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Artist's impression of `Oumuamua, the first confirmed interstellar asteroid[1]
Discovery [2][3]
Discovered by Pan-STARRS 1
Discovery site Haleakala Obs., Hawaii
Discovery date 19 October 2017
MPC designation A/2017 U1 [4]
Pronunciation /ˈməˈmə/ (About this sound listen)
Named after
(Hawaiian name)[5]
1I/2017 U1A/2017 U1 [6]
C/2017 U1 [3]P10Ee5V
Interstellar object[5]
hyperbolic asteroid [7]
Orbital characteristics[7]
Epoch 2017-Oct-31 (JD 2458057.5)
Observation arc 29 days
Aphelion n/a
Perihelion 0.25529 AU
−1.2805 AU[n 1]
Eccentricity 1.1994
26.33±0.01 km/s (interstellar)[8]
0° 40m 48.72s / day
Inclination 122.68°
Earth MOID 0.0958 AU37.3 LD
Jupiter MOID 1.455 AU
Physical characteristics
Dimensions 180 × 30 × 30 m (est. at 0.10)[9][10]
200 m (est. at 0.06–0.08)[11]
8.14±0.02 h[12]
8.10±0.42 h[11]
0.1 (spectral est.)[9]
0.06–0.08 (spectral est.)[11]
RR (red, featureless)[13]
B–V = 0.7±0.06[9]
V-R = 0.45±0.05[9]
g-r = 0.47±0.04[11]
r-i = 0.36±0.16[11]
r-J = 1.20±0.11[11]
19.7 to >27.5[8][n 2][14]

Oumuamua (formally designated 1I/ʻOumuamua previously C/2017 U1 (PANSTARRS) and A/2017 U1) is an interstellar object passing through the Solar System. It was discovered on a highly Hyperbolic trajectory by Robert Weryk on October 19, 2017 with observations made by the Pan-STARRS telescope[2] when the object was 0.2 AU (30,000,000 km; 19,000,000 mi) from Earth. Initially thought to be a Comet, it was reclassified as an Asteroid a week later. It is the first of a new class called hyperbolic asteroids.[7]

Based on a 29-day Observation arc, ʻOumuamua's Orbital eccentricity is 1.20, the highest of any object yet observed in the Solar System.[7][8] The previous record holder was C/1980 E1 with an outbound-orbit eccentricity of 1.057.[15][16][n 3] The high eccentricity of ʻOumuamua both inbound and outbound indicates that it has never been gravitationally bound to the Solar System and is an interstellar object due to its high incoming velocity. It has an inclination of 123° with respect to the Ecliptic,[n 4] and had a speed of 26.33 km/s (58,900 mph) relative to the Sun when in interstellar space, which peaked at 87.71 km/s (196,200 mph) at Perihelion.[8][n 5]


As the first known object of its type, ʻOumuamua presented a unique case for the International Astronomical Union, which assigns designations for astronomical objects. A new designation, "I", was established for interstellar objects, with ʻOumuamua being designated 1I, with rules on the eligibility of objects for I-numbers and the names to be assigned to these interstellar progress to be codified.[5]

The common name ʻOumuamua was chosen by the Pan-STARRS team. The name is Hawaiian in origin from the word meaning "scout" [17] (from ʻou, meaning "reach out for", and mua, with the reduplicated second mua for emphasis, meaning "first, in advance of"), and reflects the nature of the object as a "scout" or "messenger" from the past.[5] The first character is a Hawaiian ʻokina, not an Apostrophe.


ʻOumuamua, imaged with the William Herschel Telescope on 28 October 2017, is seen as a stationary light source in the centre of the image. Background stars appear streaked because the telescope is tracking the rapidly moving asteroid.

