ʻOumuamua is the first known 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 Haleakalā Observatory, Hawaii, on 19 October 2017, 40 days after it passed its closest point to the Sun on 9 September. When it was first observed, it was about 33 million km (21 million mi; 0.22 AU) from Earth (about 85 times as far away as the Moon), and already heading away from the Sun.

A2017U1 5gsmoothWHT enhanced.jpg
ʻOumuamua on 28 October 2017[a]
Discovery [2][3]
Discovered byRobert Weryk using Pan-STARRS 1
Discovery siteHaleakalā Obs., Hawaii
Discovery date19 October 2017
1I/2017 U1[4]
Pronunciation/ˌməˈmə/, Hawaiian: [ʔowˌmuwəˈmuwə] (About this soundlisten)
Named after
Hawaiian term for scout[4]
  • 1I
  • 1I/ʻOumuamua
  • 1I/2017 U1 (ʻOumuamua)
  • A/2017 U1[5]
  • C/2017 U1[3]
  • P10Ee5V[6]
Interstellar object[4]
Hyperbolic asteroid[7][8][9]
Orbital characteristics[7]
Epoch 2 November 2017 (JD 2458059.5)
Observation arc34 days
Perihelion0.25534±0.00007 AU
−1.2798±0.0008 AU[b]
26.33±0.01 km/s (interstellar)[10]
5.55 AU/year
0° 40m 48.72s / day
Earth MOID0.0959 AU · 37.3 LD
Jupiter MOID1.455 AU
Physical characteristics
Dimensions100–1000 m long[11][12][13]
230 m × 35 m × 35 m[14][15]
(est. at albedo 0.10)[14][15]
Tumbling (non-principal axis rotation)[16]
Reported values include:
8.10±0.02 h[17]
8.10±0.42 h[18]
0.1 (spectral est.)[14]
0.06–0.08 (spectral est.)[18]
B–V = 0.7±0.06[14]
V-R = 0.45±0.05[14]
g-r = 0.47±0.04[18]
r-i = 0.36±0.16[18]
r-J = 1.20±0.11[18]
19.7 to >27.5[10][20][c]

ʻOumuamua is a small object estimated to be between 100 and 1,000 metres (300 and 3,000 ft) long, with its width and thickness both estimated to range between 35 and 167 metres (115 and 548 ft).[11] It has a dark red color, similar to objects in the outer Solar System. Despite its close approach to the Sun, ʻOumuamua showed no signs of having a coma, but did exhibit non‑gravitational acceleration.[21][22] Nonetheless, the object could be a remnant of a disintegrated rogue comet (or exocomet), according to a NASA scientist.[23][24] The object has a rotation rate similar to the average spin rate seen in Solar System asteroids, but many valid models permit it to be more elongated than all but a few other natural bodies. While an unconsolidated object (rubble pile) would require it to be of a density similar to rocky asteroids,[25] a small amount of internal strength similar to icy comets[26] would allow a relatively low density. ʻOumuamua's light curve, assuming little systematic error, presents its motion as tumbling, rather than smoothly rotating, and moving sufficiently fast relative to the Sun that few possible models define a Solar System origin, although an Oort cloud origin cannot be excluded. Extrapolated and without further deceleration, the path of ʻOumuamua cannot be captured into a solar orbit, so it would eventually leave the Solar System and continue into interstellar space. ʻOumuamua's planetary system of origin, and the age of its excursion, are unknown.

In July 2019, astronomers reported that ʻOumuamua was a natural object. This came after speculation of ʻOumuamua being an alien object.[27][28]


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).[4]

The name comes from Hawaiian ʻoumuamua 'scout'[29] (from ʻou 'reach out for', and mua, reduplicated for emphasis 'first, in advance of'[4]), 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'.[4][30] The first character is a Hawaiian ʻokina, not an apostrophe, and is pronounced as a glottal stop; the name was chosen by the Pan-STARRS team[31] in consultation with Kaʻiu Kimura and Larry Kimura of the University of Hawaiʻi at Hilo.[32]

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.[33]


