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This issue of the Johns Hopkins APL Technical Digest, “New Horizons: Completing the initial Reconnaissance of the Solar System,” tells the story of the exploration of Pluto and the Kuiper Belt. It describes the history of New Horizons from conception through the flyby of Pluto and the Kuiper Belt object Arrokoth, as well as the scientific rationale for the mission. This discussion is followed by descriptions of the spacecraft and instruments; how the New Horizons team flew the mission across the solar system and then told the world about it; and a summary of the scientific results and possible future science investigations (subject to NASA approval) to study the outer heliosphere.

NASA’s New Horizons Pluto–Kuiper Belt mission was selected for development on November 29, 2001, following a competitive selection resulting from a NASA mission announcement of opportunity. New Horizons undertook the first exploration of the Pluto system and the Kuiper Belt. It also represents a watershed development in the scientific exploration of a new class of bodies in the solar system—dwarf planets, worlds with exotic volatiles on their surfaces, rapidly (possibly hydrodynamically) escaping atmospheres, and giant impact-derived satellite systems. It also provided other valuable contributions to planetary science, including the first dust density measurements beyond 18 au (astronomical units), cratering records that shed light on both the ancient and present-day Kuiper Belt object (KBO) impactor population down to tens of meters, and a key comparator to the puzzlingly active former dwarf planet and Neptunian satellite, Triton, which is in the same size class as the small planets like Pluto and Eris. The ~475-kg spacecraft carries seven scientific instruments, including imagers, spectrometers, a radio science instrument, a plasma and particles suite, and a dust counter built by university students. New Horizons demonstrated the ability of principal investigator–led missions to use nuclear power sources and to be launched to the outer solar system. As well, the mission has demonstrated the ability of nontraditional entities, like the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institute (SwRI), to explore the outer solar system, giving NASA new programmatic flexibility and enhancing its competitive options when selecting outer planet missions. This article, which heavily adapts and borrows from a 2008 Space Science Reviews article (vol. 140, pp. 3–21), is a historical overview of the origins of the New Horizons mission.

On July 14, 2015, the New Horizons mission accomplished the first flyby of Pluto–Charon, achieving full mission success during its primary mission. Less than 4 years later, during its first extended mission, New Horizons flew by Arrokoth, a 36-km contact binary trans-Neptunian object in the Kuiper Belt, on January 1, 2019. Along the way, New Horizons imaged numerous distant Kuiper Belt objects, performed important heliophysics science including complex Lyman-α radiation scans, and measured the dust and zodiacal light from regions never before explored. This article provides an overview of the New Horizons spacecraft and its engineering performance, as well as potential strategies for extending the mission far beyond its original design lifetime. Details on the mass and power budgets, as well as descriptions of key innovations to meet the challenges posed by the mission, offer insight into the engineering accomplishments that led to mission success. Trended data on the power, thermal, and propulsion systems substantiate projections of the mission’s potential to continue its exploration beyond the heliopause until ~2050.

The New Horizons science payload consists of seven instruments—three optical instruments, two plasma instruments, a dust sensor, and a radio science receiver/radiometer. These instruments were designed to withstand the cold conditions and low light levels in the Kuiper Belt so they could investigate the global geology, surface composition and temperature, and the atmospheric pressure, temperature, and escape rate of Pluto and its moons. The same payload was used to explore Arrokoth, the most distant object ever targeted for a flyby. The instrument suite is highly power efficient and represents a degree of miniaturization that is unprecedented in planetary exploration. This article describes the instruments and how they met challenging mission requirements with resounding success, making groundbreaking measurements and returning data that continues to shed light on the mysterious planets and smaller bodies of the outer solar system.

New Horizons was the first mission to Pluto. The spacecraft was launched on January 19, 2006, and flew by Pluto on July 14, 2015, returning historic images and data that revealed new insights about the Pluto system. It then flew by Arrokoth on January 1, 2019. But work on New Horizons began long before 2006, including many years of effort to design and propose the mission, build the spacecraft and its instruments, and develop and implement the mission operations concept. This article describes the mission operations concept for both nominal and encounter planning as well as anomaly resolution. It details the challenges of operating a spacecraft that will fly across the solar system and how the mission operations team met these challenges, including the technical hurdles in implementing science observation requirements into spacecraft commands; the balancing of spacecraft communication and science observation periods; the constraints imposed by the spacecraft subsystems; the distances (and thus time delay) between the mission operations center on Earth and the spacecraft; and the programmatic constraints to control mission operations costs for this long mission.

