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The 3rd zone: exploring the Kuiper belt.

Astronomers are finally beginning to understand the solar system's mysterious outer icy edge.

IF there was ever a watershed year in cometary science, it was 1950. At the halfway mark of the 20th century, a trio of pivotal ideas emerged almost simultaneously: Fred L. Whipple (now at the Harvard-Smithsonian Center for Astrophysics) introduced the concept of an icy cometary nucleus. The late Jan H. Oort proposed a frozen reservoir extending outward from 10,000 astronomical units (a.u.) away, now known as the Oort Cloud, where most long-period comets reside. (One a.u. is the average Earth-Sun distance.) And then there was the late Gerard P. Kuiper's highly publicized suggestion that Pluto was merely the first discovered member of a vast population of comets and "planetoids" that reside in the cold trans-Neptunian wilderness 30 to 50 a.u. from the Sun.

Over the next two decades a growing body of observational evidence vindicated both Whipple's comet-nucleus model and Oort's "comet cloud" idea. However, Kuiper's concept, now commonly referred to as the Kuiper Belt, remained stubbornly unsupported by observations--and largely faded from the stage.

However, late in the 20th century, long after Kuiper's death in 1973, the idea of a trans-Neptunian comet belt reemerged. The evidence came in both observational and theoretical forms. To begin, there was the 1977 discovery by Charles Kowal of an unusually large and distant "asteroid," 2060 Chiron, which proved to be a couple hundred kilometers across. Within two years astronomers determined that Chiron's eccentric orbit, tucked between Saturn and Uranus, is unstable. This finding was a strong indication that this oddball had originated from an even more distant region of the solar system and that many more such bodies likely orbited the Sun beyond the zone of the giant planets.

More Kuiper Belt clues came in the 1980s as computational power finally allowed astronomers to trace the evolution of cometary orbits backward over millions of revolutions to their sites of origin. From such work it became increasingly clear that most short-period comets could not come from the Oort Cloud. Why? Since the cloud is a nearly spherical reservoir, it produces comets with random orbital inclinations. This randomness is reflected in the inclinations of long-period comets, but that's not the case for short-period comets. Instead, most of the latter have orbits that lie preferentially in or near the plane of the planets. Work done in the late 1980s demonstrated convincingly that most short-period comets likely originate in a disklike reservoir beyond Neptune. This immediately harked back to Kuiper's belt of primordial bodies.

These threads of evidence motivated astronomers to search for objects hundreds of millions of kilometers beyond Neptune. But the hunt would hardly be easy. A large, Chiron-like object 40 a.u. away has a visual magnitude of 22 or fainter; more typical comets, just a few kilometers across, would be more than a thousand times (8 magnitudes) fainter still. Although the development of electronic detectors for astronomy in the 1980s made such searches feasible, spotting a slowly moving body against the plethora of faint, 22nd-magnitude stars was then very much the astronomical equivalent of finding a needle in a haystack.

However, with a prize as great as the discovery of a whole new architectural element of the solar system, the Kuiper Belt, at stake, multiple groups took up the challenge. Success came after four years of searching. In 1992 David C. Jewitt (University of Hawaii) and Jane X. Luu (now at MIT Lincoln Laboratory) discovered a faint object in a near-circular, low-inclination orbit a billion kilometers beyond Neptune. Designated 1992 Q[B.sub.1] by Brian G. Marsden of the International Astronomical Union's Minor Planet Center, it heralded a new era in planetary astronomy.

A Decade of Discoveries and Surprises

In 1993 observers found four more Kuiper Belt objects (KBOs), and more than a dozen in the following year. Now, 11 years after 1992 Q[B.sub.1]'s discovery, almost 1,000 KBOs are known.

The largest of these (besides Pluto) is Quaoar, an estimated 1,300 km in diameter (S&T: December 2002, page 24); the smallest is less than 50 km across. Based on the current discovery statistics, one can confidently extrapolate that more than 100,000 KBOs with diameters larger than 100 km should reside 30 to 50 a.u. from the Sun. The Kuiper Belt thus occupies a far greater expanse, contains a far greater mass, and has far more bodies than the asteroid belt between Mars and Jupiter.

