Cold Classical Kuiper Belt Research Articles

Formation of flattened planetesimals by gravitational collapse of rotating pebble clouds

Planetesimals are believed to form by the gravitational collapse of aerodynamically concentrated clumps of pebbles. Many properties of the objects in the cold classical Kuiper belt – such as binarity, rotation, and size distribution – are in agreement with this gravitational collapse model. Further support comes from the pebble-pile structure inferred for comet nuclei. For this study, we simulated the final assembly of a planetesimal from the gravitational collapse of a rotating clump of pebbles. We implemented a numerical method from granular dynamics to follow the collapse that includes the transition from a pebble swarm to solid cells at a high density. We compared the shapes of the simulated planetesimals with the shapes of the lobes of contact binaries and bilobed Solar System objects. We find that the gravitational collapse of slowly rotating pebble clouds naturally explains the formation of flattened ellipsoidal bodies. This result agrees well with the flattened structure of the bilobed planetesimal Arrokoth and the shapes of the components of bilobed comets.

The Hot Main Kuiper Belt Size Distribution from OSSOS

Using the absolute detection calibration and abundant detections of the Outer Solar System Origins Survey project, we provide population measurements for the main Kuiper Belt. For absolute magnitude H r < 8.3, there are 30,000 nonresonant main-belt objects, with twice as many hot-component objects than cold, and with total mass of 0.014 M ⊕, only one-seventh of which is in the cold belt (assuming a cold-object albedo about half that of hot-component objects). We show that trans-Neptunian objects with 5.5 < H r < 8.3 (rough diameters 400–100 km) have indistinguishable absolute magnitude (size) distributions, regardless of being in the cold classical Kuiper Belt (thought to be primordial) or the “hot” population (believed to be implanted after having been formed elsewhere). We discuss how this result was not apparent in previous examinations of the size distribution due to the complications of fitting assumed power-law functional forms to the detections at differing depths. This shared size distribution is surprising in light of the common paradigm that the hot-population planetesimals formed in a higher density environment much closer to the Sun, in an environment that also (probably later) formed larger (dwarf planet and bigger) objects. If this paradigm is correct, our result implies that planetesimal formation was relatively insensitive to the local disk conditions and that the subsequent planet-building process in the hot population did not modify the shape of the planetesimal size distribution in this 50–300 km range.

OSSOS. XXVI. On the Lack of Catastrophic Collisions in the Present Kuiper Belt

We investigate different conditions, including the orbital and size–frequency distribution (SFD) of the early Kuiper Belt, that can trigger catastrophic planetesimal destruction. The goal of this study is to test if there is evidence for collisional grinding in the Kuiper Belt that has occurred since its formation. This analysis has important implications for whether the present-day SFD of the cold classical trans-Neptunian objects (TNOs) is a result of collisional equilibrium or if it reflects the primordial stage of planetesimal accretion. As an input to our modeling, we use the most up-to-date debiased OSSOS++ ensemble sample of the TNO population and orbital model based on the present-day architecture of the Kuiper Belt. We calculate the specific impact energies between impactor–target pairs from different TNO groups and compare our computed energies to catastrophic disruption results from smoothed particle hydrodynamics simulations. We explore different scenarios by considering different total primordial Kuiper Belt masses and power slopes of the SFD and allowing collisions to take place over different timescales. The collisional evolution of the Kuiper Belt is a strong function of the unknown initial mass in the trans-Neptunian region, where collisional grinding of planetesimals requires a total primordial Kuiper Belt mass of M > 5 M ⊕, collision speeds as high as 3 km s−1, and collisions over at least 0.5 Gyr. We conclude that presently, most of the collisions in the trans-Neptunian region are in the cratering rather than disruption regime. Given the low collision rates among the cold classical Kuiper Belt objects, their SFD most likely represents the primordial planetesimal accretion.