ʻOumuamua is the first known example of an interstellar object, appearing to come from roughly the direction of the star Vega in the constellation Lyra,[18][19] with a hyperbolic excess velocity of 26 km/s (58,000 mph) with respect to the Sun.[n 6] This direction is close to the Solar apex, the most likely direction for approaches from objects outside the Solar System.[18] How long the object has been drifting among the stars in the Galactic disc, is unknown. 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.[20]

On October 26, 2017, two Precovery observations from the Catalina Sky Survey were found dated October 14 and October 17.[21][4] A two-week Observation arc has verified the strongly hyperbolic nature of this object.[22] Observations and conclusions on the composition, shape, behavior, and origin compiled from the Very Large Telescope in Chile were collected by Weryk and colleagues and published in Nature on November 20, as ʻOumuamua was on its escape trajectory from the Solar System.[23][24]

Presuming it is a rock with an Albedo of 10%, it would be approximately 160 m (520 ft) in diameter.[18] Spectra recorded by the 4.2 m (14 ft) William Herschel Telescope on October 25 showed that the object was featureless, and colored red like Kuiper belt objects.[13] Spectra from the Hale Telescope showed a less red color resembling comet nuclei or Trojans.[20] Its spectrum is similar to that of D-type[9] or P-type asteroids.[11]

Inbound velocity at 200 AU from the Sun[n 7]
Object Year Velocity
# of observations
and obs arc[n 8]
90377 Sedna 1746 2.66 196 in 9240 days
C/1980 E1 (Bowell) 1765 2.98 179 in 2514 days
C/1997 P2 (Spacewatch) 1779 2.99 94 in 49 days
C/2010 X1 (Elenin) 1798 2.96 2222 in 235 days
C/2012 S1 (ISON) 1801 2.99 6514 in 784 days
C/2008 J4 (McNaught) 1855 4.88 22 in 15 days[n 9]
1I/ʻOumuamua 1982 26.5 113 in 29 days

1I/ʻOumuamua has a rotation period of 7.3[25] to ~8.10 (± 0.42) hours, with a lightcurve amplitude of 1.8 mag.[11] This indicates that it is a highly elongated object with an axis ratio of at least 5.3:1, comparable to or greater than the most elongated Solar System objects.[11][12] According to astronomer David Jewitt, the object is physically unremarkable except for its highly elongated shape.[10] Assuming an albedo of 0.1 (typical for D-type asteroids), 1I/ʻOumuamua has dimensions of approximately 180 × 30 × 30 m.[10][9] Bannister et al. have suggested that it could also be a contact binary,[11] although this may not be compatible with its rapid rotation.[23] One speculation regarding its shape is that it is a result of a violent event (such as a collision or stellar explosion) possibly associated with its ejection from its system of origin.[23]

Extrapolating the orbit backward, the asteroid is calculated to have gone through Perihelion on September 9, 2017, and to have passed approximately 0.1616 AU (24,180,000 km; 15,020,000 mi) from Earth on October 14, 2017.[7] The object is small and faint, and by the end of October, had already faded to Apparent magnitude ~23.[4]

It has been speculated that the object may have been ejected from a stellar system in the Carina–Columba association, some 45 million years ago.[26] The Carina–Columba stellar association is now very far in the sky from the constellation Lyra, the direction from which the object came. About 1.3 million years ago the object may have passed the nearby star TYC4742-1027-1 within a distance of 0.16 parsecs (0.52 light-years), but its velocity is too high to have originated from this star, as it probably just passed through the Oort cloud of that system at a speed of 103 km/s (230,000 mph).[27][n 10]

It has been extrapolated that one hundred years ago, the object was 561±0.6 AU (84 billion km) from the Sun and traveling at 26 km/s (58,000 mph) with respect to the Sun. The object continued to speed up until it went through perihelion, where it peaked at 87.7 km/s (196,000 mph).[8] By the discovery date it had slowed down to 46 km/s (100,000 mph) and will continue to slow down until it reaches a speed of 26 km/s (58,000 mph) relative to the Sun.[8] This interstellar speed is within ~5 km/s of other stars within the Sun's stellar neighborhood, which also indicates an interstellar origin.[28] The object is heading away from the Sun at an angle of 66°[n 11] from the direction from whence it came. It will pass Jupiter's orbit in May 2018 and Saturn's orbit in January 2019.[1] As it leaves the Solar System, it will be approximately R.A. 23h51m and Declination +24°45', in Pegasus.[8] It will take the object roughly 20,000 years to leave the Solar System.[n 12]