KBO orbits seem to fall into three major categories. Plutinos are objects between 30 and 50 a.u. that circle the Sun in specific resonances with Neptune, completing three orbits for every two Neptunian revolutions, as Pluto does. Classical KBOs orbit at distances between 30 and 50 a.u. but are not trapped in resonances. Finally, scattered disk objects (SDOs) have highly eccentric orbits stretching far beyond 50 a.u.--some up to 1,000 a.u. or more.

As best we know, the surfaces of Kuiper Belt objects are generally dark, typically reflecting only 3 to perhaps 15 percent of the light that falls on them. This is in contrast to Pluto, which has a high albedo (reflectivity) of around 55 percent because its atmosphere regularly deposits fresh snow onto its surface. Despite being largely dark, however, Kuiper Belt objects exhibit a wide range of surface colors ranging from slightly bluish to extremely red.

It is unclear as yet whether KBOs fall into compositional groups as asteroids do (S&T: July 2001, page 44). Based on our limited understanding of their surface compositions and the recently determined densities of a few large objects, it remains likely that members of the Kuiper Belt consist primarily of mixtures of water ice and rock, with some amount of carbon-bearing organic molecules and more complex compounds as well.

In 2001 the first KBO satellites were discovered, and today astronomers know of at least nine KBOs (besides Pluto) having satellites. Paired objects seem quite common, perhaps because collisions play a key role in the Kuiper Belt. These impacts reshape surfaces and control the number of small bodies in the region. In fact, computer simulations show that impacts occur so often that all KBOs smaller than about 50 kilometers in diameter should have been destroyed in less than 4 billion years. If so, any existing objects of this size or smaller must be collisional byproducts far younger than the age of the solar system. Consequently, astronomers no longer believe that short-period comets, which originate from the Kuiper Belt, are ancient. Instead they generally accept the newer view that these objects are fragments chipped off larger bodies in relatively recent times--only millions to hundreds of millions of years ago--much like small asteroids, which are known to be chips off of larger ones created by collisions in the relatively recent past of the asteroid belt.

All those collisions have "eroded" the Kuiper Belt over time. Because its total mass is low, in the range of 0.5 to 1 Earth mass, the computer simulations clearly show that objects hundreds and thousands of kilometers across could not have formed in the present-day environment. The region simply doesn't possess anywhere near enough material for them to accumulate over the age of the solar system. In fact, in order to form the objects we find there today, the primordial Kuiper Belt must have had roughly 50 times its present mass.

These same simulations also suggest that the largest KBOs were well on their way to becoming large planets--perhaps something the size of Mars, Earth, or even Neptune--when their growth was suddenly interrupted. Astronomers now think that as Neptune formed, it disturbed the Kuiper Belt region gravitationally, much the same way Jupiter influenced the asteroid belt. Neptune's tidal tugs caused the encounters between objects in the young Kuiper Belt to become violent collisions.

This "dynamical excitation" made collisions erosive rather than accretional, thereby truncating the growth process. In addition, as pieces of the ancient Kuiper Belt crumbled into dust, they were easily blown away into interstellar space by radiation pressure from the Sun. Similar processes are observed in dusty disks surrounding many main-sequence stars in the galaxy, such as Vega and Beta Pictoris. This scenario may explain both the lack of a large planet in the belt and the relative dearth of material seen there today.

A Revolution in the Making

The Kuiper Belt's discovery finally gave us a context for Pluto's existence. It also provided strong links between our solar system and the disks seen around other stars like Fomalhaut (January issue, page 24), and it opened up what I call the "third zone" of the solar system--the ancient, icy disk of miniature worlds that dot the space beyond both the terrestrial- and giant-planet regions. But even after the past decade of rapid-fire discoveries, the Kuiper Belt remains a frontier of solar-system exploration. The hottest topics in ongoing Kuiper Belt research can be grouped around four major issues:

Is there an edge to the Kuiper Belt? With only one or two exceptions, all the Kuiper Belt objects found so far lie within 55 a.u. of the Sun. Scattered-disk objects, which stretch far beyond the classical KBOs, total perhaps 0.1 Earth mass--a negligible amount. Some argue that the apparent dearth of objects beyond 50 or 55 a.u. implies an outer edge (S&T: March 2001, page 26). Yet circumstantial evidence suggests that this drop-off may be just the beginning of a wide trough in the Kuiper Belt. Many planetary astronomers, including this author, find it unsatisfying that the solar system might be sharply truncated--after all, most processes in nature leave fuzzy or ragged edges, not sharp boundaries. It's doubtful that the primordial nebula just stopped abruptly.