Dynamics of a deforming planetary body

Self-gravitating planetary bodies possess non-spherical shapes and various density distributions. The dynamics of such bodies are treated using a rigid-body assumption, where bodies do not deform throughout their motion, or simplified force models and shapes like a sphere. These assumptions make problems simple, though it essentially ignores the response of deformation to translation and rotation, and vice versa, limiting its applicability. Describing the coupling of three dynamics modes (deformation, rotation, and translation) is essential to detailing the dynamical evolution of planetary bodies. However, a continuum mechanics approach such as finite element modeling (FEM) needs additional cautions because ill-defined body forces may cause unrealistic stress and displacement conditions. Here, we propose a theoretical framework for characterizing the dynamics of a deforming body, applicable to the numerical implementation of continuum mechanics models. We introduce the kinetics and mechanics to derive the governing equations representing all the dynamics modes. The advantage of this technique is that the equation of deformation no longer deals with translation and rotation included in body forces, which removes difficulties in computing stress and displacement fields around boundary conditions. We demonstrate this technique by considering a system that consists of two masses connected by a spring and a damper. An example of this application discusses the dynamical response of the Cold Classical Kuiper Belt Object (CCKBO) 486958 Arrokoth to the Sky Crater-forming impact event on its small lobe. When the dissipation quality factor, known as the Q factor, is about 100, a typical value for a rubble pile object, the deformation dissipation timescale is just a few days. Considering the yield stress and strain to be ∼10 kPa and ∼0.01, respectively, we derive a displacement of up to tens of meters and normal stress of up to ∼5 kPa if the impact direction consists of the components along the two lobes. Without it, the rotational state change gives no significant displacement and normal stress; however, the major part of the imparted momentum may be converted to shear.

Collisions of Small Kuiper Belt Objects With (486958) Arrokoth: Implications for Its Spin Evolution and Bulk Density

AbstractThe orientation and morphology of the bilobate, cold classical Kuiper belt object (486958) Arrokoth (formerly 2014 MU69) is consistent with a slow, tidal merger of a close binary. However, the discrepancy between Arrokoth's present‐day rotation (15.9 hr) and synchronous rotation for nominal cometary densities near ∼500 kg/m3 implies reduction (up to 30%) in post‐merger spin angular momentum. We investigate how collisions with dynamically cold and hot classical Kuiper belt objects might have affected Arrokoth's post‐merger spin. Using a dynamically equivalent triaxial ellipsoid, 5000 Monte Carlo simulations of 100 impacts each, consistent with Arrokoth's cratering record, were carried out. Starting from the assumption of critical, synchronous rotation for a given density, these simulations rarely reproduce Arrokoth's present spin period, unless its true density is near 250 kg/m3. We explore in greater depth the effects of formation of Arrokoth's largest crater (now officially named Sky, previously informally named Maryland). We adopt point‐source scaling and randomly select impact parameters that lead to the crater, using Arrokoth's full bilobate shape. Results from Sky's formation alone are similar to those considering a full range of impactor sizes, unless we adopt low cratering efficiency due to high porosity, which implies substantially larger Sky‐forming impactors. Overall, results imply that the probability of substantial angular momentum change due to impacts alone over Solar System history is unlikely, and spindown from a synchronous, tidal rotation rate to a 15.9‐hr period unlikely unless Arrokoth itself was and is a very low density object (∼250 kg/m3), though we cannot statistically rule out densities up to 400 kg/m3.

OSSOS Finds an Exponential Cutoff in the Size Distribution of the Cold Classical Kuiper Belt

Abstract The cold main classical Kuiper Belt consists of the nonresonant small solar system bodies with low orbital inclinations and orbital semimajor axes between 42.4 and 47.7 au. These objects likely formed in situ, and the population has experienced minimal collisional modification since formation. Using the Outer Solar System Origins Survey ensemble sample and characterization, combined with constraints from deeper surveys and supported by evidence from the Minor Planet Center catalog and the Deep Ecliptic Survey, we determine the absolute magnitude H r distribution of the cold classical belt from H r ≃ 5 to 12 (roughly diameters of 400–20 km). We conclude that the cold population’s H r distribution exhibits an exponential cutoff at large sizes. Exponential cutoffs at large sizes are not a natural outcome of pairwise particle accretion, but exponentially tapered power-law size distributions are a feature of numerical simulations of planetesimal formation via a streaming instability. Our observation of an exponential cutoff agrees with previous observational inferences that no large objects (D ≳ 400 km) exist in the cold population. We note that the asymptotic slope of the H r distribution is consistent with α ∼ 0.4. This asymptotic slope is also found in streaming instability modeling of planetesimal formation and is thus not necessarily associated with achieving collisional equilibrium. Studies of the trans-Neptunian region are providing the parameters that will enable future streaming-instability studies to determine the initial conditions of planetesimal formation in the ≈45 au region of the Sun’s protoplanetary disk.

