ʻOumuamua

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ʻOumuamua
Oumuamua orbit at perihelion.png
Hyperbolic trajectory of ʻOumuamua through the inner Solar System with the Sun at the focus
Discovery [1][2]
Discovered by Robert Weryk using Pan-STARRS 1
Discovery site Haleakala Obs., Hawaii
Discovery date 19 October 2017
Designations
MPC designation 1I/2017 U1[3]
Pronunciation /oʊˈmuːəˈmuːə/ (About this sound listen)
Named after
 Hawaiian term for scout[3]
  • 1I
  • 1I/ʻOumuamua
  • 1I/2017 U1 (ʻOumuamua)
  • A/2017 U1[4]
  • C/2017 U1[2]
  • P10Ee5V[5]
interstellar object[3]
hyperbolic asteroid[6]
Orbital characteristics [6]
Epoch 2 November 2017 (JD 2458059.5)
Observation arc 34 days
Perihelion 0.25534±0.00007 AU
−1.2798±0.0008 AU[n 1]
Eccentricity 1.19951±0.00018
26.33±0.01 km/s (interstellar)[7]
36.425°
0° 40m 48.72s / day
Inclination 122.69°
24.599°
241.70°
Earth MOID 0.0959 AU · 37.3 LD
Jupiter MOID 1.455 AU
Physical characteristics
Dimensions 230 × 35 × 35 m
(est. at albedo 0.10)[8][9]
Tumbling (non-principal axis rotation)[10]
Reported values include: 8.10±0.02 h[11]
8.10±0.42 h[12]
6.96+1.45
−0.39 h[13]
0.1 (spectral est.)[8]
0.06–0.08 (spectral est.)[12]
D?[8]
B–V = 0.7±0.06[8]
V-R = 0.45±0.05[8]
g-r = 0.47±0.04[12]
r-i = 0.36±0.16[12]
r-J = 1.20±0.11[12]
19.7 to >27.5[7][n 2][14]
22.08±0.445[6]

ʻOumuamua (/oʊˈmuːəˈmuːə/ (About this sound listen)) is the first known interstellar object to pass through the Solar System. Formally designated 1I/2017 U1, it was discovered by Robert Weryk using the Pan-STARRS telescope at Haleakala Observatory, Hawaii, on 19 October 2017, 40 days after it passed its closest point to the Sun. When first seen, it was about 33,000,000 km (21,000,000 mi; 0.22 AU) from Earth (about 85 times as far away as the Moon), and already heading away from the Sun. Initially assumed to be a comet, it was reclassified as an asteroid a week later, and finally (6 November 2017) as the first of the new class of interstellar object.

ʻOumuamua is a small object, estimated to be about 230 by 35 meters (800 ft × 100 ft) in size. It has a dark red color, similar to objects in the outer Solar System. ʻOumuamua showed no signs of a comet tail despite its close approach to the Sun, and has significant elongation and rotation rate, so it is thought to be a metal-rich rock with a relatively high density. ʻOumuamua is tumbling rather than smoothly rotating, and it 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 in interstellar space. ʻOumuamua's system of origin and the amount of time it has been traveling among the stars are unknown.

Nomenclature[edit]

As the first known object of its type, ʻOumuamua presented a unique case for the International Astronomical Union, which assigns designations for astronomical objects. Before its true nature was known it was classified as comet C/2017 U1 and later as asteroid A/2017 U1. Once it was unambiguously identified as an interstellar object, a new designation was created, I, with ʻOumuamua being 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).[3]

The name comes from Hawaiian ʻoumuamua, meaning 'scout',[15] (from ʻou, meaning 'reach out for', and mua, reduplicated for emphasis​, meaning 'first, in advance of'[3]) and reflects the way this object is like a scout or messenger sent from the distant past to reach out to us.[3][16] The first character is a Hawaiian ʻokina, not an apostrophe, and is represented by a single quotation mark and pronounced as a glottal stop; the name was chosen by the Pan-STARRS team[17] in consultation with Kaʻiu Kimura and Larry Kimura of the University of Hawaii at Hilo.[18]

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 science fiction novel Rendezvous with Rama by Arthur C. Clarke.[19]

Observations[edit]

ʻOumuamua, imaged here with the William Herschel Telescope on 28 October, is seen as a stationary light source in the center of the image. Background stars appear streaked because the telescope is tracking the rapidly moving asteroid.

