The Pluto system was recently explored by NASA's New Horizons spacecraft, making closest approach on 14 July 2015. Pluto's surface displays diverse landforms, terrain ages, albedos, colors, and composition gradients. Evidence is found for a water-ice crust, geologically young surface units, surface ice convection, wind streaks, volatile transport, and glacial flow. Pluto's atmosphere is highly extended, with trace hydrocarbons, a global haze layer, and a surface pressure near 10 microbars. Pluto's diverse surface geology and long-term activity raise fundamental questions about how small planets remain active many billions of years after formation. Pluto's large moon Charon displays tectonics and evidence for a heterogeneous crustal composition; its north pole displays puzzling dark terrain. Small satellites Hydra and Nix have higher albedos than expected.

The Kuiper Belt is a distant region of the outer Solar System. On 1 January 2019, the New Horizons spacecraft flew close to (486958) 2014 MU69, a cold classical Kuiper Belt object approximately 30 kilometers in diameter. Such objects have never been substantially heated by the Sun and are therefore well preserved since their formation. We describe initial results from these encounter observations. MU69 is a bilobed contact binary with a flattened shape, discrete geological units, and noticeable albedo heterogeneity. However, there is little surface color or compositional heterogeneity. No evidence for satellites, rings or other dust structures, a gas coma, or solar wind interactions was detected. MU69’s origin appears consistent with pebble cloud collapse followed by a low-velocity merger of its two lobes.

(where the solar irradiance is 1/1000 of the level near the Earth) require a radioisotope thermoelectric generator (RTG) to supply electrical power. One RTG was available for use by New Horizons. To accommodate this constraint, the spacecraft electronics were designed to operate on less than 200 W. The travel time to Pluto put additional demands on system reliability.Only after a flight time of approximately 10 years would the desired data be collected and returned to Earth. This represents the longest flight duration prior to the return of primary science data for any mission by NASA. The spacecraft system architecture provides sufficient redundancy to meet this requirement with a probability of mission success of greater than 0.85. The spacecraft is now on its way to Pluto, with an arrival date of 14 July 2015. Initial inflight tests have verified that the spacecraft will meet the design requirements.

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The New Horizons spacecraft encountered the Kuiper Belt object (KBO) Arrokoth (486958), originally designated as 2014 MU69 and formerly called “Ultima Thule,” on 2019 January 01. At 43.3 au from the Sun and 44.4 au from Earth, this was the most distant spacecraft reconnaissance of a solar system body to date. The Radio Science Experiment (REX) on New Horizons performed radiometry measurements of the KBO's thermal emission at λ = 4.2 cm in two observation slots, one before (dayside) and one after (nightside) the point of closest approach. Owing to the small size of the target, the intensity of the thermal emission was expected to be only marginally detectable. The KBO was not detected on approach because of unexpectedly large variations in the REX system temperature. A brightness temperature T b = 29 ± 5 K was derived for the nightside observation, considerably less than the predicted equilibrium temperature of ∼50 K derived for Arrokoth on the dayside. A model explaining this day−night contrast is used to constrain the global values of emissivity, thermal inertia, and electrical skin depth of the KBO. In particular, models with small values of thermal inertia and small values of electrical skin depth are excluded. Relatively high values of the effective radio emissivity (E eff > 0.8) provide better agreement with the REX observation.