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A Visitor from Interstellar Space.

WHEN THE PAN-STARRS 1 telescope atop Haleakala on Maui swept up a 20th-magnitude object on October 19th, observer Robert Weryk (University of Hawai'i) noticed right away that its motion didn't make sense. Orbital computations soon showed that it had passed within 0.26 astronomical unit (38,200,000 km) of the Sun on September 9th. But its extreme orbital eccentricity, 1.19, meant that it's following a hyperbolic trajectory that's not bound to the Sun. For the first time, observers had spotted an object visiting from interstellar space.

At first astronomers suspected it was a comet, so the International Astronomical Union's Minor Planet Center (MPC) applied the designation C/2017 U1. However, when even the deepest telescopic images revealed no hint of a coma or tail, they decided it must be asteroidal, so the designation morphed to A/2017 U1.

Behind the scenes, IAU and MPC officials mounted an unusual effort to name the new object quickly. The chosen name, 'Oumuamua, is a Hawai'ian construct combining 'ou (to reach out) and mua (meaning first or in advance of); the second mua is for emphasis.

But asteroids aren't cataloged by name alone, and this is the first of an entirely new class of object. So, at the suggestion of MPC associate director Gareth Williams, the IAU adopted the identifier "I," for interstellar. As noted in the naming announcement, "Correct forms for referring to this object are therefore: II; 11/2017 U1; 1I/'Oumuamua; and 11/2017 U1 ('Oumuamua)."

Although the arrival of a faint object from beyond the solar system caught astronomers by surprise, they quickly mobilized for follow-up observations with powerful ground-based telescopes.

One lucky break came immediately after the MPC's discovery announcement. Observer Joseph Masiero (Jet Propulsion Laboratory) just happened to be in the middle of a run with the 5-m Hale Telescope at Palomar Observatory, and he quickly obtained a spectrum at visible and near-infrared wavelengths. 'Oumuamua is slightly reddish overall, with the kind of unremarkable spectrum that a rocky surface would exhibit after being "weathered" by long-term exposure to space radiation.

While its spectrum seems reasonable, its shape borders on bizarre. Based on pooled, rapid-response observations from five big telescopes in Hawai'i and Chile, the light curve for 11/2017 Ul shows a 2V2-magnitude range. As Karen Meech (University of Hawai'i) and others explain in the November 20th Nature, this wide swing implies that the object has an extremely elongated shape --maybe 10 times longer than it is wide.

Moreover, the spin rate is relatively fast, about 7.34 hours, so 'Oumuamua must be made of rocky or metallic compounds with significant tensile strength. As the Nature researchers note, "It raises the question of why the first known [interstellar object] is so unusual."

The Hubble Space Telescope might have the final word on the character of 11/2017 Ul. A team led by Meech was granted enough Hubble time to study the quickly disappearing interloper three times before the end of 2017.

This object entered the solar system moving at 26 km per second (58,000 mph). At that speed, in 10 million years it would traverse nearly 850 light-years. Eric Mamajek (Jet Propulsion Laboratory) points out that the object's incoming velocity is close to that of the mean galactic velocity of stars that lie within 25 parsecs (80 light-years) of the Sun, but it does not match the relative velocity of any of the dozen nearest systems. These characteristics all suggest that 'Oumuamua has been drifting among the stars for a very long time, perhaps billions of years.

The object entered from the direction of the constellation Lyra, close to right ascension 18h 50m, declination +35[degrees] 13'. Now it's headed out of the solar system, never to return, toward the Great Square of Pegasus at 23h 51m, +24[degrees] 44'.

Caption: 'Oumuamua appeared as a faint fuzz (in cross-hairs) as recorded by Tenagra Observatory in Arizona on October 21st.

Caption: The track of the first-known interstellar object, now named 'Oumuamua, as it passed through the inner solar system. The inset shows its location when found on October 19th.
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Title Annotation:SOLAR SYSTEM
Author:Beatty, J. Kelly
Publication:Sky & Telescope
Date:Feb 1, 2018
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