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'Oumuamua
'Oumuamua Transparent
Discovered by Robert Weryk using Pan-STARRS 1
Date of Discovery 19 October 2017
ʻOumuamua 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. It is expected to leave our solar system in 2019.

Indications of origin Edit

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 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 ʻOumuamua'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.

Appearance, shape, and composition Edit

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. 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 × 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 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 compoundsthat 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".

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