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Exploring Pluto-Charon and the Kuiper Belt

and onward to the kuipers
by Alan Stern
Boulder - Mar 22, 2002
The exploration of the outer solar system began in the early 1970s with the launch of Pioneers 10 and 11. These two, small spacecraft served as trailblazers for the larger missions that followed. Despite their small size - just 600 pounds each - Pioneers 10 and 11 made history by being the first spacecraft to cross the asteroid belt, the first to visit Jupiter, the first to use Jupiter's powerful gravity to slingshot onward, and (for Pioneer 11) the first to explore Saturn.

After Pioneer, NASA launched arguably its most successful robotic explorations of new worlds, with Voyagers 1 and 2, which were launched in 1977. These craft, weighing in at almost a ton, were far more capable than the more primitive Pioneers. The Voyagers boasted larger fuel supplies, a dramatic increase in onboard intelligence, deft pointing controls (necessary for high resolution imaging), and a suite of a dozen scientific sensors. By the end of the 1980s, Voyagers 1 and 2 had conducted far more extensive explorations of the Jupiter and Saturn systems than had the Pioneers, and Voyager 2 had gone on to reconnoiter Uranus and Neptune and their fascinating satellite systems.

Following on Voyager's success, NASA moved from first-time reconnaissance to detailed survey missions in the outer solar system. The first of these was the highly successful Galileo orbiter/probe mission, which reached the Jupiter system in late 1995. Even more sophisticated still than the Voyagers, Galileo is nearly 20-feet tall and weighed 6,000 pounds at launch; its Jupiter entry probe alone weighed 700 pounds. Galileo mission's six-year orbital tour has deepened our knowledge of Jupiter, its moons and magnetosphere, in ways that no flyby mission could. NASA's other outer solar system survey mission is called Cassini. Cassini is a Saturn orbiter, designed to explore the Saturn system in even greater detail than Galileo explored Jupiter. Cassini is 22- feet tall and weighed over 12,000 pounds at launch. It carries a European-built probe to parachute to the surface of Saturn's planet-like moon Titan. Cassini was launched in 1997, and conducted a fruitful flyby of the Jupiter system in late 2000; it is scheduled to arrive in orbit about Saturn in July of 2004.

But what about the Pluto system? Why didn't NASA send one of its Pioneers or Voyagers to explore it? NASA did have a plan to send Voyager 1 to Pluto after Saturn; it would have arrived in 1986. But this plan was abandoned around the time the Voyagers launched, because it was impossible to achieve a close flyby of Saturn's fascinating satellite Titan and still reach Pluto, given Voyager's limited fuel supply. A first, detailed look at Titan was one of the major goals of the Voyager program, and Voyager 2's assignment, to reach Uranus and Neptune, took it the wrong direction to reach Pluto. So, the exploration of Pluto had to be sacrificed.

Of course, in the late 1970s, very little was known about Pluto, and the Kuiper Belt hadn't yet been discovered. I believe that if we knew then what we know now, the 1970s' decision to abandon Pluto would not have been made. Hindsight, of course, is 20:20. No one then even suspected that Pluto and its satellite Charon would turn out to be so interesting and so fundamental to so many areas of solar system science. Nor could anyone have confidently forecast the modern-day realization that Pluto-Charon and the Kuiper Belt constitute the third major realm of our solar system.

Planet 9: A New Kind Of World
The Pluto-Charon system is the only planet-satellite system in our solar system that has not been explored by spacecraft. Therefore, the state of knowledge about this system is necessarily more primitive than about any other planet. Despite this, however, many basic facts are established. These include the radius, mass, and density of Pluto (each known to better than 10%) and the radius of Charon (known to 5%), and the mass and density of Charon (known to about 25%). Importantly, Charon is almost precisely half the size of Pluto. Because the system barycenter is known to be outside Pluto (between the two bodies), the pair constitutes a true double planet - something unique in our solar system.

Pluto-Charon orbit the Sun in an elliptical, inclined, 248-year orbit that is in the 3:2 mean motion resonance of Neptune. Perihelion was reached in 1989; the system is now receding from the Sun. The planet and satellite share a polar obliquity of 120 deg. Pluto-Charon have reached complete spin-spin-orbit synchronicity; the pair are the only fully tidally evolved planet-satellite pair in the solar system. Pluto's density, very near, 2 gm cm-3, indicates its bulk composition is dominated by hydrated rock, but contains up to 30% water ice. Light organics and other exotic materials are predicted to be abundant minor constituents.

