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Oumuamua's Dramatic Visit.

The space rock from beyond the solar system has left us amazed and perplexed.

The Minor Planet Center announces dozens of new solar system discoveries every week. Most notices are mundane, describing the orbits, appearances, and sky locations of unremarkable, newly detected comets and asteroids. But last October 25th, the MPC's Electronic Circular 2017-U181 reported the discovery of a singular outsider: a faint object rushing through the inner solar system that, based on its trajectory, was making a one-time visit from the stars.

Postdoc Robert Weryk (University of Hawai'i, Manoa) had made the discovery a week earlier on October 19th, while looking for near-Earth objects with the aid of the 1.8-meter Pan-STARRS 1 scope. This robotically controlled telescope of the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) program sits near the summit of the Haleakala volcano on Maui. Its primary charge is to locate thousands of new asteroids and identify those that might threaten Earth (S&T: June 2018, p. 12).

Weryk says he noticed the object "in the process of doing my daily job." (His official title is Planetary Defense Researcher.) The body was moving very fast across the sky, which suggested it was close to Earth. Checking the archive, Weryk also found it in images from the previous night. "When I put the data together to compute an orbit, the orbit didn't make sense. It wasn't moving as you'd expect a near-Earth asteroid to move."

Weryk quickly contacted a colleague in Italy, Marco Micheli (European Space Agency), who had serendipitously gathered data with an ESA telescope that also showed the object. "When the three nights were put together," Weryk says, "it strongly suggested that we had something with an origin outside our solar system."

To the Telescopes!

The reason Weryk and his colleagues suspected they had an interstellar visitor on their hands was the object's orbit. It was notably hyperbolic, shaped like a flattened U instead of the elongated loops or parabolic paths that comets usually follow. Occasionally a planet will torque a comet into a slightly hyperbolic trajectory, but planetary scientists had never seen one so extreme. It meant that, unlike our solar system's planets, asteroids, and comets, the body wasn't gravitationally bound to the Sun.

Time was very short. When first spotted by Pan-STARRS, the interstellar visitor--which was initially estimated from its brightness to be more than 100 meters (300 ft) across--was already well on its way out of the solar system. It had reached perihelion on September 9th, with a searing close approach that brought it four times nearer to the Sun than Earth comes. Outward bound, it passed below Earth's orbit on October 10th and shortly thereafter cruised by our planet at a distance only 63 times that of the Moon. By the time the MPC announcement went out, it was nearly as far from the Sun as Mars.

The Pan-STARRS astronomers turned to Karen Meech (University of Hawai'i, Manoa) to take the lead in organizing their follow-up efforts. While no roadmap existed for what to do when confronted with an object arriving from the interstellar depths, Meech had successfully coordinated worldwide ground-based observing programs in support of several NASA missions and had a good idea of how to tackle the challenge.

"It meant dropping everything and really working around the clock," Meech says. Her team immediately submitted emergency requests for observing time to facilities including the Very Large Telescope, the Gemini South Telescope, and the Hubble Space Telescope and were approved. Over the coming nights, the world's most capable instruments turned to the mysterious object.

They weren't alone. Within minutes of the Minor Planet Center's October 25th announcement that a comet of possible interstellar origin had been located, other telescopes also began chasing the object. Joseph Masiero (Jet Propulsion Laboratory) happened to be observing with the venerable 5.1-meter Hale Telescope at Palomar Observatory. Under less-than-perfect conditions, he obtained spectra indicating a dull red overall color. Early observations also revealed the object looked point-like, with no hint of cometary activity.

Early returns also came from a group headed by David Jewitt (University of California, Los Angeles) and Jane Luu (MIT Lincoln Laboratory), who in 1992 were the discoverers of 15760 Albion, the first Kuiper Belt object (besides Pluto). The team including Meech and Weryk also elicited critical results by piecing together several successive nights of observations from different telescopes. Most importantly, all this effort revealed that the body is not a comet in the traditional sense. Even in the deepest exposures, it appeared as a completely star-like point; scientists estimated that it could be spewing out no more than a spoonful of powdery dust per second.

A Class of Its Own

With measurements obtained over a series of nights, observers constructed a light curve of how the object's brightness changed with time. That plot was illuminating. First, the body is clearly rotating, executing a full spin roughly every 8 hours. Second, the light curve shows a startling degree of variation during the course of a single rotation, as well as from one rotation to the next. The most likely interpretation is that the object has a highly elongated shape, like a weaver's shuttle, with estimates of its aspect ratio (length vs. width) ranging up to 10:1. This measurement strongly suggests that it's a monolithic object with some material strength: Were it a loose pile of rubble, as many asteroids are, it wouldn't be able to prevent itself from flying apart as it rotates.

