|Discovered by||Robert Weryk using Pan-STARRS 1|
|Discovery site||Haleakala Obs., Hawaii|
|Discovery date||19 October 2017|
|MPC designation||1I/2017 U1|
|Hawaiian term for scout|
|Orbital characteristics |
|Epoch 2 November 2017 (JD 2458059.5)|
|Observation arc||34 days|
|±0.0008 AU −1.2798[Note 1]|
Average orbital speed
|±0.01 km/s 26.33(interstellar)|
|0° 40m 48.72s / day|
|Earth MOID||0.0959 AU · 37.3 LD|
|Jupiter MOID||1.455 AU|
790–3,280 ft long
230 m × 35 m × 35 m
755 ft × 115 ft × 115 ft
(est. at albedo 0.10)
Tumbling (non-principal axis rotation)|
Reported values include: ±0.02 8.10h
±0.42 h 8.10
−0.39 h 6.96
0.1 (spectral est.)|
0.06–0.08 (spectral est.)
B–V = ±0.060.7
V-R = ±0.050.45
g-r = ±0.040.47
r-i = ±0.160.36
r-J = ±0.111.20
|19.7 to >27.5[Note 2]|
ʻOumuamua (// ( listen)) is 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 a small object, estimated to be about 230 m–1,000 m × 35 m–167 m × 35 m–167 m (755 ft–3,281 ft × 115 ft–548 ft × 115 ft–548 ft) in size. It has a dark red color, similar to objects in the outer Solar System. ʻOumuamua showed no signs of a comet 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.
In September 2018, astronomers described several possible home star systems from which 'Oumuamua may have begun its interstellar journey. However, the object did not pass particularly close to any of the objects, making its origin from any one of them improbable.
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).
The name comes from Hawaiian ʻoumuamua, meaning 'scout' (from ʻou, meaning 'reach out for', and mua, reduplicated for emphasis, meaning 'first, in advance of'), 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". 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 in consultation with Kaʻiu Kimura and Larry Kimura of the University of Hawaii at Hilo.
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.
Observations and conclusions concerning the trajectory of ʻOumuamua were primarily obtained with data from the Pan-STARRS1 Telescope, part of the Spaceguard Survey, and the Canada-France-Hawaii Telescope (CFHT), and its composition and shape from the Very Large Telescope and the Gemini South telescope in Chile, 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. Post announcement, the space-based telescopes Hubble and Spitzer joined in the observations.
ʻOumuamua is small and dark. It was not seen in STEREO HI-1A observations near its perihelion on 9 September 2017, limiting its brightness to ~13.5 mag. By the end of October, ʻOumuamua had already faded to apparent magnitude ~23, and by mid-December 2017, it was expected to be too faint and fast moving to be studied by even the largest ground-based telescopes.
ʻOumuamua was compared to the fictional alien spacecraft Rama because of its interstellar origin. Adding to the coincidence, both the real and the fictional objects are unusually elongated and limited in size. However, ʻOumuamua has a reddish hue and unsteady brightness, which are typical of asteroids.
The SETI Institute's radio telescope, the Allen Telescope Array, examined ʻOumuamua, but detected no unusual radio emissions. More detailed observations, using the Breakthrough Listen hardware and the Green Bank Telescope, were planned; 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.
ʻOumuamua appears to have come from roughly the direction of Vega in the constellation Lyra. 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. On 26 October, two precovery observations from the Catalina Sky Survey were found dated 14 and 17 October. A two-week observation arc had verified a strongly hyperbolic trajectory. It has a hyperbolic excess velocity (velocity at infinity, ) of 26.33 km/s (94,800 km/h), its speed relative to the Sun when in interstellar space.[Note 3]
|1 AU||9 August 2017||49.67|
|Perihelion||9 September 2017||87.71|
|1 AU||10 October 2017||49.67[Note 4]|
By mid November, astronomers were certain that it was an interstellar object. Based on observations spanning 34 days, ʻOumuamua's orbital eccentricity is 1.20, the highest ever observed. An eccentricity 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,[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, and therefore ʻOumuamua can only be of interstellar origin.
|# of observations|
and obs arc[Note 6]
|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 7]|
|1I/2017 U1 (ʻOumuamua)||26.5||121 in 34 days|
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.[Note 8] The object is now heading away from the Sun towards Pegasus at an angle of 66° from the direction of its approach.[Note 9]
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. 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. As it leaves the Solar System it will be approximately right ascension 23h51m and declination +24°45', in Pegasus. It will continue to slow down until it reaches a speed of 26.33 km/s relative to the Sun, the same speed it had before its approach to the Solar System. It will take the object roughly 20,000 years to leave the Solar System completely.[Note 10]
Indications of origin
Accounting for Vega's proper motion, it would have taken ʻOumuamua 600,000 years to reach the Solar System from Vega. But as a nearby star, Vega was not in the same part of the sky at that time. Astronomers calculate that one hundred years ago the 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. 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. In fact, the strong correlation between ʻOumuamua's velocity and the local standard of rest might mean that it has circulated the Milky Way several times and thus may have originated from an entirely different part of the galaxy.