Pluto's surface is highly reflective, with a globally averaged normal albedo of 55%. The surface color is red, much like Triton. Reflectance spectroscopy has identified N2, CO, CH4, and H2O frosts on the surface, with N2 being the dominant constituent. Other light organics resulting from ice radiolysis and other processes are widely expected to be present. Photometric measurements have revealed a complex lightcurve, with an amplitude higher than any other planet in the solar system. The surface has been mapped crudely (at ~500 km resolution) by HST; the maps reveal polar caps and other high-contrast surface units. Thermal measurements indicate steep surface temperature gradients, with bright (presumably sublimation-cooled) areas being near 40 K, and dark (purely radiative equilibrium?) units being near 60 K. What makes Pluto so scientifically interesting?

To begin, Pluto and its moon Charon (itself fully half Pluto's size) constitute the only true double planet in our solar system. Among the things we want to understand about this system are two basics. Firstly, how did the pair form? It is thought that Pluto-Charon formed in a titanic collision between worlds in the ancient past, much like the Earth-Moon system. Thus, Pluto-Charon is widely expected to shed light on formation models of the Earth and Moon. We also want to understand why Pluto and Charon are so different in appearance. Pluto has a highly reflective surface, distinct markings, a complex surface composition that includes a variety of volatile surface ices, and an atmosphere. Yet Charon's surface is far less reflective, with indistinct markings, and no apparent atmosphere. Is this sharp dichotomy between these two neighboring worlds a consequence of differing evolution, perhaps owing to their differing sizes and compositions, or is it a consequence of their mode of origin?

Another key attraction involves Pluto's pivotal context relative to other bodies in the outer solar system. As alluded to above, Pluto is the largest known planetary embryo, and therefore offers to teach us a good deal about the formation of planets, particularly in the outer solar system. Of equal importance, Pluto's density, size, and surface composition are strikingly similar to Neptune's large satellite Triton. One of the great surprises of Voyager 2's exploration of the Neptune system was the discovery of ongoing and vigorous internal activity on Triton. Will Pluto also display evidence of internal activity? Our present-day understanding of planetary processes indicates they should not, but Triton's activity was simultaneously an exciting surprise.

Yet another allure Pluto offers is its rich atmosphere and the intimate coupling between its atmospheric and surface properties. Pluto's atmosphere, though substantially thinner than Mars's, generates snows, the seasonal transport of volatiles across the planet, and perhaps even complex weather. And while Earth's atmosphere contains only one gas that can undergo phase transitions to other states (solid, liquid), water, Pluto's contains three - nitrogen, carbon monoxide, and methane. This bizarre and complex ensemble of cryogenic gases, combined with Pluto's rakish 120 deg polar tilt and its highly eccentric orbit, suggests Pluto has the most complex seasonal patterns of any planet in our solar system. Further still, Pluto's low gravity causes its atmosphere to escape - to bleed off - at a rate much like a comet. In fact, Pluto's atmospheric escape rate is so high that it is believed to be escaping hydrodynamically. In hydrodynamic escape, the kinetic energy of a large fraction of the atmospheric molecules is high enough to escape the planet. Although hydrodynamic escape is not seen on any of the other planets today, it is believed that this phenomenon was responsible for the early, rapid loss of hydrogen from the Earth's atmosphere, contributing to Earth's habitability. Pluto offers the only site in the solar system to study this important process today.

A Revolution At Sol's Frontier
When most adults alive today learned the geography of the solar system, just as when NASA planned the Voyager missions, Pluto appeared only an oddity, a footnote to the architecture of the solar system. Why? Because until the 1990s, our planetary system appeared to consist of four small, inner, rocky planets, an asteroid belt, four outer, gas giant planets, and little Pluto- an icy enigma.

In the late 1980s, however, dynamicists modeling the orbits of short- period comets discovered that something like the belt of formation debris that astronomer Gerard Kuiper had postulated in about 1950 was required to explain why these comets orbit so close to the plane of the solar system. This circumstantial evidence for the so-called Kuiper Belt drove observers back to their telescopes in search of other bodies lying beyond Neptune.

By this time, telescopes were being equipped with modern, CCD detectors that made searches far more sensitive than work done previously using photographic plates. As a result, by 1992 the Kuiper Belt had been discovered. The first Kuiper Belt Object (KBO) found was almost 10,000 times fainter than Pluto, and about ten times smaller in size - no wonder so many previous searches has failed to find any cohort population in which to place Pluto in context.

The 600+ KBOs discovered in the ten years since 1992 range in size from about 50 to over 1200 kilometers in radius. The largest is half the size of Pluto! By the late 1990s, studies of KBOs in the belt and those that have escaped and are now orbiting between the giant planets revealed that the population displays a wide range of surface colors, surface compositions, and shapes.

So too, it became obvious that the surfaces and interiors of KBOs likely harbor huge quantities of organics and water ice - bodies from the Kuiper Belt may have helped in seeding the early Earth (and Mars) with these raw materials so necessary for the initiation of biology. In 2001 it was discovered that a few percent of all KBOs have satellites of their own. In essence, the Kuiper Belt has turned out to be the big brother to the asteroid belt, with far more objects, and notably-, far more large objects (over 100 kilometers in size) than the asteroid belt.

