 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.^[3][16][17] In November 2019, astronomers from Yale University noted that the comet (including coma and tail), was "14 times the size of Earth", presented an image comparing the comet size with the size of planet Earth [...] and stated, “It’s humbling to realize how small Earth is next to this visitor from another solar system.”^[18]

2I/보리소프
2019년 10월 촬영된 보리소프[b]
발견
발견자	게나디 보리소프
발견일	2019년 8월 30일
궤도 성질 (역기점 20 December 2019[3])
궤도 긴반지름(a)	−0.8512±0.0002 au[c]
근일점(q)	2.00664±0.00004 au
지구와의 MOID	1.09244 au
목성과의 MOID	2.38974 au
궤도 경사(i)	44.0530°±0.0006°
궤도 이심률(e)	3.3573±0.0004 (JPL) 3.357 (MPC)[4]
승교점 경도(Ω)	308.1498°±0.0004°
근일점 편각(ω)	209.1242°±0.0019°
물리적 성질
지름	≈2 km[5][6][7] 1.4–6.6 km[8] 450–550 m[5] (for an albedo of 0.04) ≈2–16 km[9] 1.5 km[10]
근일점 근접일
최근 접근날짜	2019년 12월 8일[3][4][11]

Nomenclature

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.^[d] As the second observed interstellar interloper after 1I/ʻOumuamua, it was given the "2I" designation, where "I" stands for interstellar.^[7][12] 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.^[19]
• 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.^[20] Astronomers such as Davide Farnocchia, Bill Gray, and David Tholen were already confident that the comet was interstellar.^[20]
• 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.^[7][12]

Characteristics

Physical characteristics

In this observation, the NASA/ESA Hubble Space Telescope revisited the comet shortly after its closest approach to the Sun in December 2019.

Unlike ʻOumuamua, which had an asteroidal appearance, 2I/Borisov's nucleus is surrounded by a coma, a cloud of dust and gas.^[21] 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.^[22] 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.^[23]

With 2I/Borisov's nucleus hidden by its coma, only rough estimates of its size are available. An initial estimate of the diameter of 2I/Borisov's nucleus, ranging from 2 to 16 km, was made by Karen Meech and her team at the University of Hawaii.^[9] Guzik et al. estimated a size of about 2.0 km, assuming an albedo of 0.04 and that 30% of the surface was active.^[5] Siraj and Loeb argued that the nucleus was likely small, on the order of 1 km, based on the expected size distributions of comets and the mass of comets ejected per star.^[6] Using the production rate of cyanide and assuming the fraction of its surface that is active is greater than 4%, Fitzsimmons et al. estimated that the nucleus is between 1.4 and 6.6 km in diameter.^[8] The comet will 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.^[24]

2I/Borisov's composition appears to be similar to that of Solar System comets. A preliminary (low-resolution) visible spectrum of 2I/Borisov was similar to typical Oort Cloud comets.^[25][26] Its color indexes also resemble the Solar System's long period comets.^[22] Emissions at 388 nm indicated the presence of cyanide (formula CN), which is typically the first detected in Solar System comets including comet Halley^[8]. This was the first detection of gas emissions from an interstellar object.^[27] The non-detection of diatomic carbon has also been reported in October 2019, with the ratio C₂ to CN being less than either 0.095^[28] or 0.3.^[29] The diatomic carbon was positively detected in November 2019, with measured C₂ to CN ratio of 0.2±0.1.^[30] This resembles a carbon-chain depleted group of comets, most of which are Jupiter family comets.^[28] Atomic oxygen has also been detected, from this observers estimated an outgassing of water at a rate similar to Solar System comets.^[31][32]

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.^[5] 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.^[33][22] If 2I/Borisov disintegrates, it is possible that it leaves behind an inactive remnant, similar to 'Oumuamua.^[22][34]

Trajectory

Trajectory across sky in cylindrical cooordinates, 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 (${\displaystyle v_{\infty }}$)^[e]

Object	Velocity
1I/2017 U1 (ʻOumuamua)	26.33 km/s 5.55 au/년[35]
2I/2019 Q4 (Borisov)	32.2 km/s 6.79 au/년[36]

