Worlds Enough by Joel Davis
* * * *
In recent issues we've featured several stories set in John Barnes's "Springer" universe, in which our everyday spatial constraints are unlamented parts of the dim past. Our lead novella for March is in a sense the climax of the series, revealing parts of that history heretofore only hinted at--at both the grand and the intensely personal scales. And I mean "intense" quite literally--whether or not you've read other stories in the series.
Joel Davis, who has done a number of science fact articles for us in the past, returns with "Worlds Enough," an up-to-the-moment survey of the farthest reaches of the Solar System, which are considerably more distant and a lot less empty than we used to think. In addition, we have stories by Larry Niven, Grey Rollins, Carl Frederick, and Henry Melton--and, of course, the dramatic conclusion of Karl Schroeder's serial Sun of Suns.
* * * *
The Solar System is bigger and less sharply defined than you may have thought.
* * * *
In 1930--the same year that a science fiction magazine later known as Analog began publication--a 22-year-young astronomer named Clyde Tombaugh spotted our solar system's ninth planet, Pluto. Seventy-five years later, it's clear that Tombaugh discovered not just the ninth planet. He unknowingly found the first known member of an entirely new family of solar system objects: the Kuiper Belt, the source of so-called short period comets and what planetary scientist and Kuiper Belt expert Dr. S. Alan Stern of the Southwest Research Institute in Boulder, Colorado calls "the solar system's attic."
The first object to be unambiguously identified as a member of the Kuiper Belt was found in 1992. There had been hints and allegations long before then; Tombaugh's discovery, for example, falls into that "if only we'd known..." category. But the discovery of 1992 QB1 by astronomers David Jewitt and Jane Luu was unmistakable, and was followed by more discoveries, more questions, and more surprises. Our understanding of the solar system has been undergoing a true revolution. It's now clear that the outer solar system is home to billions and perhaps trillions of small worlds, ranging in diameter from a few kilometers to the size of the planet Pluto. And perhaps even larger.
Today nearly a thousand objects have been discovered beyond Neptune. Collectively they're referred to as trans-Neptunian objects or Kuiper Belt objects (KBOs). All that we know about these new worldlets we've learned from afar, with telescopes and the sophisticated instruments attached to them. It's been enough to give us a first-order picture of this remarkable--and mysterious--region at the far edge of the solar system. What we need now (obviously!) is a visit to the Kuiper Belt with a dedicated space probe--a Pioneer or a Voyager. And now there is one: the New Horizons Pluto-Kuiper Belt mission.
* * * *

Location of the Kuiper Belt. The tilted orbit is that of Pluto. 
Illustration courtesy of Applied Physics Laboratory, Johns Hopkins University
* * * *
Whose Idea Was This? 
The first detailed published speculations about a cometary reservoir beyond Neptune were made by Irish amateur astronomer Kenneth Edgeworth in an article published in 1943. In an article published in 1949 Edgeworth reiterated his speculations. Meanwhile, the Dutch-American astronomer Gerard Kuiper was also wondering about the possible existence of objects beyond Pluto. In 1951 he wrote a book chapter[1] that included his speculations about the origin of short period comets. These are comets with orbital periods less than 200 years, and with orbits having inclinations[2] close to the plane of the solar system. Like Edgeworth, Kuiper believed that the apparent "sharp edge" of the outer system at Pluto couldn't be real. What better place, Kuiper reasoned, for short-period comets to come from? He suggested that Pluto's massive gravitational pull[3] would occasionally jostle a proto-cometary snowball out of its distant orbit, sending it plunging sunward. The hypothetical comet nursery was eventually named for Kuiper, mainly because he was better known in the astronomical community and more widely published.
* * * *
[Footnote 1: The book was Astrophysics; a Topical Symposium Commemorating the Fiftieth Anniversary of the Yerkes Observatory and a Half Century of Progress in Astrophysics, published by McGraw-Hill and edited by J. Allen Hynek (the same J. Allen Hynek who later became famous--or infamous--for his connection to UFOs and Project Bluebook).]
[Footnote 2: Orbital inclination (symbolized with i) is measured in degrees with respect to the plane of the ecliptic, which is defined by the plane of the Earth's orbit.]
