NASA's New Horizons Mission to Pluto and Beyond

November 2014
Artist's concept of the New Horizons spacecraft as it approaches Pluto and its three moons in summer 2015.

NASA’s New Horizons spacecraft, launched in 2006, is now in the final stage of its journey to Pluto. This is a flyby mission, like the Voyager 1 and 2 missions to Jupiter, Saturn, Uranus, and Neptune in the 1980s. The spacecraft swept across the orbit of Neptune in late August and will get closest to Pluto on July 14, 2015. New Horizons is now in rare territory, being just the fifth probe to venture so far from the Sun. Its heliocentric distance is 4.5 x 109 km, and it is hurtling along at 15 km per second.

The Johns Hopkins Applied Physics Laboratory (APL), Baltimore, Maryland, designed, built, and operates the spacecraft, and manages the mission for NASA. At APL, Principal Investigator Dr. Alan Stern (Southwest Research Institute) leads the mission team. For ScienceWatch, Dr. Stern offered an upbeat assessment of what the mission will achieve: "Most incredible of all, I think, will be the detailed imaging of Pluto and its entourage of satellites—the current count is five but we may find more as we get closer. They are the real stars in this story. Pluto and its moons are billions of years old but completely new in our consciousness as real places, with real personalities and histories of their own."

After the Pluto flyby, New Horizons will venture a billion miles beyond the orbit of Neptune, deep into the mysterious Kuiper Belt, a graveyard of remnants from the formation of the solar system, where hopefully it will examine one or two icy relics of the early solar system.

To find out how planetary scientists are preparing for the historic encounter ScienceWatch made two searches of Thomson Reuters Web of Science to assess the impact of papers on the outer solar system. The publication dates searched covered the present century (2001-2014), encompassing non-review papers indexed in the Web of Science category of Astronomy & Astrophysics.


This search was conducted on the topic “Pluto” by examining the fields “Title,” “Abstract,” “Author Keywords,” and “Keywords Plus.” The selection of 10 high impact papers in Table 1 was made by manually filtering papers in which the major part of the science is directly relevant to New Horizons and its onboard instrumentation. Collectively, these papers have been cited 802 times.

Table 1: Highly Cited Papers in Anticipation of the New Horizons Mission

Rank Paper Citations
1 G.M. Bernstein, et al., “The size distribution of trans-Neptunian bodies,” Astronom. J., 128(3): 1364-90, 2004. 199
2 K. Zahnle, et al., “Cratering rates in the outer Solar System,” Icarus, 163(2): 263-89, 2003 185
3 M.H. Moore, R.L. Hudson, “Infrared study of ion-irradiated N-2-dominated ices relevant to Triton and Pluto: Formation of HCN and HNC,” Icarus, 161(2): 486-500, 2003. 73
4 W.M. Grundy, B. Schmitt, E. Quirico, “The temperature-dependent spectrum of methane ice I between 0.7 and 5 μm and opportunities for near-infrared remote thermometry,” Icarus, 155(2): 486-96, 2002. 73
5 J. Licandro, et al., “The methane ice-rich surface of large TNO 2005 FY(9): a pluto-twin in the tran-neptunian belt?,” Astronom. & Astrophys., 445(3): L35-8, 2006. 64
6 H. Hussman, F. Sohl, T. Spohn, “Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian obejcts,” Icarus, 185(1): 258-73, 2006. 56
7 W.M. Grundy, M.W. Buie, “Distribution and evolution of CH(4), N(2), and CO ices on Pluto’s surface, 1995 to 1998,” Icarus, 153(2): 248-63, 2001. 48
8 M.W. Buie, et al., “Orbits and photometry of Pluto’s satellites, Charon, S/2005 P1, and S/2005 P2,” Astronom. J., 132(1): 290-8, 2006. 45
9 J.L. Elliot, et al., “Changes in Pluto’s atmosphere, 1988-2006,” Astronom. J., 134(1): 1-13, 2007 33
10 E. Lellouch, et al., “Pluto’s lower atmosphere structure and methane abundance from high-resolution spectroscopy and stellar occultations,” Astronom. & Astrophys., 495(3): L17-21, 2009. 32
SOURCE: Thomson Reuters Web of Science

There are three categories of planets in the solar system. Mercury, Venus, Earth and Mars are the inner four “terrestrial” planets, small and rocky. Jupiter, Saturn, Uranus and Neptune add weight as the outer four “gas giants.” Pluto is in a third classification as the largest of the “ice dwarfs” at the edge of the planetary system. What the New Horizons mission is all about is mapping the surface composition, characterizing the geomorphology, and probing the neutral atmosphere and its escape rate.

In Table 1, Papers #1 and #2 on the size distribution of objects in the outer solar system, and the cratering rate, are relevant to New Horizons because they give a historical account of the remnants of the formation of the solar system. A fundamental purpose of New Horizons is to investigate the environment of the trans-Neptunian solar system

The seven instruments on New Horizons include an imaging spectrometer to probe atmospheric composition and planet structure. The key papers in Table 1 concerning the atmosphere are #9 on changes in Pluto’s atmosphere 1988 to 2006, and #10 on the lower atmosphere.

