?Oumuamua ( ( listen)) is the first known interstellar object to pass 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. Initially assumed to be a comet, it was reclassified as an asteroid a week later, and finally (6 November 2017) as the first of the new class of interstellar object.
?Oumuamua is a small object, estimated to be about 230 by 35 meters (800 ft × 100 ft) in size. It has a dark red color, similar to objects in the outer Solar System. ?Oumuamua showed no signs of a comet tail despite its close approach to the Sun, and has significant elongation and rotation rate, so it is thought to be a metal-rich rock with a relatively high density. ?Oumuamua is tumbling rather than smoothly rotating, and it is moving so fast relative to the Sun that there is no chance it originated in the Solar System. It also means that ?Oumuamua cannot be captured into a solar orbit, so it will eventually leave the Solar System and resume traveling in interstellar space. ?Oumuamua's system of origin and the amount of time it has spent traveling amongst the stars are unknown.
Video ?Oumuamua
Nomenclature
As the first known object of its type, ?Oumuamua presented a unique case for the International Astronomical Union, which assigns designations for astronomical objects. Before its true nature was known it was classified as comet C/2017 U1 and later as asteroid A/2017 U1. 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 (?Oumuamua).
The name comes from Hawaiian ?oumuamua, meaning 'scout', (from ?ou, 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 us. The first character is a Hawaiian ?okina, 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 Ka?iu 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 science fiction novel Rendezvous with Rama (1973) by Arthur C. Clarke.
Maps ?Oumuamua
Observations
Observations and conclusions concerning the trajectory of ?Oumuamua were primarily obtained with data from the Pan-STARRS1 Telescope 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.
?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 by mid-December 2017, it was expected to be 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 and limited in size. However ?Oumuamua has a reddish hue and unsteady brightness which is 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 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.
Trajectory
?Oumuamua is the first known interstellar object to visit the Solar System and it appears to come from roughly the direction of the star Vega in the constellation Lyra. The incoming direction of motion of ?Oumuamua 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 (58,900 mph), its speed relative to the Sun when in interstellar space.
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. 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, ?Oumuamua's eccentricity is so high it could not have been obtained through an encounter with any of the Sun's planets, known or unknown. Even undiscovered planets, if any exist, could not account for ?Oumuamua'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 ?Oumuamua's speed to the observed value, and therefore ?Oumuamua can only be of interstellar origin. ?Oumuamua 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 (196,200 mph) 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. The object is now heading away from the Sun (towards Pegasus) at an angle of 66° from the direction of its approach.
On the outward leg of its journey through the Solar System, ?Oumuamua 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 will pass above Jupiter's orbit in May 2018, Saturn's orbit in January 2019, and Neptune's orbit in 2022. As it leaves the Solar System it will be approximately right ascension (RA) 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.
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 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 M dwarf stars. This velocity profile also indicates an extrasolar origin, but appears to rule out the closest dozen of stars. In fact, the strong correlation between ?Oumuamua's velocity and the local standard of rest, might mean that it has circulated the galaxy several times and thus may have originated from an entirely different part of the Milky Way.
It is unknown how long the object has been traveling among the stars. The Solar System is likely the first star 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 (Carina or Columba specifically), 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 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 (230,000 mph).
According to one hypothesis, ?Oumuamua could be a fragment from a tidally disrupted planet.
Asteroidal nature
Initially, ?Oumuamua was announced as comet C/2017 U1 (PANSTARRS) on 25 October 2017 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 (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. 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.
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.
?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, (±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. 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 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 ?Oumuamua 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, ?Oumuamua has dimensions of approximately 230 m × 35 m × 35 m (800 ft × 100 ft × 100 ft) with an average diameter of about 110 m (360 ft). 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 ?Oumuamua "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".
Continuing observations
In December 2017, Harvard University Astronomy Professor Avi Loeb, 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 asteroid 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.
Hypothetical space missions
?Oumuamua 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 ?Oumuamua. Several options for sending a spacecraft to ?Oumuamua 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 ?Oumuamua pass inside the orbit of Earth each year, and 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.
See also
- Interstellar object
- Hyperbolic asteroid
- A/2017 U7, a non-interstellar hyperbolic asteroid discovered 10 days after ?Oumuamua, announced in March 2018
- A/2018 C2, another non-interstellar hyperbolic asteroid, announced in March 2018
Notes
References
External links
- "Oumuamua". NASA web site.
- Talk about A/2017 U1 from 31 October 2017. SETI Institute at Facebook Live.
- Interstellar Asteroid A/2017 U1 (update 7 November 2017) on YouTube (time 3:31 min.)
- "Spitzer DDT observations of the interstellar comet A/2017 U1". - Proposal #13249
- "Planet 1I/2017 U1". Exoplanet.eu.
- ?Oumuamua at the JPL Small-Body Database
- Close approach · Discovery · Ephemeris · Orbit diagram · Orbital elements · Physical parameters
- "A Glimpse of ?Oumuamua". NYT (Video - 2:53). Narrated by Dennis Overbye; Produced by Jonathan Corum and Jason Drakeford. 12 December 2017.
Source of the article : Wikipedia
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