[Footnote 3: At that time astronomers still believed that Pluto was a rather large and massive planet.]
* * * *
But in the 1950s there was no way to prove Kuiper's and Edgeworth's hypotheses about a cometary reservoir beyond Neptune or Pluto. Not even the largest telescopes with the most sensitive cameras and photographic plates could ever image such tiny objects at so a great distance. The Kuiper Belt would remain an untestable and unconfirmed hypothesis for another four decades.
Hints and Allegations 
Even before Edgeworth and Kuiper first suggested the existence of a belt of comets-in-waiting beyond Neptune, there were hints that the trans-Neptunian region contained more than just empty space:
Triton. Neptune's first known moon, discovered in 1846, follows a retrograde orbit (it moves clockwise around Neptune as viewed from above). It's dynamically impossible for moons in retrograde orbits to form in the same region as their planet, so Triton must be a captured body.
Nereid. Neptune's second known moon (discovered in 1949 by Gerard Kuiper!) has a highly elliptical orbit, and is also a body captured gravitationally by Neptune.
Pluto. The ninth planet has the most eccentric[4] orbit of any planet in the solar system. At times Pluto is closer to the Sun than Neptune, most recently from 1979 to 1999. Its orbit is also inclined to the plane of the solar system by a whopping 17.2 degrees. Equally intriguing, Pluto's in a 3:2 mean-motion resonance with Neptune. This curious gravitational interaction means that for every three orbits Neptune makes, Pluto makes two. Finally, Pluto turned out to be much too small (2,320 km in diameter, smaller than Earth's moon) to cause the apparent oddities in Neptune's orbit that had led to the search for a "Planet X" in the first place.
* * * *
[Footnote 4: Eccentricity is a measure of how far an orbit is from circularity. The less eccentric an orbit, the closer it is to a perfect circle; the more eccentric, the more elliptical the orbit.]
* * * *
2060 Chiron. Discovered in 1977 by Charles Kowal and named for the mythical Greek centaur, Chiron was originally thought to be the first known object in an asteroid belt between Saturn and Uranus. But its orbit turned out to be quite weird, taking Chiron from a perihelion[5] just inside Saturn's orbit to an aphelion just inside the orbit of Uranus. This is not a stable orbit over the age of the solar system; Chiron could not have originally formed in this region. Computer simulations showed that within a few million years Chiron will pass so close to Saturn that its orbit will be severely perturbed. Several years after its discovery Chiron began showing cometary activity, and many astronomers now consider it to be a really huge cometary nucleus rather than an asteroid.
* * * *
[Footnote 5: Perihelion is the point on an object's orbit closest to the Sun. Aphelion is the point on the orbit furthest from the Sun.]
* * * *
Charon. Named for the ferryman of Hades in Greek mythology, Pluto's only known moon was discovered in 1978 by astronomer James Christy of the U.S. Naval Observatory while examining photos of Pluto. Charon is about 1,272 km in diameter, a bit less than half that of Pluto. The two are gravitationally locked, each keeping the same face to the other. Because of that and their similar sizes, some astronomers consider Pluto and Charon as a double planet. Others consider it the first Kuiper Belt object discovered--long before anyone even knew of the Kuiper Belt's existence!
The flybys of the outer planets (Jupiter, Saturn, Uranus, and Neptune) in the 1970s and 1980s by Pioneer 10 and 11 and Voyager 1 and 2 revealed a wealth of new information about those planets. In the process, the space probes discovered new moons circling those distant gas giants and revealed astonishing new information about many of the previously known moons. They also revolutionized our understanding of Saturn's ring system, discovered ring systems encircling Jupiter and Uranus, and ring arcs around Neptune. But these probes did not resolve the question of the Kuiper Belt's existence.
Meanwhile, special computer chips called charge-coupled devices (CCDs) had begun a revolution in Earth-based astronomy by replacing traditional photographic film. CCDs convert as much as 90 percent of the light falling on them into an image. Standard photographic plates are 10 percent efficient at best. Early CCDs had very narrow fields of view, much narrower than standard photographic plates. But the field-of-view gap narrowed considerably as the technology matured. Astronomers, both professional and amateur, began discovering new planetary satellites, comets, and asteroids, including small asteroids in Earth-crossing orbits. As the twentieth century drew to a close, the solar system was looking a lot more crowded than it had when Clyde Tombaugh discovered Pluto. Soon it would turn out to be not only more crowded, but a whole lot bigger.
