# ʻOumuamua

(Redirected from 1I/ʻOumuamua)

Discovery [1][2] Interstellar comet 'Oumuamua exits the Solar System (artist concept) (animation) Robert Weryk using Haleakala Obs., Hawaii 19 October 2017 1I/2017 U1[3] IPA: [ʔouˌmuəˈmuə] 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] Epoch 2 November 2017 (JD 2458059.5) 34 days 0.25534±0.00007 AU −1.2798±0.0008 AU[Note 1] 1.19951±0.00018 26.33±0.01 km/s (interstellar)[7] 36.425° 0° 40m 48.72s / day 122.69° 24.599° 241.70° 0.0959 AU · 37.3 LD 1.455 AU 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][Note 2][14] 22.08±0.445[6]

ʻOumuamua () is a mildly active comet, and the first interstellar object detected passing 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.

ʻOumuamua is 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, but has since undergone non-gravitational acceleration consistent with comet outgassing. It has significant elongation and rotation rate, so it is thought to be metal-rich with a relatively high 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. ʻOumuamua's system of origin and the amount of time it has spent traveling amongst the stars are unknown.

## Nomenclature

Hyperbolic trajectory of ʻOumuamua through the inner Solar System with the Sun at the focus (animation)

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).[3] It has since been determined that ʻOumuamua is actually a mildly active comet.[15][16][17][18]

The name comes from Hawaiian ʻoumuamua, meaning 'scout'[19] (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 humanity. It roughly translates to "first distant messenger".[3][20] 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[21] in consultation with Kaʻiu Kimura and Larry Kimura of the University of Hawaii at Hilo.[22]

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.[23]

## Observations

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, part of the Spaceguard Survey,[24] and the Canada-France-Hawaii Telescope (CFHT), and its composition and shape from the Very Large Telescope and the Gemini South telescope in Chile,[25] 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.[26][27] Post announcement, the space-based telescopes Hubble and Spitzer joined in the observations.[28]

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,[29] 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.[25]

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.[30] However ʻOumuamua has a reddish hue and unsteady brightness which is typical of asteroids.[31][32][33]

The SETI Institute's radio telescope, the Allen Telescope Array, examined ʻOumuamua, but detected no unusual radio emissions.[34] More detailed observations, using the Breakthrough Listen hardware and the Green Bank Telescope, were planned;[30][34][35] the eventual data were searched for narrowband signals and none were found. Given the close proximity to this interstellar object, limits were placed to putative transmitters with the extremely low power of 0.08 watts.[36]

### Trajectory

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 appears to have come from roughly the direction of Vega in the constellation Lyra.[31][32][37][38] 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.[37][39] On 26 October, two precovery observations from the Catalina Sky Survey were found dated 14 and 17 October.[40][29] A two-week observation arc had verified a strongly hyperbolic trajectory.[6][26] It has a hyperbolic excess velocity (velocity at infinity, ${\displaystyle v_{\infty }\!}$) of 26.33 km/s (94,800 km/h), its speed relative to the Sun when in interstellar space.[Note 3]

ʻOumuamua speed relative to the Sun[41]
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[Note 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.[42] Based on observations spanning 34 days, ʻOumuamua's orbital eccentricity is 1.20, the highest ever observed.[43][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,[43][44][Note 5] 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, 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,[45] and therefore ʻOumuamua can only be of interstellar origin.[46]

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 (315,800 km/h) 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.[47][7][Note 6] The object is now heading away from the Sun towards Pegasus at an angle of 66° from the direction of its approach.[Note 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.[47][37][6] It passed above Jupiter's orbit in May 2018, and will pass above Saturn's orbit in January 2019 and Neptune's orbit in 2022.[47] As it leaves the Solar System it will be approximately right ascension 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.[Note 8]

#### Indications of origin

Inbound velocity at 200 AU from the Sun
compared to Oort cloud objects[41]
Object Velocity
km/s
# of observations
and obs arc[Note 9]
90377 Sedna 1.99 196 in 9240 days
C/1980 E1 (Bowell) 2.96 179 in 2514 days
C/1997 P2 (Spacewatch) 2.96 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[Note 10]
1I/2017 U1 (ʻOumuamua) 26.5 121 in 34 days

Accounting for Vega's proper motion, it would have taken ʻOumuamua 600,000 years to reach the Solar System from Vega.[26] But as a nearby star, Vega was not in the same part of the sky at that time.[37] 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 red dwarfs. This velocity profile also indicates an extrasolar origin, but appears to rule out the closest dozen of stars.[49] 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.[26]

It is unknown how long the object has been traveling among the stars.[47] 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.[50][26] 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,[51] some 45 million years ago.[52] 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.[53] 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 speed of 103 km/s (370,000 km/h).[54][Note 11]

According to one hypothesis, ʻOumuamua could be a fragment from a tidally disrupted planet.[55][Note 12] This makes 'Oumuamua a rare object, much less abundant than other extrasolar "dusty-snowball" comets or asteroids could be.

### Classification

Initially, ʻOumuamua was announced as comet C/2017 U1 (PANSTARRS) 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.[Note 13] 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.[citation needed] It is difficult to say which scenario is more likely due to the chaotic nature of small body dynamics,[citation needed] 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.[50]

On 27 June 2018, astronomers reported that ʻOumuamua is 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.[15][16][17][18]

### Appearance, shape, and composition

Light curve from 25–27 October 2017 with dotted line from a model with 10:1 elongation
Simulation of ʻOumuamua spinning and tumbling through space, and the resultant light curve. In reality, in all observations of ʻOumuamua the asteroid only constituted a single pixel and its shape was inferred from the light curve.
Artist's impression of ʻOumuamua

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.[56] Spectra from the Hale Telescope showed a less-red color resembling comet nuclei or Trojans.[50] Its spectrum is similar to that of D-type asteroids.[8]

ʻOumuamua is rotating around a non-principal axis, a type of movement known as tumbling.[10][57] 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.[58][Note 14] 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.[59][60]

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.[27] 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.[27] 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".[28][61]

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.[27][62][63] 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.[64][65] This possibility is inferred from spectroscopic characterization and its dark and reddened color,[64][66] and from the expected effects of interstellar radiation.[66] 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".[66]

### Continuing observations

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,[61] although earlier limited observations by other radio telescopes such as the SETI Institute's Allen Telescope Array had produced no such results.[34] 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.[67][68]

## Hypothetical space missions

ʻOumuamua is traveling too fast for any existing spacecraft to reach.[69] The Initiative for Interstellar Studies (i4is) has launched Project Lyra for assessing the feasibility of a mission to ʻOumuamua.[70] 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 (2.1×106 km; 1.3×106 mi) in order to take advantage of the Oberth effect.[39] 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".[39] 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.[39][70] Astronomers estimate that several interstellar objects similar to ʻOumuamua pass inside the orbit of Earth each year,[47] and that 10,000 are passing inside the orbit of Neptune on any given day.[71] If correct, 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.[72]

## Notes

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.[37]
6. ^ Comet C/2012 S1 (ISON) peaked at 377 km/s (1,360,000 km/h) at perihelion[48] because it passed 0.0124 AU from the Sun (20 times closer than ʻOumuamua).
7. ^ According to the formula: ${\displaystyle 2\,\theta {_{\infty }}=2\cos ^{-1}(-1/e)}$
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. ^ See also arXiv:1801.02658v2 [astro-ph.EP], Roman R.Ravikov, 16th January 2018 'Oumuamua 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.
13. ^ 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.
14. ^ 1865 Cerberus has a lightcurve amplitude of 2.3 magnitudes.

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