As seen from Earth, the comet was in the northern sky from September until mid-November. It crossed the ecliptic plane on October 26 near the star Regulus^[37], 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 will be near the inner edge of the asteroid belt.^[f] In late December, it makes it closest approach to Earth,^[16][17] 1.9 au, and will have a solar elongation of about 80°.^[11] Due to its 44° orbital inclination, 2I/Borisov does 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.^[36] It will leave the Solar System in the direction of Telescopium.^[36] In interstellar space, 2I/Borisov takes roughly 9000 years to travel a light-year relative to the Sun.^[g]

2I/Borisov's trajectory is extremely hyperbolic, having an orbital eccentricity of 3.36.^[3][4][11] 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.^[38][h] 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.^[39] These two parameters are important indicators of 2I/Borisov's interstellar origin.^[36][40] For comparison, the Voyager 1 spacecraft, which is leaving the Solar System, is traveling at 16.9 km/s (3.57 au/년).^[41] 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.^[36]

2I/Borisov — orbit diagrams

The trajectory of Borisov (yellow) as it crosses the ecliptic plane; 'Oumuamua's trajectory (red) shown for comparison

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

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

Observation

Discovery

JPL Scout eccentricity ranges^[42]

# of observations	Observation arc (hours)	Eccentricity range
81	225	0.9–1.6
99	272	2.0–4.2
127	289	2.8–4.7
142	298	2.8–4.5
151	302	2.9–4.5

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.^[43] The discovery has been compared to the discovery of Pluto by Clyde Tombaugh.^[44] 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°.^[45] Borisov described his discovery thus:^[46]

“

I observed it on August 29, but it was August 30 Greenwich Time.[i] I saw a moving object in the frame, it moved in a direction that was slightly different from that of main asteroids.[j] 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,[k] 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.[l] 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.^[19] Later using 151 observations over 12 days, NASA Jet Propulsion Laboratory's Scout gave an eccentricity range of 2.9–4.5.^[47] 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 천문단위 (51×10^⁶ km)^%s%s, and a perihelion at 0.90 au around 30 December 2019.^[48] 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.^[49] 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.^[20]

2019–2020

Hubble started observations of 2I/Borisov in October 2019, two months prior to the comet's closest approach to the Sun

Unless the comet fades or disintegrates unexpectedly, it should be observable until at least September 2020.^[20] 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".^[50] 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.^[51] 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.^[52][22]

Exploration

Comet 2I/Borisov and Distant Galaxy in November 2019.^[53]

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).^{[{{{설명}}}]} 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.^[54] According to congressional testimony, NASA may need at least five years of preparation to launch such an intercepting mission.^[55]

See also

• 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

Notes

1. 2019년 10월 12일, 보리소프의 먼지 레일을 허블 우주 망원경의 Wide Field Camera 3의 UVIS F350LP 채널로 촬영하였다. ^[1][2] 촬영 시점에 보리소프는 지구에서 4억1800만km 떨어져 있었고, 시속 177,000km로 이동 중이었다.^[2]
2. 2019년 10월 12일, 보리소프의 먼지 레일을 허블 우주 망원경의 Wide Field Camera 3의 UVIS F350LP 채널로 촬영하였다. ^[1][2] 촬영 시점에 보리소프는 지구에서 4억1800만km 떨어져 있었고, 시속 177,000km로 이동 중이었다.^[2]
3. 이심률이 1보다 크면, 음의 긴반지름을 가지는데, 이는 쌍곡선, 포물선의 점근선의 교점까지의 거리이다. 근일점거리가 짧을수록, 이심률이 1보다 클 수록, 궤도 긴반지름은 0에 가까워진다. 쌍곡선 궤도 참고.
4. This is the eighth comet discovered by Gennadiy Borisov, and thus the ambiguous term "Comet Borisov" is not formally used here.
5. 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.
6. 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.
7. 299792.458 km/s / 32.2 km/s = 9310 times slower than light.
8. 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.
9. 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.
10. Of the 850,000 objects known to orbit the Sun, 756,000 (89%) are main-belt asteroids.
11. NEO Rating for calculating the probability that a new object is a near-Earth candidate.
12. The NEO Confirmation Page for confirming near-Earth objects and potentially hazardous objects.