It is unknown how long the object has been traveling among the stars. The Solar System is likely the first star system that ʻOumuamua has closely encountered since being ejected from its birth star system, potentially several billion years ago. It has been speculated that the object may have been ejected from a stellar system in one of the local kinematic associations of young stars (specifically, Carina or Columba) within a range of about 100 parsecs, some 45 million years ago. The Carina and Columba associations are now very far in the sky from the Lyra constellation, the direction from which ʻOumuamua came when it entered the Solar System. Others have speculated that it was ejected from a white dwarf system and that its volatiles were lost when its star became a red giant. About 1.3 million years ago the object may have passed within a distance of 0.16 parsecs (0.52 light-years) to the nearby star TYC 4742-1027-1, but its velocity is too high to have originated from that star system, and it probably just passed through the system's Oort cloud at a speed of 103 km/s (370,000 km/h).[Note 11] A more recent study (August 2018) using Gaia Data Release 2 has updated the possible past close encounters and has identified four stars that 'Oumuamua passed relatively close to and at moderately low velocities in the past few million years.  This study also identifies future close encounters of 'Oumuamua on its outgoing trajectory from the Sun.
According to one hypothesis, ʻOumuamua could be a fragment from a tidally disrupted planet.[Note 12] This makes 'Oumuamua a rare object, much less abundant than other extrasolar "dusty-snowball" comets or asteroids could be.
Initially, ʻOumuamua was announced as comet C/2017 U1 (PANSTARRS) based on a strongly hyperbolic trajectory. In an attempt to confirm any cometary activity, very deep stacked images were taken at the Very Large Telescope later the same day, but the object showed no presence of a coma.[Note 13] Accordingly, the object was renamed A/2017 U1, becoming the first comet ever to be re-designated as an asteroid. Once it was identified as an interstellar object, it was designated 1I/2017 U1, the first member of a new class of objects. The lack of a coma limits the amount of surface ice to a few square meters, and any volatiles (if they exist) must lie below a crust at least 0.5 m (1.6 ft) thick. It also indicates that the object must have formed within the frost line of its parent stellar system or have been in the inner region of that stellar system long enough for all near-surface ice to sublimate, as may be the case with damocloids. It is difficult to say which scenario is more likely due to the chaotic nature of small body dynamics, although if it formed in a similar manner to Solar System objects, its spectrum indicates that the latter scenario is true. Any meteoric activity from ʻOumuamua would have been expected to occur on 18 October 2017 coming from the constellation Sextans, but no activity was detected by the Canadian Meteor Orbit Radar.
On 27 June 2018, astronomers reported that ʻOumuamua was thought to be a mildly active comet, and not an asteroid, as previously thought. This was determined by measuring a non-gravitational boost to ʻOumuamua's acceleration, consistent with comet outgassing. However, studies submitted in October 2018 suggest that the object is neither an asteroid nor a comet.
Appearance, shape, and composition
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. Spectra from the Hale Telescope showed a less-red color resembling comet nuclei or Trojans. Its spectrum is similar to that of D-type asteroids.
ʻOumuamua is rotating around a non-principal axis, a type of movement known as tumbling. This accounts for the various rotation periods reported, such as 8.10 hours, (±0.42 hours) (±0.02 hours) with a lightcurve amplitude of 1.5–2.1 magnitudes, whereas Meech et al. reported a rotation period of 7.3 hours and a lightcurve amplitude of 2.5 magnitudes.[Note 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.
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. 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. Assuming an albedo of 10% (typical for D-type asteroids) and a 6:1 ratio, ʻOumuamua has dimensions of approximately 230 m–1,000 m × 35 m–167 m × 35 m–167 m (755 ft–3,281 ft × 115 ft–548 ft × 115 ft–548 ft) with an average diameter of about 110 m (360 ft). According to astronomer David Jewitt, the object is physically unremarkable except for its highly elongated shape. Bannister et al. have suggested that it could also be a contact binary, although this may not be compatible with its rapid rotation. One speculation regarding its shape is that it is a result of a violent event (such as a collision or stellar explosion) that caused its ejection from its system of origin. JPL News reported that ʻOumuamua "is up to one-quarter mile (400 meters) long and highly-elongated-perhaps 10 times as long as it is wide".
