اوموآموا (انگلیسی: ʻOumuamua) (بهطور رسمی 1I /'Oumuamua ثبت شده؛ و پیش از آن C / 2017 U1 (PANSTARRS) و A / 2017 U1)، نخستین جرم بین ستارهای شناختهشدهای است که مسیر حرکت آن از منظومه شمسی میگذرد.اوموآمو در یک مسیر بسیار سهموی (هیپربولیک) توسط رابرت وریک در ۱۹ اکتبر ۲۰۱۷، چهل روز پس از دور زدن خورشید کشف شد. نخستین رصد توسط تلسکوپ Pan-STARRS؛ هنگامی که جسم در فاصلهٔ تنها ۰٫۲ واحد نجومی (۳۰٬۰۰۰٬۰۰۰ کیلومتر; ۱۹٬۰۰۰٬۰۰۰ مایل) از زمین بود انجام شد.
در ابتدا تصور میشد که این جسم یک دنبالهدار است، که یک هفته بعد به عنوان یک سیارک طبقهبندی شد. این نخستین سیارک از کلاس جدیدی از آنهاست که به نام سیارکهای هیپربولیک شناخته میشود
به عنوان اولین شیء شناختهشده از نوع خود، اوموآموا یک مورد منحصربهفرد برای اتحادیه بینالمللی اخترشناسی؛ که عهدهدار اختصاص نام برای اشیاء آسمانیاست، بود. یک معرفهٔ اختصاصی جدید؛ (آی "I") حرف آغازین (Interstellar)، برای اشیاء بین ستارهای ایجاد شد، و 'اوموآموا به عنوان آیِ شمارهٔ یک (1I) تعیین شد
، و قوانین مربوط به واجد شرایط بودن اشیاء برای اختصاص یک شمارهٔ (I) به آنها، و نیز نامهایی که میتواند به این اشیاء بین ستارهای اختصاص داده شود، هنوز سرگرم کدگذاری و تدوین است. "فرمهای درستی که برای اشاره به این جسم وجود دارد: 1I؛ 1I / 2017 U1؛ 1I /'اوموآموا؛ و 1I / 2017 U1 ('Oumuamua)." است.
مشاهدات و نتیجهگیریهای مربوط به مسیر'اوموآموا'، عمدتاً با دادههای تلسکوپ Pan-STARRS1 و تلسکوپ تلسکوپ هاوایی-فرانسه-کانادا (CFHT)، و ترکیب و شکل آن از تلسکوپ ویالتی و تلسکوپ جمنای جنوبی در شیلی، و همچنین تلسکوپ کک در هاوایی به دست آمدهاست. این اطلاعات توسط (Karen J. Meech کارن جین میچ)، (رابرت وریک Robert Weryk) و همکاران آنها گردآوری شد و در ماه نوامبر در مجلهٔ نیچر ۲۰ نوامبر منتشر شد.
ʻOumuamua is a small object, estimated to be about 100 m–1,000 m × 35 m–167 m × 35 m–167 m (328 ft–3,281 ft × 115 ft–548 ft × 115 ft–548 ft) in size. It has a dark red color, similar to objects in the outer Solar System. ʻOumuamua showed no signs of a comet coma (atmosphere) despite its close approach to the Sun, but underwent non-gravitational acceleration. This effect is seen in many icy comets, although other reasons have been suggested. Nonetheless, the object could be a remnant of a disintegrated interstellar comet (or exocomet), according to a NASA scientist. The object has a rotation rate similar to the average spin rate seen in Solar System asteroids, but is more elongated than all but a few other natural bodies. While a strengthless object (rubble pile) would require it to be of a density similar to rocky asteroids, a small amount of internal strength similar to icy comets would allow a relatively low density. ʻOumuamua is tumbling, rather than smoothly rotating, and is moving so fast relative to the Sun that there is no chance it originated in the Solar System. It also means that ʻOumuamua cannot be captured into a solar orbit, so it will eventually leave the Solar System and resume traveling through interstellar space. It will take the object roughly 20,000 years to travel the Solar System before exiting.[Note 3] ʻOumuamua's planetary system of origin and the amount of time it has spent traveling amongst the stars are unknown.