Col-OSSOS: The Distinct Color Distribution of Single and Binary Cold Classical KBOs

Abstract The cold classical Kuiper Belt Objects (KBOs) possess a high, ≳30% binary fraction. Widely separated and dynamically fragile, these binary systems have been useful in tracing the origins of KBOs. A new class of binaries was recently identified by their colors. The so-called blue binaries are unanimously members of the less red compositional class, and exhibit a 100% binary fraction. They appear to be push-out survivors, emplaced in the classical region during Neptune’s phases of outward migration. The presence of these binary systems implies that the majority of objects that formed near the cold classical region formed as binaries. Here we present new optical color measurements of cold classical KBOs from the Colors of the Outer Solar System Origins Survey, including colors of a blue binary discovered by the Solar System Origins Legacy Survey—2015 RJ277. The increased size of the colors sample has resulted in order-of-magnitude decrease in the probability that the binaries and singles sample share the same color distribution. From the Anderson–Darling statistic, this probability is only a 0.3%, while it is only 0.002% when utilizing the difference of means statistic. We find a hint that the blue binaries have inflated free inclinations compared to their red counterparts, consistent with the push-out origin for these bodies.

The origin of (47171) Lempo-like Kuiper belt triple systems during binary–binary interactions

ABSTRACT The trans-Neptunian triple Lempo–Paha–Hiisi is composed of a tight inner binary with components of similar size and an outer companion about half their size orbiting 10 times further away. Large trans-Neptunian objects like Pluto also have multiple small moons, but Lempo’s structure is unique in the Solar system, and the place and timing of its origin is still a subject of debate. We propose a new formation mechanism able to form a large number of systems like Lempo–Paha–Hiisi, which involves binary–binary close encounters in the primordial planetesimal disc at 30–40 au. Some of these systems were then implanted in different populations of the trans-Neptunian region during Neptune’s outward migration. Our results strongly support that the 4:7 resonant multiple object Manwë–Thorondor was once a triple system similar to Lempo–Paha–Hiisi, but the orbit of the inner binary evolved by tides, becoming a contact binary. As with Lempo–Paha–Hiisi, it should have originated in the planetesimal disc below 30–40 au. Triple systems like Lempo–Paha–Hiisi or Manwë–Thorondor could not have formed in situ and the existence of this kind of system is not expected in the cold classical Kuiper belt.

Sublimation as an effective mechanism for flattened lobes of (486958) Arrokoth

The New Horizons spacecraft's flyby of Kuiper Belt Object (KBO) (486958) Arrokoth revealed a bilobed shape with highly flattened lobes both aligned to its equatorial plane, and a rotational axis almost aligned to the orbital plane (obliquity ~99 deg). Arrokoth belongs to the Cold Classical Kuiper Belt Object population that occupies dynamically undisturbed orbits around the Sun, and as such, is a primitive object that formed in situ. Therefore, whether its shape is primordial or evolutionary carries important implications for understanding the evolution of both KBOs and potentially their dynamically derived objects, Centaurs and Jupiter Family Comets (JFC). Applying our mass loss driven shape evolution model (MONET), here we suggest that the current shape of Arrokoth could be of evolutionary origin due to volatile outgassing in a timescale of about 1 to 100 Myr, while its spin state would not significantly affected. We further argue that such a process may be ubiquitous in the evolution of the shape of KBOs shortly after their formation. This shape changing process could also be reactivated when KBOs dynamically evolve to become Centaurs and then JFCs and receive dramatically increased solar heating.

The formation of the cold classical Kuiper Belt by a short range transport mechanism

The Classical Kuiper Belt is populated by a group of objects with low inclination orbits, reddish colors and usually belonging to a binary system. This so called Cold Classical Kuiper Belt is considered to have been formed in situ from primordial ice pebbles that coagulated into planetesimals hundreds of kilometers in diameter. According to this scenario, the accretion of pebbles into large planetesimals would have occurred through the streaming instability mechanism that would be effective in the primordial Solar System disk of gas and solids. Nevertheless other objects with the same color characteristics as those found in the Cold Classical Kuiper Belt can be encountered also past the 2:1 mean motion resonance with Neptune as scattered or detached objects. Here I propose a mechanism that can account for both the cold Classical Kuiper Belt objects and other reddish objects outside the Classical Kuiper Belt. According to the proposed scenario, reddish objects were primordially in the outer portion of the planetesimal disk which was however truncated somewhere below ~42 au. In this manner the cold Classical Kuiper Belt and its scattered / detached counterpart were respectively transported outwards by a short range or slightly scattered to their present locations. Resonant objects were also formed by the same process. This mechanism is aimed at explaining the distribution of all objects that share the same color characteristics as coming from a common origin in the outer borders of the primordial planetesimal disk. According to the scenario here proposed the Cold Classical Kuiper Belt would have been formed ~4 au inside its present location with a total mass 20 − 100 times as large as its present value.