Jewitt and Luu and 1992 QB 1 
In 1987 David Jewitt, then an assistant professor at MIT working in planetary astronomy, began looking for distant objects in the solar system, with the assistance of graduate student Jane Luu. Their initial searches used then-standard astronomical technology--telescopes and photographic plates. They repeatedly photographed large areas of the night skies in both the northern and southern hemispheres, then spent tedious hours comparing the images using a blink comparator, "blinking" first one plate and then the other to see if anything "moved." This was the same technique and the same kind of machine that Tombaugh had used to find Pluto in 1930. They were unsuccessful, so they tried the new CCD technology, using another telescope at Kitt Peak and a CCD chip with a very small field of view. The results were the same.
The two pressed on, though, continuing their search for objects beyond Neptune. They had a new base of operations: the Institute for Astronomy at the University of Hawaii, with its access to the world-class cluster of telescopes atop Mauna Kea. Other groups began looking as well. Between 1989 and 1991, several teams of astronomers at various other institutions tried and failed to find distant solar system objects. As did Jewitt and Luu. But CCD technology was improving by leaps and bounds. Larger and more sensitive chips became available, and Jewitt and Luu took full advantage of them.
On the night of August 30, 1992, they hit pay dirt. Using a 2.24 m telescope on Mauna Kea, they spotted an object that seemed to move as they blinked two successive images of the same part of the sky. Additional observations in the following weeks verified their discovery: a tiny planetesimal[6] in an orbit beyond Neptune, and it was officially announced in September. The Minor Planet Center in Cambridge, MA, which keeps track of these things, gave the object the preliminary designation 1992 QB1. We now know that 1992 QB1 is a fairly small body, probably about 200 to 250 km in diameter assuming an albedo or surface reflectivity of 0.04 (similar to a cometary nucleus). Its orbit has a semimajor axis[7] of 43.7 AU[8], with perihelion at 40.9 AU (about 1.6 billion kilometers beyond Neptune) and aphelion at about 46.6 AU. It has an inclination of 2.2 degrees and a minuscule eccentricity of 0.065. 1992 QB1 appears to be a rather typical classical Kuiper Belt object.
* * * *
[Footnote 6: Small bodies formed in the early solar system by accretion of dust and ice.]
[Footnote 7: The average distance from the object to its primary, such as a planet or comet from the Sun.]
[Footnote 8: Astronomical unit, the mean distance from the Sun to the Earth, about 149.6 million km.]
* * * *
The largest known objects in the Kuiper Belt 
Name/Designation: Pluto
Diameter (km): 2,320
Semi-major axis (AU): 39.4
Discovered: 1930
* * * *
Name/Designation: 90482 Orcus
Diameter (km): ~1,600
Semi-major axis (AU): 45
Discovered: 2004
* * * *
Name/Designation: Charon
Diameter (km): 1,270
Semi-major axis (AU): 39.4
Discovered: 1978
* * * *
Name/Designation: 90377 Sedna
Diameter (km): 1,000-1,500
Semi-major axis (AU): 532
Discovered: 2004
* * * *
Name/Designation: 50000 Quaoar
Diameter (km): 1,000-1,400
Semi-major axis (AU): 43.25
Discovered: 2002
* * * *
Name/Designation: 55636 2002 TX300
Diameter (km): ~965
Semi-major axis (AU): 43.19
Discovered: 2002
* * * *
Name/Designation: 55637 2002 UX25
Diameter (km): ~910
Semi-major axis (AU): 42.71
Discovered: 2002
* * * *
Name/Designation: 28978 Ixion
Diameter (km): 890-1,220
Semi-major axis (AU): 39.39
Discovered: 2001
* * * *
Name/Designation: 20000 Varuna
Diameter (km): 860-1,260
Semi-major axis (AU): 43.23
Discovered: 2000
* * * *
Name/Designation: 55565 2002 AW197
Diameter (km): 770-1,010
Semi-major axis (AU): 47.52
Discovered: 2002
* * * *
The Borderland of Sol 
Almost a thousand planetesimals belonging to the Kuiper Belt have been discovered since 1992, and astronomers have steadily gathered basic information about them. The data acquired so far are giving planetary scientists new insights into the formation and evolution of the solar system's outermost regions. This is the area that science fiction writer Larry Niven once called "The Borderland of Sol."
