An article for AGU’s EOS (Transactions of the American Geophysical Union)


New Horizons: NASA’s Pluto-Kuiper Belt Mission



Alan Stern

Southwest Research Institute

Department of Space Studies

1050 Walnut Street, No. 426

Boulder, CO 80302


[303]546-9687 (fax)



Andy Cheng

Johns Hopkins Applied Physics Laboratory

Johns Hopkins Road

Laurel, MD 20723


[240]228-6670 (fax)



Revised: 25 February 2002


The trans-Neptunian region, containing the binary planet Pluto-Charon and the myriad planetary embryos of the Kuiper Belt, is a scientific and intellectual frontier. In recent years, the Pluto-Charon system itself has become recognized as a key element for understanding the origin of the outer solar system. So too, it has become apparent that Pluto-Charon is a scientific wonderland offering insights into exotic dynamics, the nature of primitive organic material, complex volatile transport processes, hydrodynamic atmospheric escape, as well as rich surface and atmospheric chemistry. Pluto’s size, density, albedo, surface composition, and atmosphere also make it a unique (and likely more primitive) comparator to Neptune’s, large and complex icy satellite Triton. Further, the discovery of the Kuiper Belt (KB), within which Pluto-Charon orbits, has fueled a revolution in our understanding of the origin, architecture, and richness of the deep outer solar system. Together, Pluto-Charon and the Kuiper Belt provide an exciting frontier for first-time planetary reconnaissance, with rich possibilities for illuminating the origin of the outer solar system, the nature of binary worlds, the interior and surface evolution of small bodies, and the physics of cryogenic atmospheres.


In this article we describe the Pluto-Kuiper Belt mission recently selected for development by NASA. This mission, called New Horizons, involves a team consisting of the Southwest Research Institute (SwRI), the Johns-Hopkins Applied Physics Laboratory (APL), Ball Aerospace, NASA/Goddard Space Flight Center (NASA/GSFC), Stanford University, and scientists from over a dozen other U.S. universities, NASA centers, and research institutions, including the Jet Propulsion Laboratory (JPL). The goals of the New Horizons mission are to reconnoiter the Pluto-Charon and the Kuiper Belt; exploration of the Jupiter system is also planned, as is a limited cruise science program. In total, the mission will include up to five science flybys (including Jupiter). These topics are of broad interest to planetary science and geophysics.


The selection of the New Horizons team culminated a yearlong NASA process to compete the PKB mission. This process, announced in December 2000 began with the January 2001 release of a NASA Office of Space Science Announcement of Opportunity (AO) calling for complete mission proposals due in April 2001. It continued with peer review leading to a down-select announced by NASA in June 2001 of two mission study team finalists from the suite of submitted proposals. These two teams then conducted detailed, NASA-funded Phase A studies of the PKB mission requirements and implementation techniques. After Phase A study submission by the two finalist teams in September 2001, and site visits and detailed peer review of the Phase A study reports in October 2001, NASA selected of New Horizons as the winning PKB mission team in November, 2001.


The entire PKB selection process, which was recommended by NASA’s Solar System Exploration Subcommittee (SSES) and Space Science Applications Advisory Committee (SSAAC) advisory committees, represented the first time that an outer planets mission was competitively selected. The end result of this process was a mission with substantially greater scientific return but lower cost than the sole-sourced Pluto Kuiper Express (PKE) mission that NASA was forced to cancel in 2000 owing to unacceptable projected cost increases.



Pluto-Charon and the Kuiper Belt: State of Knowledge Summary


Pluto-Charon. Because the Pluto-Charon system is the only planet-satellite system in our solar system that has not been explored by spacecraft, the state of knowledge about this system is necessarily more primitive than at 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 7%), 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 constitute a true double planet— something unique in our knowledge of the solar system.