CO Depletion in Protoplanetary Disks: A Unified Picture Combining Physical Sequestration and Chemical Processing

Abstract The gas-phase CO abundance (relative to hydrogen) in protoplanetary disks decreases by up to two orders of magnitude from its interstellar medium value of ∼10−4, even after accounting for freeze-out and photodissociation. Previous studies have shown that while local chemical processing of CO and the sequestration of CO ice on solids in the midplane can both contribute, neither of these processes appears capable of consistently reaching the observed depletion factors on the relevant timescale of 1–3 Myr. In this study, we model these processes simultaneously by including a compact chemical network (centered on carbon and oxygen) to 2D (r + z) simulations of the outer (r &gt; 20 au) disk regions that include turbulent diffusion, pebble formation, and pebble dynamics. In general, we find that the CO/H2 abundance is a complex function of time and location. Focusing on CO in the warm molecular layer, we find that only the most complete model (with chemistry and pebble evolution included) can reach depletion factors consistent with observations. In the absence of pressure traps, highly efficient planetesimal formation, or high cosmic-ray ionization rates, this model also predicts a resurgence of CO vapor interior to the CO ice-line. We show the impact of physical and chemical processes on the elemental (C/O) and (C/H) ratios (in the gas and ice phases), discuss the use of CO as a disk mass tracer, and, finally, connect our predicted pebble ice compositions to those of pristine planetesimals as found in the Cold Classical Kuiper Belt and debris disks.

Evolution of MU69 from a binary planetesimal into contact by Kozai-Lidov oscillations and nebular drag

The New Horizons flyby of the cold classical Kuiper Belt object MU69 showed it to be a contact binary. The existence of other contact binaries in the 1–10 km range raises the question of how common these bodies are and how they evolved into contact. Here we consider that the pre-contact lobes of MU69 formed as a binary embedded in the Solar nebula, and calculate its subsequent orbital evolution in the presence of gas drag. We find that the sub-Keplerian wind of the disk brings the drag timescales for 10 km bodies to under 1 Myr for quadratic-velocity drag, which is valid in the asteroid belt. In the Kuiper belt, however, the drag is linear with velocity and the effect of the wind cancels out as the angular momentum gained in half an orbit is exactly lost in the other half; the drag timescales for 10 km bodies remain ≳10 Myr. In this situation we find that a combination of nebular drag and Kozai-Lidov oscillations is a promising channel for collapse. We analytically solve the hierarchical three-body problem with nebular drag and implement it into a Kozai cycles plus tidal friction model. The permanent quadrupoles of the pre-merger lobes make the Kozai oscillations stochastic, and we find that when gas drag is included the shrinking of the semimajor axis more easily allows the stochastic fluctuations to bring the system into contact. Evolution to contact happens very rapidly (within 104 yr) in the pure, double-average quadrupole, Kozai region between ≈85 − 95∘, and within 3 Myr in the drag-assisted region beyond it. The synergy between J2 and gas drag widens the window of contact to 80∘ − 100∘ initial inclination, over a larger range of semimajor axes than Kozai and J2 alone. As such, the model predicts a low initial occurrence of binaries in the asteroid belt, and an initial contact binary fraction of about 10% for the cold classicals in the Kuiper belt. The speed at contact is the orbital velocity; if contact happens at pericenter at high eccentricity, it deviates from the escape velocity only because of the oblateness, independently of the semimajor axis. For MU69, the oblateness leads to a 30% decrease in contact velocity with respect to the escape velocity, the latter scaling with the square root of the density. For mean densities in the range 0.3–0.5 g cm−3, the contact velocity should be 3.3 − 4.2 m s−1, in line with the observational evidence from the lack of deformation features and estimate of the tensile strength.