Observations and conclusions concerning the trajectory of ʻOumuamua were primarily obtained with data from the Pan-STARRS1 Telescope and the Canada-France-Hawaii Telescope (CFHT), and its composition and shape from the Very Large Telescope and the Gemini South telescope in Chile,[20] as well as the Keck II telescope in Hawaii. These were collected by Karen J. Meech, Robert Weryk and their colleagues and published in Nature on 20 November.[21][22] Post announcement, the space-based telescopes Hubble and Spitzer joined in the observations.[23]

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.[12] By the end of October ʻOumuamua had already faded to apparent magnitude ~23,[24] and by mid-December 2017, it was expected to be too faint and fast moving to be studied by even the largest ground-based telescopes.[20]

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 and limited in size.[25] However ʻOumuamua has a reddish hue and unsteady brightness which is typical of asteroids.[26][27][28]

The SETI Institute's radio telescope, the Allen Telescope Array, examined ʻOumuamua, but detected no unusual radio emissions.[29] More detailed observations, using the Breakthrough Listen hardware and the Green Bank Telescope, were planned.[25][29][30]

Trajectory[edit]

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 6 September and 16 October 2017, and moving northward again towards its destination in Pegasus.
ʻOumuamua's hyperbolic trajectory over the Solar System
Animation of ʻOumuamua passing through the Solar System

ʻOumuamua is the first known interstellar object to visit the Solar System and it appears to come from roughly the direction of the star Vega in the constellation Lyra.[26][27][31][32] The incoming direction of motion of ʻOumuamua is 6° from the solar apex (the direction of the Sun's movement relative to local stars), which is the most likely direction for approaches from objects outside the Solar System.[31][33] On 26 October, two precovery observations from the Catalina Sky Survey were found dated 14 and 17 October.[34][24] A two-week observation arc had verified a strongly hyperbolic trajectory.[6][21] It has a hyperbolic excess velocity (velocity at infinity, ) of 26.33 km/s (58,900 mph), its speed relative to the Sun when in interstellar space.[n 3]

ʻOumuamua speed relative to the Sun[35]
Distance Date Velocity
km/s
2300 AU 1605 26.34
1000 AU 1839 26.35
100 AU 2000 26.67
10 AU 2016 29.50
1 AU 9 August 2017 49.67
Perihelion 9 September 2017 87.71[7]
1 AU 10 October 2017 49.67[n 4]
10 AU 2019 29.51
100 AU 2034 26.65
1000 AU 2196 26.36
2300 AU 2430 26.32

By mid November, astronomers were certain that it was an interstellar object.[36] Based on observations spanning 34 days, ʻOumuamua's orbital eccentricity is 1.20, the highest ever observed.[37][7] An eccentricity above 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,[37][38][n 5] ʻOumuamua's eccentricity is so high it could not have been obtained through an encounter with any of the Sun's planets, known or unknown. Even undiscovered planets, if any exist, could not account for ʻOumuamua's trajectory – any undiscovered planet must be far from the Sun and hence moving slowly according to Kepler's laws of planetary motion. Encounters with such a planet could not boost ʻOumuamua's speed to the observed value,[39] and therefore ʻOumuamua can only be of interstellar origin.

ʻOumuamua entered the Solar System from above 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 (196,200 mph) as it passed below the ecliptic on 6 September and made a sharp turn upward 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.[40][7][n 6] The object is now heading away from the Sun (towards Pegasus) at an angle of 66° from the direction of its approach.[n 7]

On the outward leg of its journey through the Solar System, ʻOumuamua passed below 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 above the ecliptic on 16 October and passed above the orbit of Mars on 1 November.[40][31][6] It will pass above Jupiter's orbit in May 2018, Saturn's orbit in January 2019, and Neptune's orbit in 2022.[40] As it leaves the Solar System it will be approximately right ascension (RA) 23h51m and declination +24°45', in Pegasus.[7] 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.[7] It will take the object roughly 20,000 years to leave the Solar System completely.[n 8]

Indications of origin[edit]

Inbound velocity at 200 AU from the Sun
compared to Oort cloud objects[35]
Object Velocity
km/s		 # of observations
and obs arc[n 9]
90377 Sedna 2.66 196 in 9240 days
C/1980 E1 (Bowell) 2.98 179 in 2514 days
C/1997 P2 (Spacewatch) 2.99 94 in 49 days
C/2010 X1 (Elenin) 2.96 2222 in 235 days
C/2012 S1 (ISON) 2.99 6514 in 784 days
C/2008 J4 (McNaught) 4.88 22 in 15 days[n 10]
1I/2017 U1 (ʻOumuamua) 26.5 115 in 34 days