Papers #3, 4, 5, and 7 are about the properties of methane ice. Pluto’s surface has a crust of nitrogen and methane ice overlying water ice. The key paper in this quartet is #7 by William Grundy and Marc Buie, which presents near-infrared spectra of the planet Pluto obtained at Lowell Observatory on 83 nights during 1995-1998. The paper sketches a uniquely detailed picture of the physical distribution of CO, N2, and CH4 ices on Pluto’s surface.

Will Grundy (Lowell Observatory, Flagstaff, Arizona) is a co-investigator on New Horizons, heading the mission’s surface composition team. He offered the following forward look: “With the New Horizons encounter, we'll receive a huge flood of much more detailed information all at once, not at all like we're used to. The whole nature of how we work with the data will change dramatically, when there's far too much for one person to look at all at once on their computer screen.”


The Kuiper Belt is the largest structure in the planetary system, extending from 4.5 x 1010 km (Neptune’s orbit) to 7.5 x 1010 km. David Jewitt (then at the University of Hawaii) and his doctoral student Jane Luu (University of California, Berkeley) discovered the first Kuiper Belt object (KBO) in 1992. In recognition of their discovery they received in 2012 the prestigious Shaw Prize in Astronomy as well as the Kavli Prize in Astrophysics.

The number of known KBOs exceeds 1,000, and there are suggestions that it includes more than 100,000 objects with a diameter greater than 100 km. Most KBOs have surfaces of frozen volatiles, principally methane, ammonia, and water, with similarities to comets, and in contrast to the asteroids, which are rocky.

To assess the impact of research on the Kuiper Belt, ScienceWatch sought papers published between 2001 and 2013, with the search terms “Kuiper Belt” and its acronyms. This 21st century analysis produced 1,186 papers that have been cited 24,063 times in 6,898 articles. Figure 1 shows that since 2003 the number of papers published each year has averaged about 100. The yearly count of citations grew rapidly before settling at 2,500-plus from 2009.

Figure 1. Annual number of published papers (left) and collective citations (right) to papers on the Kuiper Belt. (SOURCE: Thomson Reuters Web of Science)

Table 2 lists the 10 most highly cited Kuiper Belt papers published in the last five years (2009-2013). The paper heading the leader board is an account of the observables resulting from the dynamical evolution of the Kuiper Belt. The modeling shows how the giant planets excited the orbits of many KBOs, causing them to evolve on to cometary orbits and impact the terrestrial planets in the Late Heavy Bombardment approximately 4 billion years ago. Paper #1 should be read alongside #2, which looks at the hot and cold populations of KBOs, and also #6. Paper #5, by David Jewitt considers the Centaurs, a group of about 200 minor planets orbiting between Jupiter and Neptune. They have recently been expelled from the Kuiper Belt.

Table 2: High-Impact Papers on The Kuiper Belt, 2009-2013

Rank Paper Citations
1 M. Booth, et al., “The history of the Solar system’s  debris disc: observable properties of the Kuiper belt,” Month. Not. Royal Astron. Soc., 399(1): 385-98, 2009. 51
2 W.C. Fraser, J.J. Kavelaars, “The size distribution of the Kuiper belt objects for D ≥ 10 km,” Astronom. J., 137(1): 72-82, 2009. 51
3 M.J. Brucker, et al., “High albedos of low inclination Classical Kuiper belt objects,” Icarus, 201(1): 284-94, 2009. 46
4 J.N. Cuzzi, et al., “Towards initial mass functions for asteroids and Kuiper Belt Objects,” Icarus, 208(2): 518-38, 2010. 43
5 D. Jewitt, “The Active Centaurs,” Astronom. J., 137(5): 4296-312, 2009. 42
6 W.C. Fraser, et al., “The luminosity function of the hot and cold Kuiper belt populations,” Icarus, 210(2): 944-55, 2010. 35
7 S.D. Benecchi, et al., “The correlated colors of transneptunian binaries,” Icarus, 200(1): 292-303, 2009. 35
8 J. Lebreton, et al., “An icy Kuiper belt around the young solar-type star HD 181327,” Astron. & Astrophys., 539(A117): 2012. 32
9 C. Elroa, et al., “Cold DUst around NEarby Stars (DUNES). First results. A resolved exo-Kuiper belt around the solar-like star ζ2 Ret,” Astron. & Astrophys., 518(L131), 2010. 29
10 J.J. Kavelaars, et al., “The Canada-France ecliptic plane survey L-3 data release: The orbital structure of the Kuiper belt,” Astronom. J., 137(6): 4917-35, 2009. 28
SOURCE: Thomson Reuters Web of Science

The formation process of KBOs and asteroids is the main thrust of #4, which studies the accretion of 10-100 km size planetesimals. Paper #10 is the source of the estimate that the size of the main classical Kuiper Belt population is around 105.  If New Horizons can glimpse just a couple of these objects, our knowledge of the history of the outer solar system will be greatly improved.

New Horizons is a historic mission of exploration. When the spacecraft arrives at the icy dwarf Pluto, the United States will become the first nation to reach every planet from Mercury to Pluto with a space probe. The last NASA launch to an unexplored planet occurred 30 years ago, with Voyager. New Horizons will complete the reconnaissance of the planets in the solar system, and will surely open a new chapter on understanding the origin of the solar system.

Simon Mitton is a Life Fellow of St. Edmund’s College, University of Cambridge, UK.

The data and citation records included in this report are from Thomson Reuters Web of ScienceTM. Web of ScienceTM is a registered trademark of Thomson Reuters. All rights reserved.