Photometry of Kuiper belt object (486958) Arrokoth from New Horizons LORRI

On January 1st 2019, the New Horizons spacecraft flew by the classical Kuiper belt object (486958) Arrokoth (provisionally designated 2014 MU69), possibly the most primitive object ever explored by a spacecraft. The I/F of Arrokoth is analyzed and fit with a photometric function that is a linear combination of the Lommel-Seeliger (lunar) and Lambert photometric functions. Arrokoth has a geometric albedo of pv=0.21−0.04+0.05 at a wavelength of 550 nm and ≈0.24 at 610 nm. Arrokoth's geometric albedo is greater than the median but consistent with a distribution of cold classical Kuiper belt objects whose geometric albedos were determined by fitting a thermal model to radiometric observations. Thus, Arrokoth's geometric albedo adds to the orbital and spectral evidence that it is a cold classical Kuiper belt object. Maps of the normal reflectance and hemispherical albedo of Arrokoth are presented. The normal reflectance of Arrokoth's surface varies with location, ranging from ≈0.10–0.40 at 610 nm with an approximately Gaussian distribution. Both Arrokoth's extrema dark and extrema bright surfaces are correlated to topographic depressions. Arrokoth has a bilobate shape and the two lobes have similar normal reflectance distributions: both are approximately Gaussian, peak at ≈0.25 at 610 nm, and range from ≈0.10–0.40, which is consistent with co-formation and co-evolution of the two lobes. The hemispherical albedo of Arrokoth varies substantially with both incidence angle and location, the average hemispherical albedo at 610 nm is 0.063 ± 0.015. The Bond albedo of Arrokoth at 610 nm is 0.062 ± 0.015.

The geology and geophysics of Kuiper Belt object (486958) Arrokoth.

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.

Dynamics of Planetary Systems within Star Clusters: Aspects of the Solar System’s Early Evolution

Abstract Most planetary systems—including our own—are born within stellar clusters, where interactions with neighboring stars can help shape the system architecture. This paper develops an orbit-averaged formalism to characterize the cluster’s mean-field effects, as well as the physics of long-period stellar encounters. Our secular approach allows for an analytic description of the dynamical consequences of the cluster environment on its constituent planetary systems. We analyze special cases of the resulting Hamiltonian, corresponding to eccentricity evolution driven by planar encounters, as well as hyperbolic perturbations upon dissipative disks. We subsequently apply our results to the early evolution of our solar system, where the cluster’s collective potential perturbs the solar system’s plane, and stellar encounters act to increase the velocity dispersion of the Kuiper Belt. Our results are twofold. First, we find that cluster effects can alter the mean plane of the solar system by ≲1° and are thus insufficient to explain the ψ ≈ 6° obliquity of the Sun. Second, we delineate the extent to which stellar flybys excite the orbital dispersion of the cold classical Kuiper Belt and show that while stellar flybys may grow the cold belt’s inclination by the observed amount, the resulting distribution is incompatible with the data. Correspondingly, our calculations place an upper limit on the product of the stellar number density and residence time of the Sun in its birth cluster, η τ ≲ 2 × 104 Myr pc−3.

Probabilistic evolution of pairs of trans-Neptunian objects in close orbits

AbstractWe have studied the probabilistic evolution of four candidates for young pairs of trans-Neptunian objects: 2003 QL91 – 2015 VA173, 1999 HV11 – 2015 VF172, 2002 CY154 – 2005 EW318 and 2013 SD101 – 2015 VY170 over 10 Myr in the past. All pairs belong to cold Classical Kuiper Belt objects. We concluded that the age of the considered pairs exceeds 10 Myr.

Light Curves and Rotational Properties of the Pristine Cold Classical Kuiper Belt Objects

Abstract We present a survey of the rotational and physical properties of the dynamically low inclination Cold Classical (CC) trans-Neptunian objects (TNOs). The CCs are primordial planetesimals and contain information about our solar system and planet formation over the first 100 million years after the Sun’s formation. We obtained partial/complete light curves for 42 CCs. We use statistical tests to derive general properties about the shape and rotational frequency distributions of the CCs and infer that they have slower rotations and are more elongated/deformed than the other TNOs. On the basis of the full light curves, the mean rotational period of the CCs is 9.48 ± 1.53 hr compared to 8.45 ± 0.58 hr for the rest of the TNOs. About 65% of the TNOs have a light-curve amplitude below 0.2 mag compared to the 36% of CCs with small amplitude. We present the full light curve of one likely contact binary, 2004 VC131, with a potential density of 1 g cm−3 for a mass ratio of 0.4. We have hints that 2004 MU8 and 2004 VU75 are perhaps potential contact binaries, on the basis of their sparse light curves, but more data are needed to confirm this finding. Assuming equal-sized binaries, we find that ∼10%–25% of the CCs could be contact binaries, suggesting a deficit of contact binaries in this population compared to previous estimates and to the (∼40%–50%) possible contact binaries in the Plutino population. These estimates are lower limits and may increase if nonequal-sized contact binaries are considered. Finally, we put in context the results of the New Horizons flyby of 2014 MU69.