How was the Kuiper Belt formed? Alan Stern is serious when he calls the Kuiper Belt the solar system's attic. For many of us, the attic of our house is a convenient storage space. It's where we toss all the stuff that we're not using and don't have a place for in the living room, bedroom, or even the closets. So up it goes, and out of sight.
The same appears to be true of the Kuiper Belt. The solar system formed 4.7 billion years ago from a disk of dust, ice and gas. Dust and ice particles stuck together, soon growing large enough for their gravitation to begin pulling in still more material. Planetesimals closer to the infant Sun were mostly rock, and soon merged into the four inner terrestrial planets. Much further out, planetesimals composed of both rock and ice grew. Several became huge--the infant Jupiter, Saturn, Uranus, and Neptune. In the region now occupied by Uranus and Neptune, planetesimals would have formed mostly from dust and ices of water, methane, and other simple organic molecules. These primordial objects would have soon settled into slightly elliptical orbits on the plane of the solar system.
Within a few million years, Jupiter had become massive enough for its gravitational field to exert a powerful influence on the evolving solar system. Computer simulations show that Jupiter's gravity would violently disrupt the orbits of planetesimals within its region of influence. Some would be thrown farther inward; others would be flung entirely out of the solar system. And still others would be tossed into orbits taking them dozens and even hundreds of AU from the Sun.
More important for the Kuiper Belt, though, was the formation of Neptune. Other computer simulations indicate that Uranus and Neptune may have formed closer to the Sun than they are today. As their gravitational fields threw planetesimals inward towards Jupiter, they slowly spiraled outward (an example of Newton's Third Law). As it did so, Neptune's gravitational field in particular affected the orbits of many planetesimals in the regions beyond its original location. Some became trapped in orbits affected by mean-motion resonance with Neptune; others were tossed even further outward. And some remained mostly where they were, but in orbits with greater eccentricity or inclination that before.
* * * *

Orbit of Quaoar, a classical KBO. 
Illustration courtesy of Chad Trujillo, Gemini Observatory
* * * *
This is a fairly simplified picture of the Kuiper Belt's formation--a confusing and chaotic time in the solar system's early history. There are still many puzzles that await resolution. For example: Why does the Kuiper Belt have a fairly sharp boundary at about 50 AU? Is it a real edge, or merely a dip or "trough"? What creates the Kuiper Belt's dynamic structure?
What does the Kuiper Belt look like? Astronomers generally agree that the Kuiper Belt has three main structural components: the classical Kuiper Belt, a scattered disk, and an extended scattered disk.
The classical Kuiper Belt stretches from Neptune's orbit (about 30 AU) to a fairly sharp boundary at about 50 AU from the Sun. This appears to be real; it corresponds closely to the 2:1 mean-motion resonance point with Neptune, the strongest gravitational resonance at that distance. "This might be because the radial migration of Neptune shoved pre-existing disk objects out that far and no more," says Jewitt. "The edge could also be caused by tidal truncation from a passing star or conceivably from a distant, unknown "˜planet'" or planet-sized KBO.
The vast majority of KBOs found so far are in this region. Orbits in the classical Kuiper Belt are stable over the age of the solar system, mainly because their perihelia are far enough from Neptune that its gravity can't scatter or disrupt them. About 10 to 25 percent are called Plutinos; like Pluto, they're in 3:2 mean-motion resonance with Neptune. Other KBOs are in orbits with different resonance relationships with Neptune. "I consider them a sub-population of the classical belt," says Brett Gladman of the University of British Columbia, the discoverer or co-discoverer of many KBOs. Jewitt considers them to be a separate component of the Kuiper Belt. "Resonant objects are clearly a stand-out group on plots of semi-major axis versus eccentricity," he says.