Accounting for Vega's proper motion, it would have taken ʻOumuamua 600,000 years to reach the Solar System from Vega.[21] But as a nearby star, Vega was not in the same part of the sky at that time.[31] Astronomers calculate that one hundred years ago, the asteroid 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.[7] 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 M dwarf stars. This velocity profile also indicates an extrasolar origin, but appears to rule out the closest dozen of stars.[42] In fact, the strong correlation between ʻOumuamua's velocity and the local standard of rest, might mean that it has circulated the galaxy several times and thus may have originated from an entirely different part of the Milky Way.[21]

It is unknown how long the object has been traveling among the stars.[40] 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.[43][21] 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 (Carina or Columba specifically), within a range of about 100 parsecs,[44] some 45 million years ago.[45] 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 star became a red giant.[46] 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 systems Oort cloud at a speed of 103 km/s (230,000 mph).[47][n 11]

According to one hypothesis, ʻOumuamua could be a fragment from a tidally disrupted planet.[48]

Asteroidal nature[edit]

Initially, ʻOumuamua was announced as comet C/2017 U1 (PANSTARRS) on 25 October 2017 based on a strongly hyperbolic trajectory.[2] 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.[n 12] Accordingly, the object was renamed A/2017 U1, becoming the first comet ever to be re-designated as an asteroid.[4] Once it was identified as an interstellar object, it was designated 1I/2017 U1, the first member of a new class of objects.[3] 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.[8] 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. 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.[43]

Appearance, shape, and composition[edit]

Spectra recorded by the 4.2 m (14 ft) William Herschel Telescope on 25 October showed that the object was featureless, and colored red like Kuiper belt objects.[49] Spectra from the Hale Telescope showed a less-red color resembling comet nuclei or Trojans.[43] Its spectrum is similar to that of D-type asteroids.[8]

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.[10][50] This accounts for the various rotation periods reported, such as 8.10 hours, (±0.42 hours)[12] (±0.02 hours)[11] with a lightcurve amplitude of 1.5–2.1 magnitudes,[11] whereas Meech et al. reported a rotation period of 7.3 hours and a lightcurve amplitude of 2.5 magnitudes.[51][n 13] 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.[10]

Artist's impression of ʻOumuamua

The large variations on the light curves indicate that ʻOumuamua is a highly elongated object, comparable to or greater than the most elongated Solar System objects.[12][11] 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 or triaxial ellipsoid shape are precisely known. The longest-to-shortest axis ratio could be 5:1 or greater.[10] Assuming an albedo of 10% (typical for D-type asteroids) and a 6:1 ratio, ʻOumuamua has dimensions of approximately 230 m × 35 m × 35 m (800 ft × 100 ft × 100 ft) with an average diameter of about 110 m (360 ft).[8][9] According to astronomer David Jewitt, the object is physically unremarkable except for its highly elongated shape.[9] Bannister et al. have suggested that it could also be a contact binary,[12] although this may not be compatible with its rapid rotation.[22] 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.[22] JPL News reported that ʻOumuamua "is up to one-quarter mile (400 meters) long and highly-elongated-perhaps 10 times as long as it is wide".[23][52]

Light curve observations suggest the asteroid may be composed of dense metal-rich rock that has been reddened by millions of years of exposure to cosmic rays.[22][53][54] 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.[55][56] This possibility is inferred from spectroscopic characterization and its dark and reddened color,[55][57] and from the expected effects of interstellar radiation.[57] 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".[57]

Continuing observations[edit]

In December 2017, Harvard University Astronomy Professor Avi Loeb, an adviser to the Breakthrough Listen Project, cited ʻOumuamua's unusually elongated shape as one of the reasons why the Green Bank Telescope in West Virginia would listen for radio emissions from it to see if there were any unexpected signs that it might be of artificial origin,[52] although earlier limited observations by other radio telescopes such as the SETI Institute's Allen Telescope Array had produced no such results.[29] On 13 December 2017, the Green Bank Telescope observed the asteroid for six hours across four bands of radio frequency. No radio signals from ʻOumuamua were detected in this very limited scanning range, but observations are ongoing.[58][59]

Hypothetical space missions[edit]