Pluto-Charon orbit the Sun in an elliptical, inclined, 248-year orbit. This orbit is in 3:2 mean motion resonance with Neptune, which may indicate Pluto (along with Neptune) migrated outward several AU in the distant past. Perihelion was reached in 1989; the system is now receding from the Sun. The planet and satellite share a polar obliquity of 122 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. Models based on Pluto’s density, which is very near 2 gm cm-3, indicates its bulk composition is dominated by hydrated rock, but contains up to 35% water ice. Light organics and other materials are predicted to be abundant minor constituents.


Pluto’s surface is the most highly reflective of the planets, 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 of almost 30%,  higher than any other planet in the solar system. The surface has been mapped crudely (500 km resolution) by HST; the maps reveal polar caps and other high-contrast surface units. Thermal measurements indicate steep surface temperature contrasts, with bright areas being near 40 K, and dark units being near 60 K.


Pluto’s atmosphere was discovered by stellar occultation techniques. Its base surface pressure is at least 3 and perhaps as great as 0.150 millibars; the upper atmosphere has a temperature of 106 K owing to a near-surface inversion, but the details of this thermal structure are indeterminate. Hazes and/or discrete clouds may be present in the atmosphere. Model calculations predict an N2 dominated atmosphere, with traces of CH4, CO, and a complex suite of photolysis products. Owing to Pluto’s high orbital eccentricity and its high axial tilt, strong thermal forcing results. As a result of coupled ice/atmosphere sublimation thermal balance, strong seasonal pressure cycles have been predicted, including possible seasonal atmospheric collapse around 2020. Escape rate calculations indicate that Pluto’s atmosphere is likely to be in hydrodynamic escape, unlike any other planet (but like the early Earth and Mars).


Charon’s average surface albedo (35%) is much darker than Pluto’s; its surface color is gray (neutrally reflecting), and it has only a low amplitude (8%) lightcurve. Its surface composition appears to be dominated by water ice, but new absorption features in the mid-infrared have been detected in recent years, indicating the presence of other, as yet unidentified, surface constituents (possibly including ammonia or ammonia-hydrates). There has been no definitive detection of an atmosphere.


The origin of the Pluto-Charon binary is thought caused by a giant impact, much like the Earth-Moon system. The evidence for this hypothesis is based on the system’s high specific angular momentum, its high axial obliquity, and the large mass ratio of the binary. Pluto itself is thought to have been grown in heliocentric orbit during the epoch of planetary growth in the Kuiper Belt, some 4 Gyr ago. As such, and owing to its size, it is expected to represent a key sample of the bulk composition of planetesimals in the trans-Neptunian region.



The Kuiper Belt. The existence of the Kuiper Belt was first predicted by mid-20th century astronomers such as Kenneth Edgeworth and Gerard Kuiper. These and other astronomers of the 1930s, 1940s, and 1950s postulated that a debris belt of material left over from planetary formation might orbit the Sun beyond Neptune. However, the telescope and photographic technology of the mid-20th century was too primitive to give astronomers much hope of finding small bodies at these great distances.. By the late 1980s cometary astronomers, however, found strong evidence in the inclination distribution of the Jupiter family comets that they are coming from a disk-like reservoir just beyond Neptune’s orbit. As a result, a number of searches were begun in the late 1980s for the belt of material that Kuiper predicted. The first Kuiper Belt Object (KBO) was subsequently discovered in 1992. This object, designated 1992QB1, is more than 1000 times fainter than Pluto, and probably about 10 to 15 times smaller in radius.


Over 500 KBOs have now been discovered, with estimated diameters ranging from 50 to 1200 km. It is expected that the KBO size distribution includes still smaller objects (comets) and larger objects (perhaps even up to Pluto’s size).


Based on the amount of sky left to be searched and the number of faint, distant objects being found in faint CCD images, it is estimated that over 100,000 KBOs with diameters >50 km may orbit in a disk- or belt-like structure that stretches from 30 to at least 55 Astronomical Units (AU) from the Sun. This large population means that the Kuiper Belt is an even greater collection of objects than the asteroid belt between Mars and Jupiter (the main asteroid belt has only about 1000 objects as large as 50  km).