By mid November 2017, astronomers were certain that it was of extrasolar origin[29][30] as analysis by Jean Schneider demonstrates, the orbit is almost impossible to achieve from within our solar system.[31]

Asteroidal nature

1I/ʻOumuamua velocity with respect to the Sun
Distance Year Velocity
2300 AU 1605 26.34
1000 AU 1839 26.35
100 AU 2000 26.66
10 AU 2016 29.50
1 AU 2017 49.67
Perihelion 2017 87.7[8]
1 AU 2017 49.67[n 13]
10 AU 2019 29.50
100 AU 2034 26.64
1000 AU 2196 26.35
2300 AU 2430 26.32

Initially, 1I/ʻOumuamua was announced as a new comet C/2017 U1 (PANSTARRS) on 25 October 2017.[3] In an attempt to confirm the cometary activity, very deep stacked images were taken at the Very Large Telescope (VLT) later the same day, but the object showed no presence of a coma.[n 14] Accordingly, it was renamed A/2017 U1, becoming the first comet ever to be re-designated as an asteroid.[6]

Constraints on any possible activity imply that at most a few square meters of 1I/ʻOumuamua's surface may be covered by ice, and that any Volatiles (if they exist) must lie below a crust at least 0.5 m (1.6 ft) thick.[9] Nothing was seen in STEREO HI-1A observations near 1I/ʻOumuamua's perihelion on September 9, 2017.[11] The lack of a coma indicates that it must have formed within the frost line of the stellar system of origin or have been in the inner region of that stellar system long enough for all ice to sublime, as may be the case with damocloids. Analysis of its spectrum indicates that the latter is likely true.[32][33]

Nominal trajectory of 1I/ʻOumuamua (click for animation)
Path of 1I/ʻOumuamua, the first observed interstellar visitor from beyond the Solar System
Hyperbolic orbit of ʻOumuamua through the inner Solar System, with position on October 25, 2017

Potential space mission

The Initiative for Interstellar Studies (i4is) has studied potential options for sending a spacecraft to 1I/ʻOumuamua, perhaps using first a Jupiter flyby followed by a close solar flyby at 3 solar radii in order to take advantage of the Oberth effect.[34] More advanced options of using solar, laser electric, and laser sail propulsion, based on Breakthrough Starshot technology, are also considered. The problem 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. If the investigative craft goes too fast, it would not be able to get into orbit or land on the asteroid and would simply fly by it, moving at many asteroid diameters per second. The authors conclude that, although challenging, an encounter mission would be feasible using near-term technology.

Evidence of interstellar origin

The strongly hyperbolic passage of ʻOumuamua during the inbound trajectory indicates it has never undergone significant change due to gravitational forces within the Solar System, in addition the high velocity of the object is not possible to attribute to gravitational forces of the Solar System.[29][35][36]


  1. Objects in hyperbolic orbits 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 HST for fast-moving objects) around 1 January 2018.
  3. Unlike 1I/ʻOumuamua, C/1980 E1 got its high eccentricity due to a close encounter with Jupiter. Its inbound-orbit eccentricity was <1.
  4. Relative to the plane of the Solar System's ecliptic. The number is greater than 90, indicating that it orbits in the opposite direction to the planets at an inclination of about 60 degrees.
  5. Comet C/2012 S1 (ISON) peaked at 377 km/s during perihelion because it passed 0.0124 AU from the Sun (20 times closer than 2017 U1).
  6. For comparison, comet C/1980 E1 will only be moving 4.2 km/s when it is 500 AU from the Sun.
  7. Results produced with JPL Horizons. Observer Location: @sun / Table settings option "22. Speed wrt Sun". Option "39. Range & range-rate" for uncertainties.
  8. Orbits computed with only a handful of observations can be unreliable. Short arcs can result in computer generated orbits rejecting some data unnecessarily.
  9. 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.
  10. 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 parsec (447±43 light-years). It is not known if an encounter actually occurred.
  11. According to the formula 2×acos(1/e)
  12. Generically define the edge of the Solar System at 2 light-years (130,000 astronomical units), note that an Astronomical unit = 149597870.7 km, and that the comet is moving at 26.33 km/s. It will take the comet 23000 years to reach that distance. (130000 AU / 26.33 / 60 seconds-per-min / 60 min-per-hour / 24 hr-per-day / 365.25 days-per-year = 23405 years)
  13. 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.
  14. According to CBET 4450, none of the observers had detected any sign of activity. The initial classification as a comet was based on the object's orbit.