As the first known object of its type, ʻOumuamua presented a unique case for the International Astronomical Union, which assigns designations for astronomical objects. Originally classified as comet C/2017 U1, it was later reclassified as asteroid A/2017 U1, due to the absence of a coma. Once it was unambiguously identified as coming from outside the Solar System, a new designation was created: I, for Interstellar object. ʻOumuamua, as the first object so identified, was designated 1I, with rules on the eligibility of objects for I-numbers, and the names to be assigned to these interstellar objects, yet to be codified. The object may be referred to as 1I; 1I/2017 U1; 1I/ʻOumuamua; or 1I/2017 U1 (ʻOumuamua).
Before the official name was decided upon, the name Rama was suggested, the name given to an alien spacecraft discovered under similar circumstances in the 1973 science fiction novel Rendezvous with Rama by Arthur C. Clarke.
ʻOumuamua will fade to 34th apparent magnitude by 2020
ʻOumuamua is small and dark. It was not seen in STEREO HI-1A observations near its perihelion on 9 September 2017, limiting its brightness to ~13.5 mag. By the end of October, ʻOumuamua had already faded to apparent magnitude ~23, and in mid-December 2017, it was too faint and fast moving to be studied by even the largest ground-based telescopes.
ʻOumuamua was compared to the fictional alien spacecraft Rama because of its interstellar origin. Adding to the coincidence, both the real and the fictional objects are unusually elongated. ʻOumuamua has a reddish hue and unsteady brightness, which are typical of asteroids.
Seen from Earth, the apparent trajectory makes annual retrograde loops in the sky, with its origin in Lyra, temporarily moving south of the ecliptic between 2 September and 22 October 2017, and moving northward again towards its destination in Pegasus.
ʻOumuamua's hyperbolic trajectory over the Solar System
By mid-November, astronomers were certain that it was an interstellar object. Based on observations spanning 34 days, ʻOumuamua's orbital eccentricity is 1.20, the highest ever observed. An eccentricity exceeding 1.0 means an object exceeds the Sun's escape velocity, is not bound to the Solar System and may escape to interstellar space. While an eccentricity slightly above 1.0 can be obtained by encounters with planets, as happened with the previous record holder, C/1980 E1,[Note 6] ʻOumuamua's eccentricity is so high that it could not have been obtained through an encounter with any of the planets in the Solar System. Even undiscovered planets in the Solar System, if any should exist, could not account for ʻOumuamua's trajectory nor boost its speed to the observed value. For these reasons, ʻOumuamua can only be of interstellar origin.
Animation of ʻOumuamua passing through the Solar System
Inbound velocity at 200 AU from the Sun compared to Oort cloud objects
ʻOumuamua entered the Solar System from north of the plane of the ecliptic. The pull of the Sun's gravity caused it to speed up until it reached its maximum speed of 87.71 km/s (315,800 km/h) as it passed south of the ecliptic on 6 September and made a sharp turn northward at its closest approach to the Sun (perihelion) on 9 September at a distance of 0.255 AU (38,100,000 km; 23,700,000 mi) from the Sun, i.e., about 17% closer than Mercury's closest approach to the Sun.[Note 9] The object is now heading away from the Sun towards Pegasus towards a vanishing point 66° from the direction of its approach.[Note 10] This implies that a hypothetical observer near the Sun facing towards ʻOumuamua will eventually rotate through 294 degrees, while the direction of motion of ʻOumuamua (relative to the Sun) will eventually have turned by 114 degrees.