This is very much like the argument that Kuiper himself invoked in 1950 to posit the Kuiper Belt in the first place, differing only in that he was displeased with the prospect of a sharp edge at Neptune's orbit. Current deep Kuiper Belt surveys, reaching to roughly 27th magnitude, fairly convincingly rule out any massive extension of the Kuiper Belt lying inside 70 a.u., or perhaps 80 a.u. Therefore, if the so-called edge really is a trough, then it must be very wide. Whether we will find a well-populated Kuiper Belt, say 100 or more a.u. from the Sun (like those seen around some main-sequence stars with disks), remains an open question begging for still deeper surveys.

Do KBO colors separate into distinct groups? Owing to their extreme faintness, Kuiper Belt objects are notoriously hard to measure for color trends. ("Color," in this context, refers to the difference between a KBO's magnitude at two different wavelengths of visible light.) With large telescopes and great care, however, astronomers have eked out color measurements for about 100 KBOs. When Stephen C. Tegler (Northern Arizona University) and William J. Romanishin (University of Oklahoma) published the first large sample of such data in 1998, KBOs fell into two distinct "islands." One group consisted of neutral, grayish objects; the other consisted of much redder bodies. Such distinct groupings came as a surprise to some but looked much like the distinct spectral groupings identified in the asteroid belt almost 40 years beforehand. Most subsequent KBO color studies, however, suggest that the region between the two original islands is filled in--something more like a continuum from gray to very red exists.

At present the quality and quantity of color information are simply unable to unambiguously distinguish between the color-island and color-continuum possibilities. That said, with hundreds of new KBOs being discovered each year, and with more and more telescope time devoted toward their physical characteristics, the answer is probably only a few years away.

The origin of this color diversity is also of interest, and not without controversy. On one side are those who believe that KBO colors may be primordial and therefore can tell us about the objects' compositions or formation locations. Those on the other side think that mechanisms such as space-radiation "weathering" or impact processes may have caused the color variations we see.

If this latter view is correct, it has far-reaching implications--it ties in nicely with recent research showing that Kuiper Belt comets aren't the fully primordial solar-system relics we once thought they were. Sorting out the origin of color diversity may ultimately require visits from spacecraft to the Kuiper Belt (see the box below), so that small-scale surface features such as craters can be observed directly.

How do Kuiper Belt satellites form? Since 2001 we've learned that about 1 in every 50 KBOs has a satellite. This is probably a lower limit rather than the true fraction, however, since we can detect only the brightest and most widely separated pairs. Nonetheless, the duos discovered to date display some remarkable characteristics, including the fact that many are true binaries whose components are roughly the same size. Such systems, much like Pluto and Charon, could be the result of giant collisions.

But collisions can't account for the observed number of binaries unless the sat ellites, their parents, or both are more reflective and therefore smaller than present estimates. There simply aren't enough large KBOs available to produce the percentage of pairings we've found. NASA's recently launched Space Infrared Telescope Facility (SIRTF), the Hubble Space Telescope's sister scope (S&T: February issue, page 42), will determine whether higher reflectivities are the norm among KBOs with satellites. If they are, there will be strong evidence for the collisional-formation mechanism.

But what if satellite-bearing KBOs are dark and therefore large, indicating collisions are not the cause for binary KBOs? Theorists Peter M. Goldreich (Princeton University and Caltech) and Stuart J. Weidenschilling (Planetary Science Institute) have recently found several mechanisms by which paired objects can result from three-body interactions in the Kuiper Belt. In these scenarios, binaries form by gravitational capture. Although the models have weaknesses of their own--for example, they require that all the satellites formed in a very narrow window of time, billions of years ago--they may be right. Or perhaps dynamics and collisions each play a role.

Where are the comets after all? The original search for KBOs was motivated, in large part, because dynamical models of cometary orbits convinced us that short-period comets come from the trans-Neptunian region. However, all the known KBOs are tens to hundreds of times larger and thousands to millions of times more massive than kilometer-scale cometary nuclei. This begs the question: Do short-period comets come from the Kuiper Belt?