The objects in the classical Kuiper Belt have fairly circular orbits with semimajor axes from 35 to 56 AU, and form a region that looks like a doughnut. Some KBO orbits have a relatively high inclination or eccentricity. Within that doughnut is a region of orbits with low inclinations and low eccentricity. Gladman has referred to this latter region as the "dynamically cold component" of the classical Kuiper Belt, and the more inclined or eccentric orbits the "dynamically warm region." ("Cold" and "warm" refer to the relative mean random energy of orbits in each population, as if the KBOs were particles in a gas.) For reasons still unknown, the KBOs in dynamically warm orbits are grayer or more neutral in color, while those in the dynamically cold orbits are reddish in color. The problem is, there shouldn't be any dynamically "warm" component in this region; the orbits should have retained their low eccentricity and inclination for the age of the solar system. So something "stirred up" the classical Kuiper Belt early in its history, jostling some objects into dynamically warm orbits.
"We still don't know for sure what causes the correlation between inclinations and colors," says Chad Trujillo, a co-discoverer of several of the largest KBOs found so far. He thinks the two components may be two populations of KBOs superimposed on one another. "I think it's pretty easy to imagine that some inner solar system interlopers"--Gladman's dynamically warm group--"could have been superimposed onto the older, redder core population that formed beyond Neptune," he says. Trujillo also makes a sharper distinction between "the classicals" and "the resonant" KBOs, which he says "almost all [have orbits] quite a bit more elliptical than the classicals."
Other possible reasons for the dynamically warm component include those invoked to explain the sharp edge to the classical Kuiper Belt--
A planetesimal the size of Pluto or Mars passed very near or through the Kuiper Belt.
Another star passed within 50 to 200 AU of the outer parts of the Kuiper Belt during the early eons of the solar system.
As Neptune slowly migrated outward from its original location closer to the Sun, its mean-motion resonance regions disrupted the inclination of KBO orbits beyond Neptune.
The scattered disk is a region of KBOs with orbits that are highly eccentric, have large semi-major axes, and perihelia at around 35 to 40 AU, relatively near Neptune's orbit. Their aphelia are way out there: from several hundred to 3,000 AU distant.
The orbits of the scattered disk objects (SDOs) are stable on billion-year timescales, but their perihelia mean they do have a weak gravitational interaction with Neptune. In fact, that's probably why scattered belt objects are scattered. Repeated encounters with Neptune would send some of them deeper into space and pull others into orbits taking them closer to Neptune, Uranus, and the outer planets.
Finally, we should mention the Centaurs, defined as objects like 2060 Chiron with perihelia in the realm of the giant planets Jupiter and Saturn. Centaurs are not part of the scattered disk now. Says Gladman, "They have semi-major axes that are less than 30 AU, and spend most or all of their time within Neptune's orbit." But they may have originally come from the scattered disk, tossed further inward by coming too close to Neptune.
* * * *

Orbit of Sedna, a KBO that may actually be the first discovered member of the Oort Cloud. 
Illustration courtesy of Chad Trujillo, Gemini Observatory
* * * *
The extended scattered disk includes KBOs with large semi-major axes, large eccentricities, and with perihelia greater than 38 AU. These objects never come close enough to Neptune--"the big boy out there," says Jewitt--for their orbits to be jostled or distorted by its gravitational field. There are only two KBOs currently known to be part of the extended scattered disk, 90377 Sedna and (possibly) 2000 CR105. Orbits in the extended scattered disk are stable over the age of the solar system. But that also means that their orbits could not have been created by mean-motion resonances with Neptune. Some other force or forces must have put these KBOs in these orbits.
"People have successfully made models using a close stellar passage to create such orbits," says Trujillo, "so I would say it's a valid explanation. A Mars-sized body at around twice the distance to Neptune might also make such a body but I don't think anyone has modeled this case. A close stellar passage is the best explanation so far, but I think it's still much too early to say for sure."