On the outward leg of its journey through the Solar System, ʻOumuamua passed within the orbit of Earth on 14 October at a distance of approximately 0.1616 AU (24,180,000 km; 15,020,000 mi) from Earth, and went back north of the ecliptic on 16 October and passed beyond the orbit of Mars on 1 November. It passed beyond Jupiter's orbit in May 2018, beyond Saturn's orbit in January 2019, and will pass beyond Neptune's orbit in 2022. As it leaves the Solar System it will be approximately right ascension 23h51m and declination +24°45', in Pegasus. It will continue to slow down until it reaches a speed of 26.33 km/s relative to the Sun, the same speed it had before its approach to the Solar System. It will take the object roughly 20,000 years to leave the Solar System completely.[Note 11]
On 27 June 2018, astronomers reported a non-gravitational acceleration to ʻOumuamua's trajectory, potentially consistent with a push from solar radiation pressure. Initial speculation as to the cause of this acceleration pointed to comet off-gassing, whereby portions of the object are ejected as the Sun heats the surface. Although no such tail of gases was ever observed following the object, researchers estimated that enough outgassing may have increased the object's speed without the gasses being detectable. A critical re-assessment of the comet hypothesis found that, instead of the observed stability of 'Oumuamua's spin, outgassing would have caused its spin to rapidly change due to its elongated shape, resulting in the object tearing apart.
Indications of origin
Accounting for Vega's proper motion, it would have taken ʻOumuamua 600,000 years to reach the Solar System from Vega. But as a nearby star, Vega was not in the same part of the sky at that time. Astronomers calculate that one hundred years ago the object was 561 ± 0.6 AU (83.9 ± 0.090 billion km; 52.1 ± 0.056 billion mi) from the Sun and traveling at 26.33 km/s with respect to the Sun. This interstellar speed is very close to the mean motion of material in the Milky Way in the neighborhood of the Sun, also known as the local standard of rest (LSR), and especially close to the mean motion of a relatively close group of red dwarf stars. This velocity profile also indicates an extrasolar origin, but appears to rule out the closest dozen stars. In fact, the strong correlation between ʻOumuamua's velocity and the local standard of rest might mean that it has circulated the Milky Way several times and thus may have originated from an entirely different part of the galaxy.
It is unknown how long the object has been traveling among the stars. The Solar System is likely the first star system that ʻOumuamua has closely encountered since being ejected from its birth star system, potentially several billion years ago. It has been speculated that the object may have been ejected from a stellar system in one of the local kinematic associations of young stars (specifically, Carina or Columba) within a range of about 100 parsecs, some 45 million years ago. The Carina and Columba associations are now very far in the sky from the Lyra constellation, the direction from which ʻOumuamua came when it entered the Solar System. Others have speculated that it was ejected from a white dwarf system and that its volatiles were lost when its parent star became a red giant. About 1.3 million years ago the object may have passed within a distance of 0.16 parsecs (0.52 light-years) to the nearby star TYC 4742-1027-1, but its velocity is too high to have originated from that star system, and it probably just passed through the system's Oort cloud at a relative speed of about 15 km/s (54,000 km/h).[Note 12] A more recent study (August 2018) using Gaia Data Release 2 has updated the possible past close encounters and has identified four stars that ʻOumuamua passed relatively close to and at moderately low velocities in the past few million years.
This study also identifies future close encounters of ʻOumuamua on its outgoing trajectory from the Sun.
According to one hypothesis, ʻOumuamua could be a fragment from a tidally disrupted planet.[Note 13] If true, this would make ʻOumuamua a rare object, of a type much less abundant than most extrasolar "dusty-snowball" comets or asteroids.
Initially, ʻOumuamua was announced as comet C/2017 U1 (PANSTARRS) based on a strongly hyperbolic trajectory. In an attempt to confirm any cometary activity, very deep stacked images were taken at the Very Large Telescope later the same day, but the object showed no presence of a coma.[Note 14] Accordingly, the object was renamed A/2017 U1, becoming the first comet ever to be re-designated as an asteroid. Once it was identified as an interstellar object, it was designated 1I/2017 U1, the first member of a new class of objects. The lack of a coma limits the amount of surface ice to a few square meters, and any volatiles (if they exist) must lie below a crust at least 0.5 m (1.6 ft) thick. It also indicates that the object must have formed within the frost line of its parent stellar system or have been in the inner region of that stellar system long enough for all near-surface ice to sublimate, as may be the case with damocloids. It is difficult to say which scenario is more likely due to the chaotic nature of small body dynamics, although if it formed in a similar manner to Solar System objects, its spectrum indicates that the latter scenario is true. Any meteoric activity from ʻOumuamua would have been expected to occur on 18 October 2017 coming from the constellation Sextans, but no activity was detected by the Canadian Meteor Orbit Radar.