New Horizons was the first mission with primary science objectives to explore the Pluto–Charon system and, in an extended mission, to observe a Kuiper Belt object (KBO). This article summarizes the challenges in planning and targeting the New Horizons spacecraft for the Pluto encounter and how the team addressed these challenges, reducing mission risk to ensure a successful encounter that fully met its science objectives. It also presents the navigation accuracies achieved and the lessons learned, which were later applied to planning and conducting the flyby of a newly discovered KBO, Arrokoth, during New Horizons’ first extended mission.

NASA’s New Horizons (NH) mission was the first to explore Pluto and the Kuiper Belt in situ. Launched on January 19, 2006, the spacecraft successfully executed close flybys of Jupiter on February 28, 2007, Pluto on July 14, 2015, and the small Kuiper Belt object (KBO) Arrokoth on New Year’s Day 2019. NH transformed Pluto from a barely resolved astronomical target into a geologically complex and diverse world with hints of a subsurface ocean. NH data showed Charon to be very different from Pluto, with a surface dominated by water ice but sprinkled with ammonia-bearing ices near some craters, a giant chasm nearly encircling its equator, and a reddish polar hood. Pluto’s small satellites were resolved for the first time by NH, and spectra of Nix and Hydra showed deep absorption bands of water ice, strengthening the hypothesis that these small satellites formed ~4.5 billion years ago in the aftermath of the same cosmic collision that produced the Pluto–Charon binary. NH observations revealed Arrokoth to be a “contact binary” with two highly flattened lobes and an organicrich surface showing traces of methanol ice but essentially devoid of water ice. The NH Arrokoth data provide some of the first detailed evidence for the “pebble cloud collapse” streaming instability model of planetesimal formation in the solar nebula. NH returned unique scientific results during its flyby of Jupiter; data from its plasma instruments during the mission’s cruise phases are revealing new insights into the solar wind and the outer heliosphere, and NH observations of distant (i.e., non-flyby) KBOs provide data at geometries unattainable from Earth-based facilities, enabling unique results on light scattering from KBO surfaces. The NH instruments and spacecraft are still as capable as they were at launch more than 17 years ago, and they are continuing their exploration of the outer solar system and beyond during a recently approved second extended mission phase.

New Horizons’ historic flyby of Pluto in July 2015 generated headlines worldwide and set records for both the farthest planet ever explored and, arguably, the largest public engagement of any NASA science mission in history. From live road shows and innovative social media campaigns to documentaries and live television broadcasts, the mission brought the public along on this great voyage of exploration. The engagement and communications program began long before the spacecraft was even launched, and it had to include plans to generate awareness and maintain excitement through a decade-long cruise to Pluto. The excitement and outreach continued as New Horizons achieved another historic flyby—of the Kuiper Belt object 2014 MU69 (since named Arrokoth) on January 1, 2019. As more records fell—this time, for the farthest world ever explored, more than 6.5 billion kilometers from Earth—the team applied its best practices and lessons learned from the Pluto flyby.

This article gives an overview of New Horizons’ past, present, and future exploration of the heliosphere, including descriptions of the planned future investigations by the plasma and particle instruments Solar Wind Around Pluto (SWAP) and Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI). These investigations include the evolution of the solar wind, pickup ions, energetic particles, and galactic cosmic rays in the outer heliosphere, as well as the propagation of the solar disturbances throughout the heliosphere. The article also presents the observation plans for the ultraviolet spectrograph Alice, which consist of all-sky imaging in search for signatures of the hydrogen wall and interstellar clouds beyond the heliopause and also measurements of the hydrogen column density between New Horizons and other spacecraft in the inner solar system. In addition, it discusses the past measurements of circumsolar dust by the Venetia Burney Student Dust Counter (VBSDC) and the search for interstellar dust grains. Lastly, it presents an overview of the planned observations by the Long Range Reconnaissance Imager (LORRI), including of distant Kuiper Belt objects and the cosmic optical background.

Exploration is a human endeavor, and New Horizons is no exception. The technical and science achievements documented reflect the dedication of a talented group of people and the family members who supported their dream to accomplish what many thought was impossible. The images that follow highlight the effort that culminated in New Horizons delivering a new understanding of our solar system’s most distant worlds.