Beyond the extended scattered disk is the inner region of the Oort Cloud. First postulated by astronomer Jan Oort in 1950, the Oort Cloud is the source of long period comets, which have orbital periods greater than 200 years and enter the inner solar system from every part of the sky. The Oort Cloud is thought to consist of an inner flattened region that ranges from 1,000 to 5,000 AU, and an outer spherical region that stretches out to 50,000 or even 100,000 AU--a third of the way to the nearest star.
"The observational difference between the extended disk and the inner Oort cloud is a subtle one," says Alan Stern. "It's more semantic than anything; they probably blend into one another. As we learn more about KBOs we can determine which population they belong to by their physical characteristics. We can't really distinguish yet from their orbits alone."
What are KBOs made of? KBOs are so distant and small that determining the exact composition of their surfaces and interiors is quite difficult. Astronomers instead gather information on the surface colors of KBOs, and then make informed guesses about their composition. KBOs range in color from somewhat neutral (gray, that is, reflecting all wavelengths more or less equally) to very red. The reddish KBOs may be covered by large areas of tholins, polymers formed when ultraviolet light strikes simple organic compounds like ethane and methane. Or, they may have the kind of blackened crusts seen on some comet nuclei. The more neutral-colored objects may have more ices on their surface. Collisions with other objects dig craters in dark, crusty surfaces and throw up fresher icy material, including water, carbon monoxide, carbon dioxide, and methane ices. Some of the larger KBOs may even have active ice volcanoes like Triton. The basic belief among planetary scientists is that most KBOs are probably made of various kinds of ices mixed with dust and some organic material. In this respect KBOs appear to have much the same composition as comets--an argument consistent with the Kuiper Belt as the source of short period comets.
How big are they? There are at least a half-dozen known KBOs that may be a thousand kilometers or more in diameter. The largest are 90482 Orcus (up to 1,600 km in diameter), 90377 Sedna (1,000-1,500 km), and 50000 Quaoar (1,000-1,400 km). And there likely are a lot more big ones out there.
"There are probably a hundred thousand Kuiper Belt objects that are a hundred kilometers or more in diameter," says Stern. "There are at least dozens of objects--and maybe as many as a thousand--that are half the size of Pluto or bigger." That's as many as a thousand Charons, and Sednas, and Quaoars. But wait, says Stern; there's more. "I'm a hundred percent sure in my bones that there's a least one Pluto- or Mars-sized object in the Kuiper Belt. And I'd say there's a fifty percent chance that there's one Kuiper Belt object the size of Earth."
Trujillo agrees with this assessment, but adds, "I bet that if there is a Mars-sized KBO, it is probably very distant. I would be very surprised if it were, say, within 50 AU and we didn't know about it already."
And smaller ones? Estimates range into the billions.
* * * *
New Horizons Primary Instrument Uses 
Name: Ralph
Instrument Type: Imager/Imaging spectrometer
Primary Uses: *Panchromatic photometric/geologic mapping (1 km resolution); *3-color and CH4 mapping (3 km resolution); *Composition mapping (7 km resolution); *Thermal mapping (2-20 km resolution)
* * * *
Name: Alice
Instrument Type: UV imaging spectrometer
Primary Uses: *Atmospheric composition; *Upper atmosphere pressure & temperature profiles
* * * *
Name: REX
Instrument Type: Radio science, radiometry
Primary Uses: *Lower atmosphere pressure & temperature profiles; *Disk averaged brightness temperatures; *Masses, J2s
* * * *
Name: LORRI
Instrument Type: High-resolution imager
Primary Uses: *Pluto far-side mapping (35-40 km resolution); *High-resolution geology (50 m resolution; *Distant/early start to encounter imagery (5x farther than Ralph)
* * * *
Name: SWAP
Instrument Type: In situ plasma spectrometer
Primary Uses: *Atmospheric escape rate
* * * *
Name: PEPSSI
Instrument Type: In situ particle spectrometer
Primary Uses: *Pickup ion composition
* * * *
Name: SDC
Instrument Type: In situ dust counter
Primary Uses: *1st solar system dust density profile beyond 18 AU
* * * *
New Horizons 
Pluto is the only planet not visited by a space probe from Earth. And no one even knew for sure that the Kuiper Belt existed until 13 years ago. Planetary scientists have wanted to send a spacecraft to Pluto since the 1970s. Now, after years of ups and downs, cancellations and resurrections, a spacecraft from Earth is about to journey to the borderland of Sol. Its primary target: Pluto and Charon. Its secondary target: one or more Kuiper Belt objects. The mission is named New Horizons Pluto-Kuiper Belt. At the time this issue of Analog went to press, it was ready to be launched during a 36-day window beginning January 11, 2006 from Cape Canaveral atop an Atlas V booster.