ʻOumuamua is traveling too fast for any existing spacecraft to reach.[60] The Initiative for Interstellar Studies (i4is) has launched Project Lyra for assessing the feasibility of a mission to ʻOumuamua.[61] Several options for sending a spacecraft to ʻOumuamua within a time-frame of 5 to 10 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.[33] 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 'Oumuamua would be "highly desirable, due to the minimal science return from a hyper-velocity encounter".[33] If the investigative craft goes too fast, it would not be able to get into orbit or land on the asteroid and would fly past it. The authors conclude that, although challenging, an encounter mission would be feasible using near-term technology.[33][61] Astronomers estimate that several interstellar objects similar to ʻOumuamua pass inside the orbit of Earth each year.[40] If true, this provides possible opportunities for future studies of interstellar objects, although with the current space technology, close visits and orbital missions are impossible due to their high speeds.[62]

Notes[edit]

  1. ^ Objects on hyperbolic trajectories have negative semimajor axis, giving them a positive orbital energy.
  2. ^ 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.
  3. ^ For comparison, comet C/1980 E1 will only be moving 4.2 km/s when it is 500 AU from the Sun.
  4. ^ 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 major semi-axis.
  5. ^ 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.[31]
  6. ^ Comet C/2012 S1 (ISON) peaked at 377 km/s (840,000 mph) at perihelion[41] because it passed 0.0124 AU from the Sun (20 times closer than ʻOumuamua).
  7. ^ According to the formula:
  8. ^ 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 trillion kilometers) 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)
  9. ^ Orbits computed with only a handful of observations can be unreliable. Short arcs can result in computer generated orbits rejecting some data unnecessarily.
  10. ^ Other orbital solutions show C/2008 J4 entering the Solar System @ 3.5 ± 1.3 km/s. JPL #10 shows that on 1855-Mar-24 C/2008 J4 was moving 4.88 ± 1.8 km/s.
  11. ^ 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.
  12. ^ According to Central Bureau for Astronomical Telegrams's CBET 4450, none of the observers had detected any sign of cometary activity. The initial classification as a comet was based on the object's orbit.
  13. ^ 1865 Cerberus has a lightcurve amplitude of 2.3 magnitudes.

References[edit]