Based on analogy to cometary nuclei and recently-obtained millimeter wave detections, the surfaces of Kuiper Belt Objects are expected to be very dark, typically reflecting only 3% to 10% of the light that falls on them. It has been found the KBOs have a wide range of surface colors, varying from almost gray to very red, but it is not clear whether this is due to genetic differences among KBOs or evolutionary affects (e.g., space weathering, collisional resurfacing). There is some evidence for water ice and more exotic ices on KBOs. It is also not known if KBOs fall into compositional groups as the asteroid do, though some observing groups have claimed evidence to this effect. It is believed KBOs consist primarily of mixtures of water ice and rock, with some amount of organic and other complex compounds as well. Most KBOs rotate on their axes in a few hours, but some take days to rotate. In 2001 the first KBO satellites were discovered.


Collisional processes are known to play a key role in the Kuiper Belt. One significant result of collisional modeling is that KBOs smaller than ~50 km in diameter cannot have survived the collisional bombardment over time and therefore must be younger than the age of the solar system. As a result it is now widely accepted that the Jupiter Family comets, which have their source region in the Kuiper Belt, are chips off KBOs created in comparatively recent times by collisions in the Kuiper Belt.


Computer simulations indicate that the KBOs formed along with Pluto early in the history of the solar system. The total mass of the present-day Kuiper Belt is low, in the range of 0.5 to 1 Earth mass. This is known to be too low to have been able to form the KBOs in the age of the solar system. It is therefore surmised that the primordial Kuiper Belt was many (e.g., 50) times its present day mass. This mass estimate indicates that the primordial solar nebula extended uninterrupted beyond Neptune’s distance (30 AU), at least to the present-day edge of the main Kuiper Belt (55 AU). It is not clear if the relative dearth of large KBOs seen beyond 55 AU is due to a real edge in the Kuiper Belt near this distance, a decrease in the size and/or albedo of large KBOs, or simply a gap which may stretch only a few tens of AU with a larger, even more massive belt lying beyond.


Based on the sizes and orbits of KBOs, it appears that the Kuiper Belt was well on its way to growing one or more large planets, perhaps even something the size of the Earth, or even Neptune, when the growth process was interrupted. It is believed that the formation of Neptune is what disturbed the region gravitationally and interrupted this growth.  One consequence of this disturbance is that Neptune’s gravitational influence caused collisions between objects in the young Kuiper Belt to become very violent. As a result, much of the mass in the Kuiper Belt was eroded into dust and subsequently blown away into interstellar space. Similar processes have been observed to be taking place in what appear to be Kuiper Belts around many stars in the galaxy, such as Vega and Fomalhaut. This strong connection between the Kuiper Belt and other solar systems adds impetus to the desire to explore the Kuiper belt and KBOs further.



Mission and Payload Description


The first exploration of the Pluto-Charon system and the Kuiper Belt promises to be a scientific watershed. It will provide valuable insights into the origin of the outer solar system and the ancient outer solar nebula, the origin and evolution of planet–satellite systems presumably formed by giant impacts, and the comparative geology, geochemistry, tidal evolution, atmospheres, and volatile transport mechanics of icy worlds.


The New Horizons mission begins with the launch of a Discovery-class interplanetary spacecraft in January 2006 onto a trajectory that reaches Pluto-Charon via a March, 2007 Jupiter Gravity Assist (JGA). Pluto-Charon can be reached as early as 2015 or 2016, depending on the launch vehicle selected by NASA. Multiple KBOs will be encountered in the five succeeding years after the Pluto-Charon encounter.


The New Horizons spacecraft design mass is 416 kg, including propellant for a 290 m/s propulsion budget. The spacecraft subsystems are based on APL’s Discovery/CONTOUR spacecraft. CONTOUR is scheduled for launch in July 2002. CONTOUR itself, a multiple comet flyby spacecraft, is based in part on APL’s TIMED earth orbiter mission (launched in 2001). Use of CONTOUR design heritage reduced schedule and cost risk, allowing a substantial, 22% dry mass margin, a healthy 20% power margin at Pluto encounter, and a significant, multi-year margin against the NASA AO’s 2020 Pluto-Charon arrival date limit.


