When the New Horizons team chose Arrokoth as the spacecraft's second target after Pluto, they were looking for the most primitive, unaltered object they could reach with remaining fuel. Arrokoth — formally 2014 MU69, discovered only four years before the flyby — sits in the cold classical Kuiper Belt, a dynamically quiet region of the outer solar system where objects have been largely undisturbed since the formation of the solar system 4.5 billion years ago. What they found when the spacecraft swept past at 3:33 AM UTC on January 1, 2019, changing the universe's most distant flyby record in the process, was more revealing than they expected.
Arrokoth is a contact binary — two lobes pressed together, shaped like a somewhat lopsided snowman. The larger lobe, nicknamed Ultima, is roughly 20 kilometers in its longest dimension. The smaller, Thule, is about 14 kilometers. They are joined at a neck perhaps 4 kilometers wide. The surface is uniformly red-tinged by irradiated organic compounds called tholins — the same chemistry that colors the surface of Pluto's moon Charon and countless other outer solar system bodies. No impact craters of any size punctuate the lobes. The neck between them is smooth and undeformed, with no evidence of compressional ridges or the kind of damage you would expect from two objects slamming together.
How Arrokoth formed
The smoothness is the key clue. Before New Horizons visited Arrokoth, the dominant model for planetesimal formation was hierarchical accretion: small particles stick together, growing pebbles stick to each other, pebbles stick to boulders, boulders accumulate into planetesimals. In this picture, each stage involves increasingly energetic collisions, and a contact binary like Arrokoth should bear the scars of a fairly violent merger. The smooth, undeformed neck says the collision speed between the two lobes was very low — perhaps a few centimeters per second, barely faster than a person walks.
The preferred explanation is a process called pebble cloud collapse, sometimes called the streaming instability. In the early solar system's debris disk, aerodynamic interactions between gas and solid particles drive regions of the disk to locally concentrate solid material. When the local solid density exceeds a threshold, the overdense region collapses under its own gravity, forming a rotating cloud of pebbles that quickly sorts itself into a binary — two co-orbiting clumps that gradually spiral together and make contact. The contact velocity in this model is naturally very low, because both lobes formed from the same rotating cloud and share nearly the same orbital angular momentum. They drift together rather than slamming.
The Arrokoth data strongly supports this picture. The two lobes are compositionally identical — same color, same spectral features, same surface texture — meaning they formed from the same material at the same time in the same part of the nebula, not from two separately formed objects that happened to be captured into a mutual orbit. The alignment of their spin axes is also consistent with formation from a single rotating cloud. The streaming instability mechanism, which had been developed theoretically in the decade before the flyby, was effectively confirmed by a snowman at the edge of the solar system.
Where New Horizons is now
After Arrokoth, New Horizons continued outward. It is now more than 60 astronomical units from the Sun — roughly the distance of Arrokoth — in interstellar cruise. The spacecraft carries instruments still operating: the Venetia Burney Student Dust Counter detects dust grain impacts, measuring the density of dust at distances never before sampled. The ALICE ultraviolet spectrograph observes the cosmic ultraviolet background — the integrated glow of all the hydrogen in the universe, which can only be measured far from the Sun's UV emission and from the inner solar system's dust scattering. These measurements, modest in budget terms, provide data not available from any other operating instrument.
The mission team has identified potential Kuiper Belt Objects in New Horizons' reachable path, but the fuel reserves remaining after the Arrokoth flyby are limited, and no object has yet been confirmed within range that could be reached with the remaining propellant. The spacecraft will continue to operate through the late 2030s before its radioisotope thermoelectric generator output falls below the threshold needed to power instruments. By then it will be approaching the heliopause at roughly 120 to 150 AU, though it will not cross it — that distinction belongs to Voyager 1 and 2, now in interstellar space. New Horizons will follow eventually, carrying the chemical memory of what it found at the edge of everything we consider familiar.