On 27 June 2018, astronomers reported that ʻOumuamua was thought to be a mildly active comet, and not an asteroid, as previously thought. This was determined by measuring a non-gravitational boost to ʻOumuamua's acceleration, consistent with comet outgassing. However, studies submitted in October 2018 suggest that the object is neither an asteroid nor a comet, although the object could be a remnant of a disintegrated interstellar comet (or exocomet), as suggested by a NASA scientist.
Light curve from 25–27 October 2017 with dotted line from a model with 10:1 elongation
ʻOumuamua is rotating around a non-principal axis, a type of movement known as tumbling. This accounts for the various rotation periods reported, such as 8.10 hours, (±0.42 hours) (±0.02 hours) with a lightcurve amplitude of 1.5–2.1 magnitudes, whereas Meech et al. reported a rotation period of 7.3 hours and a lightcurve amplitude of 2.5 magnitudes.[Note 15] Most likely, ʻOumuamua was set tumbling by a collision in its system of origin, and remains tumbling since the time scale for dissipation of this motion is very long, at least a billion years.
Artist's impression of ʻOumuamua
Simulation of ʻOumuamua spinning and tumbling through space, and the resultant light curve. In reality, observations of ʻOumuamua detect the object as a single pixel — its shape here has been inferred from the light curve
The large variations on the light curves indicate that ʻOumuamua may be either a highly elongated object, comparable to or greater than the most elongated Solar System objects, or an extremely flat object, a pancake or oblate spheroid. However, the size and shape have not been directly observed as ʻOumuamua appears as nothing more than a point source of light even in the most powerful telescopes. Neither the albedo nor triaxial ellipsoid shape are precisely known. If cigar-shaped, the longest-to-shortest axis ratio could be 5:1 or greater. Assuming an albedo of 10% (slightly higher than typical for D-type asteroids) and a 6:1 ratio, ʻOumuamua has dimensions of approximately 100 m–1,000 m × 35 m–167 m × 35 m–167 m (328 ft–3,281 ft × 115 ft–548 ft × 115 ft–548 ft) with an average diameter of about 110 m (360 ft). According to astronomer David Jewitt, the object is physically unremarkable except for its highly elongated shape. Bannister et al. have suggested that it could also be a contact binary, although this may not be compatible with its rapid rotation. One speculation regarding its shape is that it is a result of a violent event (such as a collision or stellar explosion) that caused its ejection from its system of origin. JPL News reported that ʻOumuamua "is up to one-quarter mile, 400 m (1,300 ft), long and highly-elongated-perhaps 10 times as long as it is wide".
Light curve observations suggest the object may be composed of dense metal-rich rock that has been reddened by millions of years of exposure to cosmic rays. It is thought that its surface contains tholins, which are irradiated organic compounds that are more common in objects in the outer Solar System and can help determine the age of the surface. This possibility is inferred from spectroscopic characterization and its dark and reddened color, and from the expected effects of interstellar radiation. Despite the lack of any cometary coma when it approached the Sun, it may still contain internal ice, hidden by "an insulating mantle produced by long-term cosmic ray exposure".
In September 2018, astronomers described several possible home star systems from which ʻOumuamua may have originated.