This spacecraft will carry four complementary reconnaissance instruments. The payload consists of the PERSI Vis/IR/UV remote sensing package, the REX radio/radiometry experiment, the PAM plasma suite and the LORRI long-focal-length imager. Notably, New Horizons accommodates an infrared imaging spectrometer, which Voyager did not have, and which is essential to characterize the composition and the physical state (including temperature) of the ices on the surface. In addition, New Horizons will achieve a best imaging resolution at Pluto that is several times superior to the best achieved by Voyager at Triton, allowing, for example, better discrimination among possible geologic processes. The disk-average surface temperatures of the daysides and the nightsides of Pluto and Charon will be determined by measurement of the microwave brightness temperatures by REX; surface temperature mapping across each body will be achieved by measurement of temperature-sensitive spectral features of ices by LEISA.  Table 21provides additional detail regarding the payload and its sensor suite.


As the next mission to Jupiter, New Horizons will conduct an intensive, 4-month campaign of Jupiter system observations in early 2007. Closest approach will occur in March 2007 at a distance of 45±5 Rj (as set by the Pluto aim point); this is over three times closer than Cassini’s Jupiter flyby in 2000-2001. This encounter affords irresistible opportunities for studies such as long time base imaging studies of atmospheric and auroral dynamics, new observations of the Galilean and irregular satellites of Jupiter, and in situ exploration of the jovian magnetosphere.


During the cruise from Jupiter to Pluto, New Horizons may be able to reach a Kuiper belt escapee (a so-called Centaur object), but this depends upon groundbased searches finding a suitable target along the mission trajectory.


The Pluto-Charon encounter begins 6 months prior to closest approach. For a period of 75 days on either side of closest approach, New Horizons images will exceed the best the Hubble Space telescope can achieve at Pluto-Charon. This allows advance planning to optimize the close approach sequence, and a substantial timebase of disk-resolved images to study time-variable phenomena such as volatile transport and meteorology. 




Table 1. New Horizons payload overview.



Sensor Characteristics



Remote sensing suite

MVIC (panchromatic and four-color CCD imager, 0.4-1.0  microns, 20 microradians/pixel), LEISA (near infrared imaging spectrometer, wedged filter, 1.25-2.5 l/Dl = 600 for 2.1-2.25 microns and 300 otherwise, 62 microradians/pixel), and ALICE (UV imaging spectrometer, 500-1850 Ĺ, spectral resolution 3 Ĺ, 5 milliradians/pixel)



Uplink radio science, passive radiometry

Signal/noise power spectral density 55 db-Hz; ultrastable oscillator stability 1x10-13 in 1 second samples. Disk-averaged radiometry to ±0.1 K.

Stanford U., JHU/APL


Plasma and high energy particle spectrometers

SWAP (solar wind plasmas up to 6.5 keV, toroidal electrostatic analyzer and retarding potential analyzer), and PEPSSI (ions 1-5000 keV and electrons 20-700 keV, time-of-flight by energy to separate pickup ions)



High resolution imager

Panchromatic, narrow angle CCD imager, 0.30-0.95 microns, 5 microradians/pixel




Long focal length approach imagery will include 40 km-class mapping of the so-called farside hemispheres of Pluto and Charon 3.2 days out (one half the rotation period of Pluto-Charon). This obviates the well-known farside mapping dilemma imposed by Pluto’s slow (6.4 d) rotation for a single-spacecraft flyby mission.