Observations and conclusions concerning the trajectory of ʻOumuamua were primarily obtained with data from the Pan-STARRS1 Telescope, part of the Spaceguard Survey,[34] and the Canada–France–Hawaii Telescope (CFHT), and its composition and shape from the Very Large Telescope and the Gemini South telescope in Chile,[35] 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 2017.[36][37] Post announcement, the space-based telescopes Hubble and Spitzer joined in the observations.[38]

ʻOumuamua has faded into the 34th magnitude as of 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.[18] By the end of October, ʻOumuamua had already faded to apparent magnitude ~23,[39] and in mid-December 2017, it was too faint and fast moving to be studied by even the largest ground-based telescopes.[35]

ʻOumuamua was compared to the fictional alien spacecraft Rama due to its interstellar origin. Adding to the coincidence, both the real and the fictional objects are unusually elongated.[40] ʻOumuamua has a reddish hue and unsteady brightness, which are typical of asteroids.[41][42][43]

The SETI Institute's radio telescope, the Allen Telescope Array, examined ʻOumuamua, but detected no unusual radio emissions.[44] More detailed observations, using the Breakthrough Listen hardware and the Green Bank Telescope, were performed;[40][44][45] the 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 effective isotropically radiated power of 0.08 watts.[46]


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

ʻOumuamua appears to have come from roughly the direction of Vega in the constellation Lyra.[41][42][47][48] 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.[47][49] On 26 October, two precovery observations from the Catalina Sky Survey were found dated 14 and 17 October.[50][39] A two-week observation arc had verified a strongly hyperbolic trajectory.[7][36] It has a hyperbolic excess velocity (velocity at infinity,  ) of 26.33 km/s (94,800 km/h; 58,900 mph), its speed relative to the Sun when in interstellar space.[d]

ʻOumuamua speed relative to the Sun[51]
Distance Date Velocity
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[10]
1 AU 10 October 2017 49.67[e]
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.[52] Based on observations spanning 34 days, ʻOumuamua's orbital eccentricity is 1.20, the highest ever observed[53][10] until 2I/Borisov was discovered in August 2019. 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,[53][54][f] 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.[55][56]

Animation of ʻOumuamua passing through the Solar System
Inbound velocity at 200 AU from the Sun
compared to Oort cloud objects[51]
Object Velocity
# of observations
and obs arc[g]
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[h]
1I/2017 U1 (ʻOumuamua) 26.5 121 in 34 days
Pixel intensity based density plot animation based on reconstruction of the central bright spot observed on 28 October 2017

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; 196,200 mph) 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.[57][10][i] The object is now heading away from the Sun towards Pegasus towards a vanishing point 66° from the direction of its approach.[j] 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.[57][47][7] 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.[57] As it leaves the Solar System it will be approximately right ascension 23'51" and declination +24°45', in Pegasus.[10] It will continue to slow down until it reaches a speed of 26.33 kilometres per second (94,800 km/h; 58,900 mph) relative to the Sun, the same speed it had before its approach to the Solar System.[10]

Non-gravitational accelerationEdit

On 27 June 2018, astronomers reported a non-gravitational acceleration to ʻOumuamua's trajectory, potentially consistent with a push from solar radiation pressure.[59][60] Initial speculation as to the cause of this acceleration pointed to comet off-gassing,[22] 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.[61] A critical re-assessment of the comet hypothesis found that, instead of the observed stability of ʻOumuamua's spin, outgassing would have caused its spin to rapidly change due to its elongated shape, resulting in the object tearing apart.[8]

Indications of originEdit

Accounting for Vega's proper motion, it would have taken ʻOumuamua 600,000 years to reach the Solar System from Vega.[36] But as a nearby star, Vega was not in the same part of the sky at that time.[47] Astronomers calculate that one hundred years ago the object was 83.9 ± 0.090 billion km; 52.1 ± 0.056 billion mi (561 ± 0.6 AU) from the Sun and traveling at 26.33 km/s with respect to the Sun.[10] 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.[62] In fact, the closeness of ʻOumuamua's velocity to 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.[36]

It is unknown how long the object has been traveling among the stars.[57] The Solar System is likely the first planetary system that ʻOumuamua has closely encountered since being ejected from its birth star system, potentially several billion years ago.[63][36] 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,[64] some 45 million years ago.[65] 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.[66] 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 (34,000 mph; 54,000 km/h).[67][k] 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.[68] This study also identifies future close encounters of ʻOumuamua on its outgoing trajectory from the Sun.[69]