Binary survival in the outer solar system

As indicated by their special characteristics, the cold classical Kuiper belt objects (KBOs) formed and survived at ≃42–47 au. Notably, they show a large fraction of equal-size binaries whose formation is probably related to the accretion of KBOs themselves. These binaries are uncommon in other –hot, resonant, scattered– populations, which are thought to have been implanted from the massive disk below 30 au to >30 au during Neptune's migration. Here we highlight the possibility that equal-size binaries formed in the disk but were subsequently removed by impacts and/or dynamical effects (e.g., scattering encounters with Neptune). We determine the dependence of these processes on the size and separation of binary components. Our results indicate that tighter binaries, if they formed in the massive disk, have relatively good chances of survival (unless the disk was long-lived). In contrast, the widest binaries in the hot population, such as 2002 VF130, have a very low survival probability (<1%) even if the massive disk was short-lived. They may represent a trace of lucky survivors of a much larger population of the original disk binaries, or they formed at ∼30–40 au and dodged the impact- and encounter-related perturbations that we studied here. We find that all known satellites of the largest KBOs would survive during the dynamical implantation of these bodies in the Kuiper belt. The low orbital eccentricities of Pluto's small moons may have been excited by impacts and/or encounters of the Pluto system to Neptune.

The HST lightcurve of (486958) 2014 MU69

We report Hubble Space Telescope (HST) lightcurve observations of the New Horizons spacecraft encounter Kuiper Belt object (KBO) (486958) 2014 MU69 acquired near opposition in July 2017. In order to plan the optimum flyby sequence the New Horizons mission planners needed to learn as much as possible about the target in advance of the encounter. Specifically, from lightcurve data, encounter timing could be adjusted to accommodate a highly elongated, binary, or rapidly rotating target. HST astrometric (Porter et al., 2018) and stellar occultation (Buie et al., 2018) observations constrained MU69's orbit and diameter (21–41 km for an albedo of 0.15–0.04), respectively. Photometry from the astrometric dataset suggested a variability of ≥0.3 mags, but they did not determine the period or provide shape information. To that end we strategically spaced 24 HST orbits over 9 days to investigate rotation periods from approximately 2–100 h and to better constrain the lightcurve amplitude. Until New Horizons detected MU69 in its optical navigation images beginning in August 2018, this HST lightcurve campaign provided the most accurate photometry to date. The mean variation in our data is 0.15 magnitudes which suggests that MU69 is either nearly spherical (a:b axis ratio of 1:1.15), or its pole vector is pointed near the line of sight to Earth; this interpretation does not preclude a near-contact binary or bi-lobed object. However, image stacks do conclude that MU69 does not have a binary companion ≥2000 km with a sensitivity to 29th magnitude (an object a few km in size for an albedo of 0.04–0.15). Our data are not of sufficient signal to noise to uniquely determine the period or amplitude, however, they did provide the necessary information for spacecraft planning. We report with confidence that MU69 is not both rapidly rotating and highly elongated (which we define as a lightcurve amplitude ≥ 0.5 magnitude). Since this paper is being published post fly-by, we note that our results are consistent with the fly-by imagery and orientation of MU69 (Stern et al., 2019). The combined dataset also suggests that within the KBO lightcurve literature there are likely other objects which share a geometric configuration like MU69 resulting in an underestimate of the contact binary fraction for the Cold Classical Kuiper Belt.

Great Expectations: Plans and Predictions for New Horizons Encounter With Kuiper Belt Object 2014 MU69 (“Ultima Thule”)

AbstractThe New Horizons encounter with the cold classical Kuiper Belt object 2014 MU69 (informally named “Ultima Thule,” hereafter Ultima) on 1 January 2019 will be the first time a spacecraft has ever closely observed one of the free‐orbiting small denizens of the Kuiper Belt. Related to but not thought to have formed in the same region of the solar system as the comets that been explored so far, it will also be the most distant, and most primitive body yet visited by spacecraft. In this letter we begin with a brief overview of cold classical Kuiper Belt objects, of which Ultima is a prime example. We give a short preview of our encounter plans. We note what is currently known about Ultima from Earth‐based observations. We then review our expectations and capabilities to evaluate Ultima's composition, surface geology, structure, near space environment, small moons, rings, and the search for activity.