On 26 October 2018, Loeb and his postdoc Shmuel Bialy submitted a paper exploring the possibility of ʻOumuamua being an artificial thin solar sail accelerated by solar radiation pressure in an effort to help explain the object's non-gravitational acceleration. Other scientists have stated that the available evidence is insufficient to consider such a premise, and that a tumbling solar sail would not be able to accelerate. In response, Loeb wrote an article detailing six anomalous properties of ʻOumuamua that make it unusual, unlike any comets or asteroids seen before. A subsequent report on observations by the Spitzer Space Telescope set a tight limit on cometary outgassing of any carbon-based molecules and indicated that ʻOumuamua is at least ten times more shiny than a typical comet. A detailed podcast produced by Rob Reid provides the full details about the differences between ʻOumuamua and known comets.
ʻOumuamua was at first thought to be traveling too fast for any existing spacecraft to reach. The Initiative for Interstellar Studies (i4is) launched Project Lyra to assess the feasibility of a mission to ʻOumuamua. Several options for sending a spacecraft to ʻOumuamua within a time-frame of 5 to 25 years were suggested. One option is using first a Jupiter flyby followed by a close solar flyby at 3 solar radii (2.1×10^6 km; 1.3×10^6 mi) in order to take advantage of the Oberth effect. Different mission durations and their velocity requirements were explored with respect to the launch date, assuming direct impulsive transfer to the intercept trajectory. Using a powered Jupiter flyby, a solar Oberth maneuver and Parker Solar Probe heat shield technology, a Falcon Heavy-class launcher would be able to launch a spacecraft of dozens of kilograms towards 1I/ʻOumuamua, if launched in 2021. More advanced options of using solar, laser electric, and laser sail propulsion, based on Breakthrough Starshot technology, have also been considered. The challenge is to get to the asteroid in a reasonable amount of time (and so at a reasonable distance from Earth), and yet be able to gain useful scientific information. To do this, decelerating the spacecraft at ʻOumuamua would be "highly desirable, due to the minimal science return from a hyper-velocity encounter". If the investigative craft goes too fast, it would not be able to get into orbit or land on the object and would fly past it. The authors conclude that, although challenging, an encounter mission would be feasible using near-term technology. Seligman and Laughlin adopt a complementary approach to the Lyra study but also conclude that such missions, though challenging to mount, are both feasible and scientifically attractive.
Other interstellar objects
Astronomers estimate that several interstellar objects of extrasolar origin (like ‘Oumuamua) pass inside the orbit of Earth each year, and that 10,000 are passing inside the orbit of Neptune on any given day. If the estimate is correct, this provides future opportunities for studies of interstellar objects. However, with current space technology, close visits and orbital missions are challenging due to their high speeds, though not impossible.
On 27 November 2018, Loeb and his undergraduate student at Harvard College, Amir Siraj, proposed a search for ‘Oumuamua-like objects which are trapped in the Solar System as a result of losing orbital energy through a close encounter with Jupiter. They identified centaur candidates, such as 2017 SV13 and 2018 TL6, as captured interstellar objects that could be visited by dedicated missions. The authors pointed out that future sky surveys, such as with LSST, should find many candidates.
C/2017 U7, a non-interstellar hyperbolic comet discovered 10 days after ʻOumuamua, announced in March 2018
C/2018 C2, another non-interstellar hyperbolic comet, announced in March 2018
^5-minute exposure taken by the William Herschel Telescope on 28 October; ʻOumuamua appears as a light source in the center of the image, while background stars appear streaked due to the speed of ʻOumuamua as the telescope tracked it.
^Objects on hyperbolic trajectories have negative semimajor axis, giving them a positive orbital energy.
^Range at which the object is expected to be observable. Brightness peaked at 19.7 mag on 18 October 2017, and fades below 27.5 mag (the limit of Hubble Space Telescope for fast-moving objects) around 1 January 2018. By late 2019, it should dim to 34 mag.
^Given that the Oort cloud is the furthest reaches of the Solar System, define the edge of the Solar System at 2 light-years (130,000 astronomical units; 19 trillionkilometers) and assume an average velocity of 26.3 km/s. It will take the object 23,000 years to travel 2 light–years (1.9×1013 km / 26.3 km-per-sec / 60 seconds-per-min / 60 minutes-per-hour / 24 hours-per-day / 365.25 days-per-year = 23,000 years)
^For comparison, comet C/1980 E1 will only be moving 4.2 km/s when it is 500 AU from the Sun.