In April 2020, astronomers presented a new possible scenario for the object's origin.[70][71] According to one hypothesis, ʻOumuamua could be a fragment from a tidally disrupted planet.[72][l] If true, this would make ʻOumuamua a rare object, of a type much less abundant than most extrasolar "dusty-snowball" comets or asteroids. However, this scenario leads to cigar-shaped objects whereas ʻOumuamua's lightcurve favors a disc-like shape.[73]

In May 2020, it was proposed that the object was the first observed member of a class of small H2-ice-rich bodies that form at temperatures near 3 K in the cores of giant molecular clouds. The non-gravitational acceleration and high aspect ratio shape of ʻOumuamua might be explainable on this basis.[74] However, it was later calculated that hydrogen icebergs cannot survive their journey through interstellar space.[75]


Initially, ʻOumuamua was announced as comet C/2017 U1 (PANSTARRS) based on a strongly hyperbolic trajectory.[3] 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.[m] Accordingly, the object was renamed A/2017 U1, becoming the first comet ever to be re-designated as an asteroid.[5] Once it was identified as an interstellar object, it was designated 1I/2017 U1, the first member of a new class of objects.[4] 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.[14] 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.[63]

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.[22][76][61][77] However, studies submitted in October 2018 suggest that the object is neither an asteroid nor a comet,[8][9] although the object could be a remnant of a disintegrated interstellar comet (or exocomet), as suggested by a NASA scientist.[23][24]

Appearance, shape and compositionEdit

Spectra from the Hale Telescope on 25 October showed red color resembling comet nuclei or Trojans.[63] Higher signal to noise spectra recorded by the 4.2 m (14 ft) William Herschel Telescope later that day showed that the object was featureless, and colored red like Kuiper belt objects.[78] Spectra obtained with the 8.2 m (27 ft) Very Large Telescope the following night showed that behaviour continued into near-infrared wavelengths.[79] Its spectrum is similar to that of D-type asteroids.[14]

Light curve from 25 to 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.[16][80] This accounts for the various rotation periods reported, such as 8.10 hours (±0.42 hours[18] or ±0.02 hours[17]) by Bannister et al. and Bolin et al. with a lightcurve amplitude of 1.5–2.1 magnitudes,[17] whereas Meech et al. reported a rotation period of 7.3 hours and a lightcurve amplitude of 2.5 magnitudes.[81][n] 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.[16][82]

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,[18][17] or an extremely flat object, a pancake or oblate spheroid.[83] 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 its albedo nor its triaxial ellipsoid shape is precisely known. If cigar-shaped, the longest-to-shortest axis ratio could be 5:1 or greater.[16] Assuming an albedo of 10% (slightly higher than typical for D-type asteroids[84]) 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)[11][12][13][14][15] with an average diameter of about 110 m (360 ft).[14][15] According to astronomer David Jewitt, the object is physically unremarkable except for its highly elongated shape.[15] Bannister et al. have suggested that it could also be a contact binary,[18] although this may not be compatible with its rapid rotation.[37] 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.[37] 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".[38][85]

A 2019 paper finds the best models as either a cigar-shape, 1:8 aspect ratio, or disc-shape, 1:6 aspect ratio, with the disc more likely since its rotation doesn't require a specific orientation to see the range of brightnesses observed.[86] Monte Carlo simulations based on the available orbit determination suggest that the equatorial obliquity of ʻOumuamua could be about 93 degrees, if it has a very prolate or cigar-like shape, or close to 16 degrees, if it is very oblate or disk-like.[87]

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.[37][88][89] 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.[90][91] This possibility is inferred from spectroscopic characterization and its dark and reddened color,[90][79] and from the expected effects of interstellar radiation.[79] 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".[79]

In November 2019, some astronomers have noted that ʻOumuamua may be a "cosmic dust bunny", due to its "very lightweight and 'fluffy' conglomerate of dust and ice grains."[92][93][94]

In August 2020, astronomers reported that ʻOumuamua is not likely to have been composed of frozen hydrogen which had been proposed earlier; the compositional nature of the object continues to be unknown.[95][96]