The spacecraft-planet relative flyby speed of the Pluto-Charon encounter will be 11 km/sec. Near closest approach, New Horizons will obtain maps of both Pluto and Charon with km-scale resolution; at closest approach, images at scales as high as 25 m/pixel may be achieved (depending on the final flyby distance selected). In addition, the Group 1 objectives call for mapping the surface composition and distributions of major volatile species, for which New Horizons will obtain: (i) four-color global (dayside) maps at 1.6 km resolution, (ii) diagnostic, hyper-spectral near-infrared maps at 7 km/pixel resolution globally (dayside) and at 0.6 km/pixel for selected areas. Characterization of the neutral atmosphere and its escape rate will be accomplished by a battery of investigations including: (i) diagnostic ultraviolet airglow and solar occultation spectra to determine the mole fractions of N2, CH4, CO and Ar to 1% in total mixing ratio and to determine the temperature structure in the upper atmosphere, (ii) radio occultations at both Pluto and Charon, measuring the density/temperature structure of Pluto’s neutral atmosphere to the surface, (iii) in situ determination of the atmospheric escape rate by measuring Pluto pickup ions, and (iv) H Lyα mapping of the Pluto-Charon system in order to determine the rate of Roche-lobe flow of atmosphere from Pluto to Charon.


Numerous other scientific objectives will also be carried out during the encounter, as shown in Table 2. New Horizons will achieve the same objectives at the sample of Kuiper Belt Objects it reconnoiters as it will at Pluto-Charon.



Table 2.  New Horizons Pluto-Charon and KBO measurement objectives.

Group 1: Required by the NASA PKB AO

Characterize the global geology and morphology of Pluto and Charon

Map surface composition of Pluto and Charon

Characterize the neutral atmosphere of Pluto and its escape rate

Group 2: Highly Desired by NASA PKB AO

Characterize the time variability of Pluto's surface and atmosphere

Image Pluto and Charon in stereo

Map the terminators of Pluto and Charon with high resolution

Map the surface composition of selected areas of Pluto and Charon at high resolution

Characterize Pluto's ionosphere and solar wind interaction

Search for neutral species including H, H2, HCN, and CxHy, and other hydrocarbons and nitriles in Pluto's upper atmosphere

Search for an atmosphere around Charon

Determine bolometric Bond albedos for Pluto and Charon

Map the surface temperatures of Pluto and Charon

Group 3: Cited as Desirable in the NASA PKB AO

Characterize the energetic particle environment of Pluto and Charon

Refine bulk parameters (radii, masses, densities) and orbits of Pluto and Charon

Search for additional satellites and rings






Concluding Remarks


The New Horizons mission team is excited to initiate the development of the PKB mission for NASA. Congress funded the PKB effort sufficiently to complete the detailed design effort and to initiate certain long lead-time procurements in FY2002. Further development of the mission will require sustained funding in FY2003 and beyond. If NASA is funded to complete New Horizons and launch it, the result should be the long awaited reconnaissance of the planetary system’s third domain and final frontier— Pluto-Charon and the Kuiper Belt.


The New Horizons project intends this reconnaissance to benefit the entire community and the U.S. public. Efforts to accomplish this broad-based return include an active, well-funded Education and Public Outreach (EPO) program, a rapid (days timescale) data dissemination policy, and an $11M funding block within New Horizons for NASA-selected participating scientist and data analysis efforts by members of the U.S. national planetary science community.


The first exploration of the last known planet in the solar system is an exciting possibility, with strong prospects for providing the public with a renewed sense of drama and excitement in planetary exploration. Equally importantly, in achieving its geophysical, geochemical, geological, and atmospheric science objectives, the Pluto-Kuiper Belt mission will open the study the origin and evolution of binary worlds and Kuiper Belt Objects, and in doing so, address some of the most compelling questions in all of planetary science.


Alan Stern is the Principal Investigator of the New Horizons mission. Andy Cheng is the Project Scientist for New Horizons. Additional information on New Horizons can be found at and

Figure 1. Upper panel: The New Horizons spacecraft and key components. The high-gain Communications antenna is 2.5 m in diameter. Lower panel: Heliocentric trajectory schematic.