The Cold Classical Kuiper Belt, a class of small bodies in undisturbed orbits beyond Neptune, is composed of primitive objects preserving information about Solar System formation. In January 2019, the New Horizons spacecraft flew past one of these objects, the 36-kilometer-long contact binary (486958) Arrokoth (provisional designation 2014 MU69). Images from the flyby show that Arrokoth has no detectable rings, and no satellites (larger than 180 meters in diameter) within a radius of 8000 kilometers. Arrokoth has a lightly cratered, smooth surface with complex geological features, unlike those on previously visited Solar System bodies. The density of impact craters indicates the surface dates from the formation of the Solar System. The two lobes of the contact binary have closely aligned poles and equators, constraining their accretion mechanism.

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.

The solar system currently possesses two remnant debris disks leftover from the planetary formation era in the form of the asteroid belt and the Edgeworth–Kuiper Belt (EKB). Similar to other stellar systems, these debris disks continually generate submillimeter-sized dust grains through processes such as mutual collisions, interstellar dust grain bombardment, and sublimation/sputtering of larger grains. Here, we use recent in situ measurements by the New Horizons Student Dust Counter and an interplanetary dust dynamics model to constrain the overall structure and magnitude of the solar system’s debris disk, including the disk mass, optical depth, and surface brightness in both scattered light and thermal emission. We find that ∼99% of the solar system’s dust disk mass (grains with diameter <1 mm) is contained within EKB and Oort Cloud cometary grains outside of 30 au, with the remaining ∼1% mass in the form of Jupiter-family cometary dust within 5 au. The total disk mass is estimated to be ∼8 × 10−7 M ⊕ with a total fractional luminosity of ∼5 × 10−7, confirming our solar system as a relatively dust-poor system compared to debris disks around similar-aged FGK stars. Finally, we estimate that Kuiper Belt Object collisional events such as that which created the Haumea family could transiently increase the current surface brightness of our debris disk by a factor of only ∼6, far less than median brightnesses seen in other nearby disks. This further supports the idea that the EKB has been largely depleted of its primordial mass relative to other stellar systems by instabilities triggered by planetary migration.

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