On one hand, the dynamical case is strong, shored up by models that show how several billion comets will form over time by countless shattering collisions among KBOs. But there isn't any observational evidence for them--small comets haven't yet been directly detected in the belt. (With predicted magnitudes of 28 to 30, it is no wonder.) And though the circumstantial case for their existence is extremely strong, doubting Thomases can (and, since this is science, should!) remain skeptical until some future deep survey actually detects comet-size bodies orbiting beyond Neptune.

The Kuiper Belt: 2013

More than a half century after Kuiper's suggestion, and a decade after Jewitt and Luu spotted 1992 Q[B.sub.1], the first confirmed Kuiper Belt object, the pace of discovery in this distant region of the solar system remains brisk. Ground-based telescopes are tallying more than 100 new KBOs each year. And with the advent of larger and larger CCD cameras on larger and larger telescopes, the rate of discovery is predicted to increase. Scientists at a Kuiper Belt conference held in Chile last March estimated that several thousand KBOs would be found by the end of this decade. And if the fraction of them with satellites continues to be in the neighborhood of a few percent, then a decade from now there will be more known satellites in the Kuiper Belt than satellites of the nine planets in our solar system.

The coming decade has much promise. SIRTF will measure the albedos and true diameters of several dozen KBOs. It will also measure their surface temperatures and better determine their surface compositions. At the same time Hubble will work to refine the orbits of KBO satellites. Unfortunately, however, no known technique will make more than fuzzy maps of KBO surfaces. To do better will require visits by spacecraft.

Kuiper Belt science is still in its infancy, something akin to where asteroid studies were 50 years ago. We have learned much, but we also still have much to learn. If there is anything to count on, it is that the Kuiper Belt will continue to surprise.

New Horizons for NASA

NASA plans to make the first-ever reconnaissance of Kuiper Belt objects as a part of its New Horizons Pluto-Kuiper Belt (PKB) mission. In 2002 the National Research Council's Decadal Survey for Planetary Science ranked the up-close investigation of Pluto-Charon and the Kuiper Belt as its highest-priority mission in planetary science, citing the fundamental scientific importance of understanding this region of the solar system (S&T: October 2002, page 24). NASA authorized the construction of New Horizons earlier this year and has since selected an Atlas V rocket as the launch vehicle. New Horizons is planned to leave Earth in January 2006 and arrive at Pluto and its moon, Charon, as early as the summer of 2015.

More than 400,000 Kuiper Belt objects larger than 50 kilometers are thought to exist within 712 billion km of the Sun. Using its engine to make targeting maneuvers after flying by Pluto and Charon, New Horizons will then proceed on to one or two of these KBOs in the five-year period after the Pluto encounter. The actual target objects will not be selected until a year before the Pluto-Charon encounter. This strategy allows the New Horizons team to exploit the rapidly expanding body of knowledge about individual KBOs for an additional 12 years before deciding which ones to visit.

The 465-kilogram New Horizons spacecraft (about half the weight of Voyagers 1 and 2) will carry six instruments to investigate the geology, surface compositions, temperatures, atmospheric structure, and plasma interactions of both Pluto-Charon and the KBOs it encounters. The spacecraft will also carry a student-built dust counter to map out the distribution of small particles in the deep outer solar system.

New Horizons will map Pluto and Charon at kilometer scales--with even higher resolution in selected regions. Mission objectives also call for mapping the color and surface composition across the sunlit hemisphere of each target body it visits using visible and near-infrared spectrometers. A battery of investigations, including ultraviolet airglow and solar occultation spectroscopy, will characterize the atmosphere around Pluto and will help search for atmospheres around other KBOs. More information on New Horizons can be found at http://pluto.jhuapl.edu.--S. Alan Stern

ALAN STERN directs the Southwest Research Institute's Department of Space Studies in Boulder, Colorado, and serves as principal investigator of NASA's New Horizons mission to Pluto and the Kuiper Belt.
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Author:Stern, S.Alan
Publication:Sky & Telescope
Geographic Code:1USA
Date:Nov 1, 2003
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