  1. 1.0 1.1 "Solar System's First Interstellar Visitor Dazzles Scientists". Jet Propulsion Laboratory. 20 November 2017. Retrieved 20 November 2017. 
  2. 2.0 2.1 "Small Asteroid or Comet 'Visits' from Beyond the Solar System". NASA. 2017-10-26. Retrieved 2017-10-29. 
  3. 3.0 3.1 3.2 "MPEC 2017-U181: COMET C/2017 U1 (PANSTARRS)". Minor Planet Center. International Astronomical Union. 25 October 2017. Retrieved 25 October 2017.  (CK17U010)
  4. 4.0 4.1 4.2 "A/2017 U1". Minor Planet Center. Retrieved 9 November 2017. 
  5. 5.0 5.1 5.2 5.3 "MPEC 2017-V17 : NEW DESIGNATION SCHEME FOR INTERSTELLAR OBJECTS". Minor Planet Center. International Astronomical Union. 6 November 2017. Retrieved 6 November 2017. 
  6. 6.0 6.1 "MPEC 2017-U183: A/2017 U1". Minor Planet Center. International Astronomical Union. 25 October 2017. Retrieved 25 October 2017.  (AK17U010)
  7. 7.0 7.1 7.2 7.3 7.4 7.5 "JPL Small-Body Database Browser: ʻOumuamua (A/2017 U1)" (JPL s13 with last obs: 2017-11-12). Jet Propulsion Laboratory. Archived from the original on 2017-10-25. Retrieved 9 November 2017. 
  8. 8.0 8.1 8.2 8.3 8.4 8.5 8.6 8.7 "Pseudo-MPEC for A/2017 U1 (Fact File)". Bill Gray of Project Pluto. 26 October 2017. Retrieved 26 October 2017.  (Orbital elements)
  9. 9.0 9.1 9.2 9.3 9.4 9.5 9.6 9.7 Jewitt, David (2017). "Interstellar Interloper 1I/2017 U1: Observations from the NOT and WIYN Telescopes" (PDF). Submitted to ApJL: 11. ArXiv:1711.05687Freely accessible. 
  10. 10.0 10.1 10.2 "A Familiar-Looking Messenger from Another Solar System". National Optical Astronomy Observatory. Retrieved 15 November 2017. 
  11. 11.00 11.01 11.02 11.03 11.04 11.05 11.06 11.07 11.08 11.09 11.10 11.11 Bannister, M. T.; Schwamb, M. E. (2017). "Col-OSSOS: Colors of the Interstellar Planetesimal 1I/2017 U1 in Context with the Solar System". Submitted to ApJL: 9. ArXiv:1711.06214Freely accessible. 
  12. 12.0 12.1 Bolin, B. T. (2017). "APO Time Resolved Color Photometry of Highly-Elongated Interstellar Object 1I/ʻOumuamua". Submitted to AJ: 11. ArXiv:1711.04927Freely accessible. 
  13. 13.0 13.1 Astronomer Alan Fitzsimmons w/ 4.2-m William Herschel Telescope
  14. "Which way home? Finding the origin of our Solar System's first interstellar visitor" (PDF). NASA. Retrieved 15 November 2017. 
  15. "JPL Small-Body Database Search Engine: e > 1". JPL Small-Body Database. Retrieved 2017-10-26. 
  16. 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): 9 (2 pages). Bibcode:2017RNAAS...1....5D. ArXiv:1711.00445Freely accessible. doi:10.3847/2515-5172/aa96b4. 
  17. Pukui, Mary Kawena; Elbert, Samuel H. "Hawaiian Dictionary". Ulukau: Hawaiian Electronic Library. University of Hawaiʻi Press. Retrieved 21 November 2017. 
  18. 18.0 18.1 18.2 Beatty, Kelly. "Astronomers Spot First-Known Interstellar Comet". Sky & Telescope. Retrieved 25 October 2017. 
  19. Seidel, Jamie (October 26, 2017). "'Alien' object excites astronomers. Is it a 'visitor' from nearby star?". The New Zealand Herald. 
  20. 20.0 20.1 Ye, Quan-Zhi; Zhang, Qicheng (2017). "1I/ʻOumuamua is Hot: Imaging, Spectroscopy and Search of Meteor Activity". Submitted to ApJ Letters: 8. ArXiv:1711.02320Freely accessible. 