Moreover, the irregular brightness changes suggest that it is tumbling chaotically as it falls through space. Although some speculated that at least part of the oddness could arise from patchy reflectivity on the surface, spectra from all of the telescopes that observed the body consistently indicate a uniformly reddish surface that would be perfectly in place among the icy, carbon-rich bodies (such as the Centaurs) that inhabit our own outer solar system.

The newcomer presented a problem of nomenclature. Based on its wildly noncircular orbit, the MPC's first announcement classified it as a comet, giving it the designation C/2017 U1. Shortly thereafter, as it became clear that the body showed no hint of a coma, it was reclassified as asteroid A/2017 U1. As a storm of updated trajectory determinations proved beyond doubt that the community was witnessing an arrival from beyond the solar system, the International Astronomical Union announced an official naming scheme, with code letter "I" for interstellar. In place of the discoverer's name--in this case, Pan-STARRS--the IAU accepted the team's proposed name of 'Oumuamua, a Hawaiian word for scout (see sidebar, page 26).

With its remarkable name and its remarkable properties, 'Oumuamua was ready for stardom. On November 20th, the European Southern Observatory circulated a news release describing the Meech team's results. The document linked the undeniably weird properties of a first-ever object with a name that brings to mind the hand of some far-distant extraterrestrial agency and a stunning artist's impression of a menacing starship-like shard silhouetted against a glowing field of stars. The story, which had gained international attention after being reported first by Sky & Telescope nearly a month prior, vaulted to massive prominence. The eerie parallels to Arthur C. Clarke's novel Rendezvous with Rama were given wide remark, and in a story for NBC News, astronomer Seth Shostak (SETI Institute) memorably wondered "if it's a rock or a rocket."

To rule out the latter, the Breakthrough Listen project marshaled an effort in which the 100-m Green Bank Telescope listened for radio broadcasts from 'Oumuamua as it rushed away from the Sun. Results of the radio surveillance showed no sign of technological activity. As much as it looked like a spaceship, 'Oumuamua was just a rock.

Slingshot Victim

'Oumuamua leaves both insights and unanswered questions in its wake. It is small and faint enough that an automated survey telescope such as Pan-STARRS 1 was lucky to spot it. Had it arrived a few weeks earlier or later, the difference in Earth's orbital vantage point would likely have let it slip through undetected.

The very fact that 'Oumuamua was found hints that it is part of a very large population of similar objects pervading the galaxy--it's supremely unlikely that we would have been lucky enough to see the lone example. Granted, extrapolations that point to a vast multitude on the basis of a single example do not generally inspire a great degree of confidence. Nonetheless, by taking careful account of the total volume of space that Pan-STARRS searches, Aaron Do (University of Hawai'i, Manoa) and colleagues estimated the interstellar density of 'Oumuamua-like objects. They concluded that, if we were to slice up our galaxy into regions equal in size to the sphere circumscribed by Earth's orbit around the Sun, there should be about one interstellar voyager per region. Their calculation suggests that one or more of these bodies is nearly always in the act of rushing through the inner solar system. Equally impressively, it suggests that more than a trillion quadrillion ([10.sup.27]) of them are orbiting in the galactic disk.

Projecting 'Oumuamua's path back in time indicates that it came from the direction of the constellation Lyra. Using its exact point of origin and the speed with which it approached the solar system, astronomers further backtracked its trajectory to check whether it may have been ejected from a known nearby star system. Piotr Dybczynski (Adam Mickiewicz University, Poland) and Malgorzata Krolikowska (Polish Academy of Sciences) assessed more than 200,000 nearby stars but concluded that none is a particularly compelling candidate for 'Oumuamua's parent. They did find that, about 800,000 years ago, 'Oumuamua traversed within roughly 7 light-years of the planet-bearing red dwarf Gliese 876. But the miss distance, while intriguing, was too large to be anything but a chance close encounter; almost certainly, it was just one of thousands of similarly wide stellar passes that "Oumuamua has experienced during billions of years of galactic wandering.

Such forlorn travelers were likely set on their journeys by massive planets in their home systems. We think the same happened here. In the so-called Nice Model, the giant planets that orbit our own Sun migrated inward and outward as they were forming, launching many planetesimals into interstellar space. This haphazard rearrangement explains several features of our solar system, such as the existence of Trojan asteroids around Jupiter and Neptune and the structure of the Kuiper Belt (S&T: June 2016, p. 16).

As the giant planets moved around, they also would have wrested at least several Earth masses' worth of icy, comet-like planetesimals--comprising quadrillions of individual objects--from the Sun's grip and spewed them into interstellar space. These losses occurred via the same gravitational slingshot mechanism that enabled Jupiter and the other giant planets to send the Voyager, Pioneer, and New Horizons probes barreling out of the solar system. When replicated across the formation of all the stars in our galaxy, the process could produce a vast population of "Oumuamua-mass bodies.