^The solar escape velocity from Earth's orbit (1 AU from the Sun) is 42.1 km/s. For comparison, even 1P/Halley moves at 41.5 km/s when 1 AU from the Sun, according to the formula v = 42.1219 √1/r − 0.5/a, where r is the distance from the Sun, and a is the major semi-axis. Near-Earth asteroid 2062 Aten only moves at 29 km/s when 1 AU from the Sun because of the much smaller semi-major axis.
^Unlike ʻOumuamua, C/1980 E1's orbit got its high eccentricity of 1.057 due to a close encounter with Jupiter. Its inbound-orbit eccentricity was less than 1.
^Orbits computed with only a handful of observations can be unreliable. Short arcs can result in computer generated orbits rejecting some data unnecessarily.
^JPL #10 shows that on 1855-Mar-24 C/2008 J4 was moving 4.88 ± 1.8 km/s.
^Comet C/2012 S1 (ISON) peaked at 377 km/s (1,360,000 km/h) at perihelion because it passed 0.0124 AU from the Sun (20 times closer than ʻOumuamua).
^Given that the Oort cloud is the furthest reaches of the Solar System, define the edge of the Solar System at 2 light-years (130,000 astronomical units; 19 trillionkilometers) and assume an average velocity of 26.3 km/s. It will take the object 23,000 years to reach 2 light–years (1.9×1013 km / 26.3 km-per-sec / 60 seconds-per-min / 60 minutes-per-hour / 24 hours-per-day / 365.25 days-per-year = 23,000 years)
^This is true for the nominal position of the star. However, its actual distance is not known precisely: According to Gaia Data Release 1, the distance to TYC4742-1027-1 is 137 ± 13 parsecs (447 ± 42 light-years). It is not known if an encounter actually occurred. Update: This star has new measurements in Gaia Data Release 2, and an origins study based on this by Bailer-Jones et al. (2018) shows that TYC4742-1027-1 did not come within 2pc of ʻOumuamua.
^See also Ravikov, Roman R. (2018). "1I/2017 ʻOumuamua-like Interstellar Asteroids as Possible Messengers from the Dead Stars". arXiv:1801.02658v2 [astro-ph.EP]., ʻOumuamua is a fragment of a white-dwarf-star tidal-disruption-event. This easily explains its 6:1 or 10:1 elongation and its "refractory" composition; containing probably nickel-iron, possibly other metals, too.
^Bonnell, Jerry; Nemiroff, Robert (3 November 2017). "A/2017 U1: An Interstellar Visitor". Astronomy Picture of the Day. Archived from the original on 13 March 2019. Retrieved 13 March 2019. A point of light centered in this 5 minute exposure recorded with the William Herschel Telescope in the Canary Islands on October 28 [...] Faint background stars appear streaked because the massive 4.2 meter diameter telescope is tracking the rapidly moving A/2017 U1 in the field of view.
^Królikowska, Małgorzata; Dybczyński, Piotr A. (1 October 2013). "Near-parabolic comets observed in 2006-2010. The individualized approach to 1/a-determination and the new distribution of original and future orbits". Monthly Notices of the Royal Astronomical Society. 435 (1): 440–459. Bibcode:2013MNRAS.435..440K. doi:10.1093/mnras/stt1313. ISSN0035-8711.
^Clark, Stuart (20 November 2017). "Mysterious object confirmed to be from another solar system". The Guardian. Retrieved 21 November 2017. Astronomers are now certain that the mysterious object detected hurtling past our Sun last month is indeed from another solar system. They have named it 1I/2017 U1 (ʻOumuamua) and estimate it could be one of 10,000 others lurking undetected in our cosmic neighbourhood.