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. It is thought that its surface contains tholins, which are irradiated organic compounds that are more common in objects in the outer Solar System and can help determine the age of the surface. This possibility is inferred from spectroscopic characterization and its dark and reddened color, and from the expected effects of interstellar radiation. Despite the lack of any cometary coma when it approached the Sun, it may still contain internal ice, hidden by "an insulating mantle produced by long-term cosmic ray exposure".
In 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, although earlier limited observations by other radio telescopes such as the SETI Institute's Allen Telescope Array had produced no such results. 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.
Hypothetical space missions
ʻOumuamua is traveling too fast for any existing spacecraft to reach. The Initiative for Interstellar Studies (i4is) has launched Project Lyra for assessing the feasibility of a mission to ʻOumuamua. 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. 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". 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. Astronomers estimate that several interstellar objects similar to ʻOumuamua pass inside the orbit of Earth each year, and that 10,000 are passing inside the orbit of Neptune on any given day. 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.
- A/2017 U7, a non-interstellar hyperbolic comet discovered 10 days after ʻOumuamua, announced in March 2018
- A/2018 C2, another non-interstellar hyperbolic comet, announced in March 2018
- (514107) 2015 BZ509, an asteroid with possible interstellar origin
- Hyperbolic asteroid
- Interstellar object
- Objects on hyperbolic trajectories have negative semimajor axis, giving them a positive orbital energy.
- Range at which the object is expected to be observable. Brightness peaked at 19.7 mag on 18 October 2017, and fades below 27.5 mag (the limit of Hubble Space Telescope for fast-moving objects) around 1 January 2018. By late 2019, it should dim to 34 mag.
- For comparison, comet C/1980 E1 will only be moving 4.2 km/s when it is 500 AU from the Sun.
- The solar escape velocity from Earth's orbit (1 AU from the Sun) is 42.1 km/s. For comparison, even 1P/Halley moves at 41.5 km/s when 1 AU from the Sun, according to the formula v = 42.1219 √, 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.
- Unlike ʻOumuamua, C/1980 E1's orbit got its high eccentricity of 1.057 due to a close encounter with Jupiter. Its inbound-orbit eccentricity was less than 1.
- Orbits computed with only a handful of observations can be unreliable. Short arcs can result in computer generated orbits rejecting some data unnecessarily.
- 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.
- Comet C/2012 S1 (ISON) peaked at 377 km/s (1,360,000 km/h) at perihelion because it passed 0.0124 AU from the Sun (20 times closer than ʻOumuamua).
- According to the formula:
- 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)
- 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 'Oumuamua.
- 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.
- 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.
- 1865 Cerberus has a lightcurve amplitude of 2.3 magnitudes.
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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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Bannister, M. T.; Schwamb, M. E. (2017). "Col-OSSOS: Colors of the Interstellar Planetesimal 1I/2017 U1 in Context with the Solar System". The Astrophysical Journal. 851 (2): L38. arXiv:1711.06214. Bibcode:2017ApJ...851L..38B. doi:10.3847/2041-8213/aaa07c.
As its albedo is unknown, we do not describe 1I/‘Oumuamua as consistent with Tholen (1984) P type.
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So far limited observations of ‘Oumuamua, using facilities such as the SETI Institute’s Allen Telescope Array, have turned up nothing.
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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.
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Such outgassing is a behaviour typical for comets and contradicts the previous classification of `Oumuamua as an interstellar asteroid. “We think this is a tiny, weird comet,” commented Marco Micheli. “We can see in the data that its boost is getting smaller the farther away it travels from the Sun, which is typical for comets.”
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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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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.
- 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
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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.
- "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
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- Bartels, Meghan (25 September 2018). "'Oumuamua Isn't from Our Solar System. Now We May Know Which Star It Came From". Space.com.
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- "Project Lyra – A Mission to ʻOumuamua". I4IS. Initiative for Interstellar Studies.
- Fraser, Wesley (11 Feb 2018). "The Sky at Night: The Mystery of 'Oumuamua" (Interview). Interviewed by Chris Lintott. BBC.
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Chemical propulsion just doesn’t close the case in this scenario.
|Wikimedia Commons has media related to 1I/ʻOumuamua.|
- "Oumuamua". NASA web site.
- Talk about A/2017 U1 from 31 October 2017. SETI Institute at Facebook Live.
- on YouTube (time 3:31 min.)
- "Spitzer DDT observations of the interstellar comet A/2017 U1". – Proposal #13249
- "Planet 1I/2017 U1". Exoplanet.eu.
- ʻOumuamua at the JPL Small-Body Database
- "A Glimpse of ʻOumuamua". NYT (Video – 2:53). Narrated by Dennis Overbye; Produced by Jonathan Corum and Jason Drakeford. 12 December 2017.