Yet conundrums remain. In order for a particular planet to eject a small body from its parent system, that planet must simultaneously be relatively massive and relatively far from its parent star. Put more precisely, the escape velocity from the planetary surface must comfortably exceed the planet's own orbital velocity. Earth cannot produce solar system escapees using its gravity alone, and interestingly, neither can the vast majority of known exoplanets. Hot Jupiters and short-period super-Earths are simply too close to their parent stars to be effective, and in general, planets orbiting in "warm" regions near the star where rocky and metallic bodies form can't eject them. A flurry of 'Oumuamuas might shoot from binary-star systems or, alternately, they might owe their independent orbits to countless Neptune analogs orbiting Sun-like stars at Neptune-like distances.

Dust Problem

If passing interstellar objects have a lot of ice, we'd expect them to behave like comets when they are warmed by the Sun. 'Oumuamua, however, showed no sign whatsoever of a volatile-powered coma, indicating that it had no dust-infused ice that was heated to vaporization--despite passing perihelion just 0.26 astronomical unit from the Sun, a region where the solar radiation is 16 times stronger than at Earth. At first glance, this absence of activity would suggest 'Oumuamua didn't have any near-surface ice.

But Jewitt and his collaborators soon pointed out that it's possible the ice is there but couldn't vaporize. 'Oumuamua spent a very long time in the interstellar medium, and over the eons it would have been both continually bombarded with the relativistic particles called cosmic rays and episodically bathed in extreme-ultraviolet light from massive, young stars. Under such conditions, laboratory experiments (carried out by scientists such as Scott Sandford at NASA Ames Research Center) show that simple carbon-containing molecules gradually refashion themselves into complicated tar-like complexes that evolve to have 'Oumuamua's characteristically ruddy hue. They also serve as remarkably good thermal insulators. A beverage cooler made from comet crust would keep bottles cold for much longer than one made from Styrofoam.

Thermal modeling simulations by a number of groups show that if 'Oumuamua is indeed covered by a modest insulating layer, then the Sun's intense but brief heat likely would not have been sufficient to warm any underlying ice to the sublimation point. Moreover, 'Oumuamua's odd shape and its tumbling motion may have provided further protection. At many orientations, a cigar-like figure exposes only a small fraction of its surface area to direct irradiation, and the chaotic tumbling would have kept any one section from being regularly heated, helping to prevent isolated hot spots from blistering out.

As 'Oumuamua zipped away from the Sun, it rapidly grew dimmer, and by the end of 2017 it was about 27th magnitude. A team led by David Trilling (Northern Arizona University) trained the Spitzer Space Telescope in 'Oumuamua's direction but registered no infrared heat signature from it. Their non-detection suggests that it is both smaller than initially believed and also much more reflective at visible wavelengths --which is strange, given the hints of a tarry surface. No one really knows what to make of it.

Meech's team made a series of ground-based and HST observations of its fleeting departure, and these last looks imparted one final surprise: 'Oumuamua's trajectory out of the solar system failed to adhere to the hyperbolic arc that one would expect from Newton's Law of Gravity. Rather, something gave the departing body an extra kick.

The team's best guess is outgassing. Rocket-like gas emission that provides acceleration in a specific direction is not unexpected for a comet, but it's somewhat awkward to square that with the fact that no one ever observed any gas from 'Oumuamua. A thruster-like jet should have been visible thanks to the light scattered by dust caught up with the expelled gas. Perhaps any dust particles entrained were larger than expected and hence ineffective at scattering light.

They're still going with the bizarre 10:1 aspect ratio, too, Meech says. The spindle shape corresponds best to the observed 2 1/2-magnitude dips in the light curves, though an alternative, pancake shape is also possible (see page 24). A more definitive analysis could be obtained by completely modeling the rotation state and the shape in a consistent manner, she explains--a project that is feasible but very computationally intensive.

Preparing for the Next One

While detailed modeling will still bring results, we won't get any more direct observations of 'Oumuamua itself. Yet its appearance may herald many such visitors. Now that astronomers know what to look for, facilities such as Pan-STARRS will probably detect similar objects once every several years or so, and the upcoming Large Synoptic Survey Telescope will be sensitive enough to spot objects like 'Oumuamua at distances roughly equal to the Earth-Sun separation. As a consequence, it is likely to report a new one every several months.

Astronomers are eagerly awaiting the next arrivals to see which of 'Oumuamua's properties--its color, its spin, its shape, and its lack of coma--turn out to be emblematic of the population. If, for example, none of the incoming bodies shows evidence of evaporating ices, we will be left with an origins mystery that will be answered only by an up-close view, or even by excavating a patch of crust to determine what lies beneath.