In total, over 1,000 KBOs are likely to have been spotted by the end of next year. And that'll be just the tip of the iceberg, because only a small fraction of the sky has been surveyed for these faint objects. Based on the number of KBOs seen in every square degree of sky, it is estimated that the Kuiper Belt contains over 100,000 KBOs larger than 100 kilometers across.

The size, shape, mass, and general nature of the Kuiper Belt appears to be much like similar planetary formation debris belts seen around other nearby stars, such as Vega and Fomalhaut. This is an important link between our solar system and others, offering a nearby laboratory for planetary formation and debris belt evolution that can actually be visited by spacecraft.

The discovery and astronomical exploration of the Kuiper Belt over the past decade has fueled a revolution in our view of our planetary system. Today we recognize the Kuiper Belt as the third major population region of the planetary system, the context for Pluto, and the site of ancient planet building that was aborted at mid-term. And it revealed a unique place where we can study the embryos of outer planets, arrested in their growth in mid-stage, and presented to us for study. Nowhere else in the solar system does such an opportunity present itself.

New Horizons: Exploring New Frontiers
During the 1990s, a steady stream of scientific and public pressure put a mission to Pluto squarely on NASA's planetary exploration agenda. Late in the 1990s, NASA settled on a mid-sized mission concept, called Pluto-Kuiper Express, or PKE. PKE would launch a mid-sized, new-technology spacecraft to Pluto. But the cost of PKE doubled, and, as a result, NASA scrapped PKE in the fall of 2000.

With bold leadership, NASA's Associate Administrator for Space Science, Dr. Ed Weiler, launched a competition among universities, research labs, and aerospace industry for proposals to accomplish the first exploration of Pluto, its giant moon Charon, and the Kuiper Belt of comets and miniature worlds.

The Pluto-Kuiper Belt mission NASA selected, less than six months ago in late 2001, is called New Horizons, and it is much more than just the first mission to the last known planet in our solar system. It is also the next mission to explore the Jupiter system, a mission to explore a suite of objects in the frozen Kuiper Belt beyond Pluto, and a groundbreaking effort pioneering a new, lower-cost way of conducting outer solar system exploration. As detailed in the accompanying table, New Horizons provides more bang for less bucks than PKE could have.

Table 1. Pluto-Kuiper Belt Mission Attributes Comparison



New Horizons

Launch Date

Dec 2004 or Jan 2006

Jan 2006


Jupiter Gravity Assist

Jupiter Gravity Assist




Instrument Complement

  • Integrated UV/Vis/IR Remote Sensing
  • Radio Science
  • Integrated\UV/Vis/IR Remote Sensing
  • Radio Science
  • In situ Plasmas and Particles
  • High-Resolution Imaging
  • Data Storage

    2 Gb

    48 Gb

    Jupiter Science

    None Planned

    Extensive Plans

    Cruise Science

    None Planned

    Selected Observations


    1 (optional)

    1 (mandatory), maybe 3

    Total Cost



    A Citizens Campaign for Pluto-Kuiper Belt Exploration
    The exploration of Pluto and the Kuiper Belt by New Horizons represents both a return to first-time exploration in NASA's planetary program, and a scientific bonanza of proportions not unlike Voyager. Yet by selecting the mission by competitive bidding, NASA achieved a mission that promises to reduce mission costs down to dimes on the dollar, compared to Voyager.

    Nevertheless, the fate of New Horizons depends on whether or not the U.S. Congress funds the project to continue. Although the mission requires just 0.7% of NASA's annual budget to continue, no funding for it was provided in the 2003 NASA budget request.

    A public, grass-roots campaign to fund PKB is underway, however, led by 19-year old exploration enthusiast Ted Nichols. His web site, www.plutomission.com, provides detailed information on Pluto-Charon, the Kuiper Belt, and New Horizons; the site also provides the public with an easy-to-use way to email key Congressional appropriators to express their support for New Horizons.

    If Congress approves the development funding to construct and launch New Horizons, the exploration of Pluto-Charon and the Kuiper Belt will commence with a series of rapid-fire flyby encounters beginning just over a dozen years from now.

    Alan Stern is the Director of the Department of Space Studies at the Southwest Research Institute and the Principal Investigator of NASA's New Horizons Pluto-Kuiper Belt mission."

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    Pluto Flight Plan Trimmed To Save Time, Money and Fuel
    Laurel - Feb 21, 2002
    New Horizons mission planners have developed a new strategy that could trim nearly a year off their original schedule to send a spacecraft to the solar system's outermost planet that in addition to cutting costs will also save fuel increasing the opportunity to visit one or more Kuiper Belt Object beyond Pluto.

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