ʻOumuamua
Oumuamua 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 Haleakala 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 from Earth, and already heading away from the Sun.
Oumuamua is a small object estimated to be between long, with its width and thickness both estimated to range between. It has a dark red colour, similar to objects in the outer Solar System. Despite its close approach to the Sun, Oumuamua showed no signs of having a coma, but did exhibit non-gravitational acceleration. Nonetheless, the object could be a remnant of a disintegrated rogue comet, according to a NASA scientist. 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 would require it to be of a density similar to rocky asteroids, a small amount of internal strength similar to icy comets would allow a relatively low density. Oumuamua'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 Oumuamua cannot be captured into a solar orbit, so it would eventually leave the Solar System and continue into interstellar space. Oumuamua's planetary system of origin, and the age of its excursion, are unknown.
In July 2019, astronomers reported that Oumuamua was an object of a "purely natural origin".
Naming
As the first known object of its type, Oumuamua 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. Oumuamua, 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/Oumuamua; or 1I/2017 U1.The name comes , 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 okina, not an apostrophe, and is pronounced as a glottal stop; the name was chosen by the Pan-STARRS team in consultation with Kaiu 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
Observations and conclusions concerning the trajectory of Oumuamua were primarily obtained with data from the Pan-STARRS1 Telescope, part of the Spaceguard Survey, and the Canada–France–Hawaii Telescope, 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 2017. Post announcement, the space-based telescopes Hubble and Spitzer joined in the observations.Oumuamua 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, Oumuamua had already faded to apparent magnitude ~23, and in mid-December 2017, it was too faint and fast moving to be studied by even the largest ground-based telescopes.
Oumuamua 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. Oumuamua has a reddish hue and unsteady brightness, which are typical of asteroids.
The SETI Institute's radio telescope, the Allen Telescope Array, examined Oumuamua, but detected no unusual radio emissions. More detailed observations, using the Breakthrough Listen hardware and the Green Bank Telescope, were performed; 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.
Trajectory
Oumuamua appears to have come from roughly the direction of Vega in the constellation Lyra. The incoming direction of motion of Oumuamua is 6° from the solar apex, 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 of, its speed relative to the Sun when in interstellar space.| 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 |
| 1 AU | 10 October 2017 | 49.67 |
| 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. Based on observations spanning 34 days, Oumuamua's orbital eccentricity is 1.20, the highest ever observed 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, Oumuamua'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 Oumuamua's trajectory nor boost its speed to the observed value. For these reasons, Oumuamua can only be of interstellar origin.
| Object | Velocity km/s | # of observations and obs arc |
| 90377 Sedna | 1.99 | 196 in 9240 days |
| C/1980 E1 | 2.96 | 179 in 2514 days |
| C/1997 P2 | 2.96 | 94 in 49 days |
| C/2010 X1 | 2.96 | 2222 in 235 days |
| C/2012 S1 | 2.99 | 6514 in 784 days |
| C/2008 J4 | 4.88 | 22 in 15 days |
| 1I/2017 U1 | 26.5 | 121 in 34 days |
Oumuamua 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 as it passed south of the ecliptic on 6 September and made a sharp turn northward at its closest approach to the Sun on 9 September at a distance of from the Sun, i.e., about 17% closer than Mercury's closest approach to the Sun. The object is now heading away from the Sun towards Pegasus towards a vanishing point 66° from the direction of its approach. This implies that a hypothetical observer near the Sun facing towards Oumuamua will eventually rotate through 294 degrees, while the direction of motion of Oumuamua will eventually have turned by 114 degrees.
On the outward leg of its journey through the Solar System, Oumuamua passed within the orbit of Earth on 14 October at a distance of approximately from Earth, and went back north of the ecliptic on 16 October and passed beyond the orbit of Mars on 1 November. 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. 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.