A number of spacecraft missions that match velocities to fly alongside small bodies such as asteroids and comets have been successfully carried out, most recently the comet-chasing Rosetta (S&T: May 2017, p. 14) and the asteroid-sampling Osiris-REX and Hayabusa 2 (S&T: June 2018, p. 22). A sample-return mission to an object like 'Oumuamua would be the scientific equivalent of a grand slam, but unfortunately it's unlikely to be feasible. Pulling alongside a body on a hyperbolic trajectory and then returning to Earth requires too much energy for chemically propelled rockets to do the job.

A one-way interceptor mission, however, would be doable.A flight plan of this type has precedent with NASA's 2005-era Deep Impact mission, in which a kinetic impactor smashed into Comet Tempel 1, creating a debris plume that was observed from both a companion flyby probe and from Earth. Had 'Oumuamua been detected a few weeks before it crossed the sphere of Earth's orbit in early August on its way to its closest solar approach, and if a rocket had been ready to launch, a modest deep-space boost of a few kilometers per second would have been adequate for a high-speed rendezvous with a shard from an alien solar system. Hopefully, in coming years, there will be similarly fortuitous chances!

What's in a Name?

The term 'Oumuamua refers to a leader or scout. It comes from 'ou ("reach out for") and mua ("first, in advance of," repeated for emphasis). The discoverers chose the name in consultation with Hawaiian language experts to highlight the object as a messenger from the distant past reaching out to us. But the term also has military connotations and can refer to a leader in battle or a scout sent to survey an enemy's position.

An interesting choice for something that looks like an alien spaceship.

--Camille M. Carlisle

GREG LAUGHLIN is an astronomy professor at Yale University. He blogs about planets at oklo.org.

Caption: SOLAR SYSTEM VISITOR This artist's impression shows the first discovered interstellar body, called Oumuamua. Although estimates for the true shape vary wildly, its discoverers are sticking with this strange, highly elongated profile". In this model, it's about 400 meters (1,300 feet) long.

Caption: WHIRLWIND TOUR The track of the first-known interstellar object through the solar system. The round inset shows its location when it was discovered on October 19th. Observations when the object had passed the orbit of Jupiter indicate that it had traveled farther than expected, perhaps propelled by jets of sublimating ice (see page 25).

Caption: SKY PATH Oumuamua came tumbling out of Lyra in 2017 (not that anyone saw it), looped through the sky, and was discovered cruising near the Pisces-Cetus border on October 19th. It quickly receded in the direction of Pegasus. The trajectory spans 2012 through 2020, and the loops at its ends are a parallax effect due to Earth's yearly circuit around the Sun.

Caption: OUMUAMUA'S LIGHT CURVE Compiled by 13 telescopes, this composite light curve shows how 'Oumuamua's brightness changed as it tumbled through space. Although there is a clear periodicity, it doesn't always follow the exact same pattern. Note that although the data have been converted to visual magnitudes, many observations were taken in infrared.

Caption: JUST A SPECK This deep composite image shows 'Oumuamua (circled), surrounded by the trails of faint stars that were smeared out as the telescopes tracked the moving object. The image combines multiple images from the Very Large Telescope and the Gemini South Telescope.

Caption: ALTERNATIVE IDEAS Astronomers have suggested several different shapes for 'Oumuamua. The squatter cigar gained popularity, but the body's strange wobbling spin also leaves open the possibility that It's a pancake (S&T: July 2018, p. 8).

Caption: GRAVITY ASSIST Giant planets orbiting far from their stars can kick small bodies out of their planetary systems. When a body encounters the planet, the planet's gravity boosts the object's velocity such that it whizzes away at high speed. The new trajectory could take any of a variety of angles emanating forward from the planet's direction of motion.

Caption: IN HINDSIGHT If astronomers had discovered 'Oumuamua early enough, a mission launched in late July 2017 would have had enough time to intercept the object in late October. However, estimates put 'Oumuamua's average brightness in June 2017 at a visual magnitude of 26, too faint for Pan-STARRS to detect. (Although when seen from above 'Oumuamua crossed Earth's orbit in late July, it was not 1 a.u. from the Sun until early August.)
'Oumuamua at a Glance

Size                   ~100 meters

Perihelion             0.26 a.u.
                       (September 9, 2017)

Orbital eccentricity   1.2 (hyperbolic)

Distance from Earth    0.23 a.u.
when it crossed        (October 10, 2017)
below Earth's orbit,   90 lunar distances
outbound

Closest approach       0.16 a.u.
to Earth               (October 14, 2017)
                       63 lunar distances

Apparent magnitude     ~20
when discovered
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Title Annotation:INTERSTELLAR INTERLOPER
Author:Laughlin, Greg
Publication:Sky & Telescope
Date:Oct 1, 2018
Words:3524
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