^de la Fuente Marcos, Carlos; de la Fuente Marcos, Raúl; Aarseth, Sverre J. (2018). "Where the Solar system meets the solar neighbourhood: patterns in the distribution of radiants of observed hyperbolic minor bodies". Monthly Notices of the Royal Astronomical Society Letters. 476 (1): L1–L5. arXiv:1802.00778. Bibcode:2018MNRAS.476L...1D. doi:10.1093/mnrasl/sly019.
^Belton, M.J.S.; et al. (10 April 2018). "The Excited Spin State of 1I/2017 U1 'Oumuamua". The Astrophysical Journal. 856 (2): L21. arXiv:1804.03471. Bibcode:2018ApJ...856L..21B. doi:10.3847/2041-8213/aab370. We find that ‘Oumuamua is "cigar-shaped"', if close to its lowest rotational energy, and an extremely oblate spheroid if close to its highest energy state for its total angular momentum.
^Thomas, C. A.; Trilling, D. E.; Emery, J. P.; Mueller, M.; Hora, J. L.; Benner, L. A. M.; Bhattacharya, B.; Bottke, W. F.; Chesley, S. (1 September 2011). "ExploreNEOs. V. Average Albedo by Taxonomic Complex in the Near-Earth Asteroid Population". The Astronomical Journal. 142 (3): 85. Bibcode:2011AJ....142...85T. doi:10.1088/0004-6256/142/3/85. ISSN0004-6256.
^ abIan Sample (11 December 2017). "Astronomers to check interstellar body for signs of alien technology". The Guardian. Retrieved 12 December 2017. Green Bank telescope in West Virginia will listen for radio signals from ʻOumuamua, an object from another solar system ... "Most likely it is of natural origin, but because it is so peculiar, we would like to check if it has any sign of artificial origin, such as radio emissions," said Avi Loeb, professor of astronomy at Harvard University and an adviser to the Breakthrough Listen project. "If we do detect a signal that appears artificial in origin, we’ll know immediately." ... While many astronomers believe the object is an interstellar asteroid, its elongated shape is unlike anything seen in the asteroid belt in our own solar system. Early observations of ʻOumuamua show that it is about 400m long but only one tenth as wide. "It's curious that the first object we see from outside the solar system looks like that," said Loeb.
^ abcFitzsimmons, A.; et al. (18 December 2017). "Spectroscopy and thermal modelling of the first interstellar object 1I/2017 U1 ʻOumuamua". Nature Astronomy. 2 (2): 133. arXiv:1712.06552. Bibcode:2018NatAs...2..133F. doi:10.1038/s41550-017-0361-4. The discovery epoch photometry implies a highly elongated body with radii of ∼200×20 m when a comet-like geometric albedo of 0.04 is assumed. Here we report spectroscopic characterisation of ʻOumuamua, finding it to be variable with time but similar to organically rich surfaces found in the outer Solar System. The observable ISO population is expected to be dominated by comet-like bodies in agreement with our spectra, yet the reported inactivity implies a lack of surface ice. We show this is consistent with predictions of an insulating mantle produced by long-term cosmic ray exposure. An internal icy composition cannot therefore be ruled out by the lack of activity, even though ʻOumuamua passed within 0.25 au of the Sun.
^Hein, Andreas M.; Perakis, Nikolaos; Eubanks, T. Marshall; Hibberd, Adam; Crowl, Adam; Hayward, Kieran; Kennedy III, Robert G.; Osborne, Richard (7 January 2019). "Project Lyra: Sending a spacecraft to 1I/'Oumuamua (former A/2017 U1), the interstellar asteroid". Acta Astronautica. in press. arXiv:1711.03155. Bibcode:2017arXiv171103155H.
^Hibberd, Adam; Hein, Andreas M.; Eubanks, T. Marshall (14 February 2019). "Project Lyra: Catching 1I/'Oumuamua - Mission Opportunities After 2024". arXiv:1902.04935 [physics.space-ph].
^Siraj, Amir; Loeb, Abraham (2019). "Identifying Interstellar Objects Trapped in the Solar System through Their Orbital Paramteters". The Astrophysical Journal. 872: L10. arXiv:1811.09632. doi:10.3847/2041-8213/ab042a.