New Horizons has a mass at launch of about 465 kg, with on-board power provided by the same kind of plutonium-powered radioisotope thermal generators used by the Voyagers, Galileo, and Cassini-Huygens space probes. The spacecraft carries seven scientific instruments, including cameras, spectrometers and radio science experiments. One instrument worth special mention is the Student Dust Counter (SDC), the first instrument designed and run by college students to be carried on a planetary space probe. Alan Stern is the principal scientist for the New Horizons mission.
New Horizons will stretch spacecraft engineering to new limits. This is a long mission! After a cruise phase of about 13 months New Horizons will fly by Jupiter and its system of moons and rings, from February 25 through March 2, 2007. The flyby will give New Horizons a gravitational boost for its journey to Pluto and beyond.
Then it will cruise. For another eight years. Most of this time New Horizons will slumber, waking occasionally so its handlers back home can make sure its systems are still working normally.
In 2015 New Horizons will make its historic flyby of Pluto and Charon. Ten weeks before flyby the spacecraft's images of Pluto and Charon will exceed the best possible resolution achieved by the Hubble Space Telescope. Daily observations will begin about four weeks before closest approach. And if all goes well, New Horizons will make its flyby of Pluto-Charon on July 14, 2015, passing within 11,000 km of Pluto and about 27,000 km of Charon at a velocity of about 14 km per second.
* * * *

The New Horizons spacecraft, with major instruments labeled. 
"Ralph" and "LORRI" are the imaging instruments. 
Illustration courtesy of Applied Physics Laboratory, Johns Hopkins University
* * * *
Even before the spacecraft reaches Pluto, astronomers will find candidate KBOs for the extended mission. "We don't want to pick a KBO target until as late as possible," says Stern. He likens it to planning to take a trip to France in 2015 and eating dinner at a Parisian restaurant. Between now and 2015, thousands of new restaurants will open in Paris. So you don't pick a restaurant today; you wait until 2015 to make reservations.
Location is also important. "We need to find a KBO behind Pluto and a little to its left or right," explains Stern. "Like shooting skeet, you have to find it and lead it." After the Pluto flyby, New Horizons will still have enough maneuvering fuel for course changes that can get it to KBOs within its "trajectory cone." Stern plans to do the search between 2011 and 2013 using the Keck and Subaru telescopes. They'll look for KBOs in the right place out to 55 AU and choose one or two finalists from a short list of about a half dozen. Two weeks after the Pluto encounter New Horizons will carry out a trajectory change maneuver--a "burn"--that will aim it towards its first Kuiper Belt minor planet.
* * * *

New Horizons' path through the solar system from Pluto through KBO encounters. 
Illustration courtesy of Applied Physics Laboratory, Johns Hopkins University
* * * *
After the Pluto flyby New Horizons will cruise again, for about one to three years. The first KBO flyby could occur as early as 2016 or as late as 2019. The spacecraft will take some optical navigation images of the target KBO about four or five weeks before the flyby. With that information, flight controllers on Earth will have the spacecraft make an additional burn to put it on its final path. New Horizons should fly past its target at a velocity of between 8 to 14 km per second, possibly as close as 3,000 km.
And what will New Horizons see? What information will it glean from the quick encounter with a tiny, icy Kuiper Belt object in the distant regions of the solar system? We will probably learn much more about the shape, composition, size, geology, and mass of a KBO than we've ever known before. And, we should remember this: Every--every--flyby we've carried out has revealed stunning surprises about the planet, moon, asteroid, or comet encountered. Without exception.
So when we finally get our first up-close look at Pluto and then a Kuiper Belt object, when we first stare at a chunk of matter left over from the formation of the solar system, we should expect surprises.
We should expect to see something ... astounding.
(c)Copyright 2006 by Joel Davis