Non-gravitational acceleration
On 27 June 2018, astronomers reported a non-gravitational acceleration to Oumuamua's trajectory, potentially consistent with a push from solar radiation pressure. Initial speculation as to the cause of this acceleration pointed to comet off-gassing, 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. A critical re-assessment of the comet hypothesis found that, instead of the observed stability of Oumuamua's spin, outgassing would have caused its spin to rapidly change due to its elongated shape, resulting in the object tearing apart.Indications of origin
Accounting for Vega's proper motion, it would have taken Oumuamua 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 object was 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, 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. In fact, the closeness of Oumuamua'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.It is unknown how long the object has been traveling among the stars. The Solar System is likely the first planetary system that Oumuamua 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 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 Oumuamua 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. About 1.3 million years ago the object may have passed within a distance of 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. A more recent study 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.
In April 2020, astronomers presented a new possible scenario for the object's origin. According to one hypothesis, Oumuamua could be a fragment from a tidally disrupted planet. If true, this would make Oumuamua 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 Oumuamua's lightcurve favors a disc-like shape.
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 Oumuamua might be explainable on this basis. However, it was later calculated that hydrogen icebergs cannot survive their journey through interstellar space.
Classification
Initially, Oumuamua was announced as comet C/2017 U1 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. 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 must lie below a crust at least 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 Oumuamua 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 Oumuamua 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 Oumuamua's acceleration, consistent with comet outgassing. However, studies submitted in October 2018 suggest that the object is neither an asteroid nor a comet, although the object could be a remnant of a disintegrated interstellar comet, as suggested by a NASA scientist.
Appearance, shape and composition
Spectra from the Hale Telescope on 25b October showed red color resembling comet nuclei or Trojans. Higher signal to noise spectra recorded by the William Herschel Telescope later that day showed that the object was featureless, and colored red like Kuiper belt objects. Spectra obtained with the Very Large Telescope the following night showed that behaviour continued into near-infrared wavelengths. Its spectrum is similar to that of D-type asteroids.from 25 to 27 October 2017 with dotted line from a model with 10:1 elongation
Oumuamua 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 by Bannister et al. and Bolin et al. with a lightcurve amplitude of, whereas Meech et al. reported a rotation period of 7.3 hours and a lightcurve amplitude of 2.5 magnitudes. Most likely, Oumuamua 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 Oumuamua may be either a highly elongated object, comparable to or greater than the most elongated Solar System objects, or an extremely flat object, a pancake or oblate spheroid. However, the size and shape have not been directly observed as Oumuamua 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. Assuming an albedo of 10% and a 6:1 ratio, Oumuamua has dimensions of approximately with an average diameter of about. 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 that caused its ejection from its system of origin. JPL News reported that Oumuamua "is up to one-quarter mile,, long and highly-elongated-perhaps 10 times as long as it is wide".
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.
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. 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 November 2019, some astronomers have noted that Oumuamua may be a "cosmic dust bunny", due to its "very lightweight and 'fluffy' conglomerate of dust and ice grains."
Additional measurements
In December 2017, astronomer Avi Loeb of Harvard University, an adviser to the Breakthrough Listen Project, cited Oumuamua'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 object for six hours across four bands of radio frequency. No radio signals from Oumuamua were detected in this very limited scanning range, but observations are ongoing.In September 2018, astronomers described several possible home star systems from which Oumuamua may have originated.
Discussion
Hydrogen ice theory
It has been proposed that `Oumuamua contains a significant amount of hydrogen ice. This would point to it originating from the core of an interstellar molecular cloud, where conditions for the formation of this material might exist. 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. 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.
Hypothetical space missions
The Initiative for Interstellar Studies launched Project Lyra to assess the feasibility of a mission to Oumuamua. Several options for sending a spacecraft to Oumuamua within a time-frame of 5 to 25 years were suggested. 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 would be even more capable:
Such an interstellar precursor could easily pass by Oumuamua on its way out of the Solar System, at speeds of.
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 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 object and would fly past it. The authors conclude that, although challenging, an encounter mission would be feasible using near-term technology. 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.