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    JPL 1 (Solution date: 2017-Oct-24) Archived 25 October 2017 at the Wayback Machine.
    JPL 10 (Solution date: 2017-Nov-03) Archived 7 November 2017 at the Wayback Machine.
    JPL 14 (Solution date: 2017-Nov-21) Archived 22 November 2017 at the Wayback Machine.
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  33. ^ a b c d Hein, Andreas M.; Perakis, Nikolaos; Long, Kelvin F.; Crowl, Adam; Eubanks, Marshall; Kennedy, Robert G., III; Osborne, Richard (2017). "Project Lyra: Sending a Spacecraft to 1I/ʻOumuamua (former A/2017 U1), the Interstellar Asteroid". arXiv:1711.03155 Freely accessible [physics.space-ph]. 
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  35. ^ a b "HORIZONS Web-Interface". Solar System Dynamics Group. JPL.  Results produced with JPL Horizons On-Line Ephemeris System using Soln.date: 2017-Nov-21. Observer Location: "@sun" / Table settings: "20. Observer range & range-rate", "22. Speed wrt Sun & observer".
  36. ^ 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. 
  37. ^ a b "JPL Small-Body Database Search Engine — Constraints: e > 1". JPL Small-Body Database. Jet Propulsion Laboratory. Retrieved 26 October 2017. 
  38. ^ de la Fuente Marcos, Carlos; de la Fuente Marcos, Raúl (1 November 2017). "Pole, Pericenter, and Nodes of the Interstellar Minor Body A/2017 U1". Research Notes of the AAS. 1 (1): 5. arXiv:1711.00445 Freely accessible. Bibcode:2017RNAAS...1....5D. doi:10.3847/2515-5172/aa96b4. 
  39. ^ Jason T. Wright (7 December 2017). "On Distinguishing Interstellar Objects Like ʻOumuamua From Products of Solar System Scattering". Research Notes of the AAS. The American Astronomical Society. 1 (1): 38. Bibcode:2017RNAAS...1...38W. doi:10.3847/2515-5172/aa9f23. 
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  42. ^ Mamajek, Eric (2017). "Kinematics of the Interstellar Vagabond A/2017 U1". arXiv:1710.11364 Freely accessible [astro-ph.EP]. 
  43. ^ a b c Ye, Quan-Zhi; Zhang, Qicheng (5 December 2017). "1I/ʻOumuamua is Hot: Imaging, Spectroscopy and Search of Meteor Activity". The Astrophysical Journal Letters. 851: L5. arXiv:1711.02320 Freely accessible. doi:10.3847/2041-8213/aa9a34. 
  44. ^ Moór, A.; Szabó, Gy. M.; Kiss, L. L.; Kiss, Cs.; Ábrahám, P.; Szulágyi, J.; Kóspál, Á.; Szalai, T. (2013). "Unveiling new members in five nearby young moving groups". Monthly Notices of the Royal Astronomical Society. 435 (2): 1376–1388. arXiv:1309.1669 Freely accessible. Bibcode:2013MNRAS.435.1376M. doi:10.1093/mnras/stt1381. 
  45. ^ Gaidos, Eric; Williams, Jonathan P.; Kraus, Adam (2017). "Origin of Interstellar Object A/2017 U1 in a Nearby Young Stellar Association?". Research Notes of the AAS. 1 (1): 13. arXiv:1711.01300 Freely accessible. Bibcode:2017RNAAS...1...13G. doi:10.3847/2515-5172/aa9851. 
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  47. ^ Portegies Zwart, Simon; Pelupessy, Inti; Bedorf, Jeroen; Cai, Maxwell; Torres, Santiago (9 November 2017). "The origin of interstellar asteroidal objects like 1I/2017 U1". arXiv:1711.03558 Freely accessible [astro-ph.EP]. 
  48. ^ Ćuk, Matija (19 December 2017). "1I/ʻOumuamua as a Tidal Disruption Fragment From a Binary Star System". arXiv:1712.01823 Freely accessible [astro-ph.EP]. 
  49. ^ Fitzsimmons, Alan [@FitzsimmonsAlan] (27 October 2017). "Spectrum of A/2017 U1 obtained on Wednesday night with the @INGLaPalma 4.2m WHT. Colour is red like Kuiper Belt Objects, featureless" (Tweet) – via Twitter. 
  50. ^ Drahus, Michal; Guzik, Piotr; Waniak, Waclaw; Handzlik, Barbara; Kurowski, Sebastian; Xu, Siyi (1 December 2017). "Tumbling motion of 1I/ʻOumuamua reveals body's violent past". arXiv:1712.00437 Freely accessible [astro-ph.EP]. 
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  52. ^ a b Ian 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. 
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  55. ^ a b Matt Williams (20 November 2017). "That Interstellar Asteroid is probably pretty strange looking". Universe Today. Retrieved 20 December 2017. It’s dark and reddened surface is also an indication of tholins, which are the result of organic molecules (like methane) being irradiated by cosmic rays for millions of years. 
  56. ^ Matt Williams (24 November 2017). "Project Lyra, a mission to chase down that interstellar asteroid". Universe Today. Retrieved 20 December 2017. It was also determined to be rocky and metal rich, and to contain traces of tholins – organic molecules that have been irradiated by UV radiation.  Also here at Phys.org
  57. ^ a b c Fitzsimmons, Alan; Snodgrass, Colin; Rozitis, Ben; Yang, Bin; Hyland, Meabh; Seccull, Tom; Bannister, Michele T.; Fraser, Wesley C.; Jedicke, Robert; Lacerda, Pedro (18 December 2017). "Spectroscopy and thermal modelling of the first interstellar object 1I/2017 U1 'Oumuamua". Nature Astronomy. arXiv:1712.06552 Freely accessible [astro-ph.EP]. 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 'Oumuamua, 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 'Oumuamua passed within 0.25 au of the Sun. 
  58. ^ "Breakthrough Listen Releases Initial Results and Data from Observations of 'Oumuamua". Breakthrough Listen. 13 December 2017. Retrieved 15 December 2017. No evidence of artificial signals emanating from the object so far detected by the Green Bank Telescope, but monitoring and analysis continue. Initial data are available for public inspection in the Breakthrough Listen archive 
  59. ^ Ian Sample (15 December 2017). "Is 'Oumuamua an alien spacecraft? Initial scans show no signs of technology". The Guardian. Retrieved 15 December 2017. 
  60. ^ Clarke, Stephen (22 November 2017). "An interstellar interloper is dashing through our solar system". Astronomy Now. Retrieved 24 November 2017. 
  61. ^ a b "Project Lyra – A Mission to ʻOumuamua". I4IS. Initiative for Interstellar Studies. 
  62. ^ Berger, Eric (22 November 2017). "Chasing ʻOumuamua — unfortunately human technology isn't up to the task". Ars Technica. Retrieved 23 November 2017. Chemical propulsion just doesn’t close the case in this scenario. 

External links[edit]

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