Additional measurementsEdit

In December 2017, astronomer Avi Loeb of Harvard University, 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,[85] although earlier limited observations by other radio telescopes such as the SETI Institute's Allen Telescope Array had produced no such results.[44] On 13 December 2017, the Green Bank Telescope observed the object 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.[97][98]

In September 2018, astronomers described several possible home star systems from which ʻOumuamua may have originated.[99][100]


Hydrogen ice theoryEdit

It has been proposed that `Oumuamua contains a significant amount of hydrogen ice.[101][102] This would point to it originating from the core of an interstellar molecular cloud, where conditions for the formation of this material might exist.[103] The sun's heat would cause the hydrogen to sublimate, which would in turn propel the body. The hydrogen coma formed by this process would be difficult to detect from Earth-based telescopes, as the atmosphere blocks those wavelengths.[104] Regular water-ice comets undergo this as well, however to a much lesser extent and with a visible coma. This may explain the significant non-gravitational acceleration that `Oumuamua underwent without showing signs of coma formation. Significant mass loss caused by the sublimation would also explain the unusual cigar-like shape, comparable to how a bar of soap will always become more elongated as it is used up.

However, it was later shown that hydrogen icebergs cannot form out of small grains and that irrespective of their origin, they would quickly evaporate during their journey in interstellar space.[105]

Hypothetical space missionsEdit

The Initiative for Interstellar Studies (i4is) launched Project Lyra to assess the feasibility of a mission to ʻOumuamua.[106] Several options for sending a spacecraft to ʻOumuamua within a time-frame of 5 to 25 years were suggested.[107][108] Different mission durations and their velocity requirements were explored with respect to the launch date, assuming direct impulsive transfer to the intercept trajectory.

The Space Launch System (also being looked at for "interstellar precursor missions") would be even more capable:[109][110]

Such an interstellar precursor could easily pass by ʻOumuamua on its way out of the Solar System, at speeds of 63 km/s (39 mi/s).[111][112]

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".[49] 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.[49][106] 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.[113]

Other interstellar objectsEdit

2I/Borisov was discovered on 30 August 2019, and was soon confirmed to be an interstellar comet. Arriving from the direction of Cassiopeia, the object arrived at perihelion (closest point to the Sun) on 8 December 2019.

Alien object speculationEdit

On 26 October 2018, Loeb and his postdoc Shmuel Bialy submitted a paper exploring the possibility of ʻOumuamua being an artificial thin solar sail[114][115] accelerated by solar radiation pressure in an effort to help explain the object's non-gravitational acceleration.[59][60][116] Other scientists have stated that the available evidence is insufficient to consider such a premise,[117][118][119] and that a tumbling solar sail would not be able to accelerate.[120] In response, Loeb wrote an article detailing six anomalous properties of ʻOumuamua that make it unusual, unlike any comets or asteroids seen before.[121][122] 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.[123] A detailed podcast produced by Rob Reid provides the full details about the differences between ʻOumuamua and known comets.[124] In January 2021, Loeb is releasing a book, Extraterrestrial: The First Sign of Intelligent Life Beyond Earth, which describes the possibility that 'Oumauamua may be alien technology,[125][126] although such an explanation is considered very unlikely by most scientists.[127]

See alsoEdit

  • 514107 Kaʻepaokaʻawela, an asteroid of possible interstellar origin
  • 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


  1. ^ 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.[1]
  2. ^ Objects on hyperbolic trajectories have negative semimajor axis, giving them a positive orbital energy.
  3. ^ 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.
  4. ^ For comparison, comet C/1980 E1 will only be moving 4.2 km/s when it is 500 AU from the Sun.
  5. ^ 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.
  6. ^ 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.[47]
  7. ^ Orbits computed with only a handful of observations can be unreliable. Short arcs can result in computer generated orbits rejecting some data unnecessarily.
  8. ^ JPL #10 shows that on 1855-Mar-24 C/2008 J4 was moving 4.88±1.8 km/s.
  9. ^ Comet C/2012 S1 (ISON) peaked at 377 km/s (1,360,000 km/h) at perihelion[58] because it passed 0.0124 AU from the Sun (20 times closer than ʻOumuamua).
  10. ^ According to the formula:  
  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. 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.
  12. ^ 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.
  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.


  1. ^ 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.
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