  21. "MPEC 2017-U185: A/2017 U1". Minor Planet Center. International Astronomical Union. 26 October 2017. Retrieved 26 October 2017. 
  22. "Archive of 2017 U1 JPL SBDB solution #10 (2017-Nov-03) with a 16-day observation arc". Jet Propulsion Laboratory. 
  23. 23.0 23.1 23.2 Rincon, Paul (20 November 2017). "Bizarre shape of interstellar asteroid". BBC. Retrieved 20 November 2017. 
  24. Meech, Karen J.; Weryk, Robert; Micheli, Marco; Kleyna, Jan T.; Hainaut, Olivier R.; et al. (20 November 2017). "A brief visit from a red and extremely elongated interstellar asteroid". Nature. ISSN 1476-4687. doi:10.1038/nature25020. Retrieved 20 November 2017. 
  25. Meech, K.; et al. (November 20, 2017). "Light curve of interstellar asteroid `Oumuamua". ESO.org. Retrieved November 21, 2017. 
  26. Gaidos, Eric (2017). "Origin of Interstellar Object A/2017 U1 in a Nearby Young Stellar Association?". Research Notes of the AAS. 1: 3. Bibcode:2017RNAAS...1...13G. ArXiv:1711.01300Freely accessible. doi:10.3847/2515-5172/aa9851. 
  27. Portegies Zwart, Simon (2017). "The origin of interstellar asteroidal objects like 1I/2017 U1". 1711: 4. Bibcode:2017arXiv171103558P. ArXiv:1711.03558Freely accessible. 
  28. Mamajek, Eric (2017). "Kinematics of the Interstellar Vagabond A/2017 U1". Research Notes of the AAS. 1710: 4. Bibcode:2017arXiv171011364M. ArXiv:1710.11364Freely accessible. 
  29. 29.0 29.1 "Interstellar Asteroid FAQs". NASA Planetary Defense. Retrieved 21 November 2017. 
  30. 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(’Oumuamua) and believe it could be one of 10,000 others lurking undetected in our cosmic neighbourhood. The certainty of its interstellar origin comes from an analysis that shows its orbit is almost impossible to achieve from within our solar system. 
  31. Schneider, Jean (2017). "Is 1I/2017 U1 really of interstellar origin ?". ArXiv:1711.05735Freely accessible.  "(Submitted on 15 Nov 2017) Comments: Accepted in Research Notes of the AAS. Draft version"
  32. Laughlin, Gregory; Batygin, Konstantin (2017). "On the Consequences of the Detection of an Interstellar Asteroid" (PDF). Research Notes of the AAS. 1711: 4. Bibcode:2017arXiv171102260L. ArXiv:1711.02260Freely accessible. 
  33. Antier, Karl. "A/2017 U1, first interstellar asteroid ever detected!". International Meteor Organization. Retrieved 7 November 2017. 
  34. Hein, Andreas M.; Perakis, Nikolaos; Long, Kelvin F.; Crowl, Adam; Eubanks, Marshall; Kennedy, Robert G., III; Osborne, Richard. "Project Lyra: Sending a Spacecraft to 1I/ʻOumuamua (former A/2017 U1), the Interstellar Asteroid". ArXiv:1711.03155Freely accessible. 
  35. NASA - Trajectories Accessed November 21st, 2017
  36. Braeunig - Orbital Mechanics Basics Accessed November 21st, 2017

External links

  • Andreas M. Hein, Nikolaos Perakis, Kelvin F. Long, Adam Crowl, Marshall Eubanks, Robert G. Kennedy III, Richard Osborne. Project Lyra: Sending a Spacecraft to 1I/ʻOumuamua (former A/2017 U1), the Interstellar Asteroid https://arxiv.org/abs/1711.03155