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, are primitive objects preserving information about Solar System formation. The New Horizons spacecraft flew past one of these objects, the 36 km long contact binary (486958) Arrokoth (2014 MU69), in January 2019. Images from the flyby show that Arrokoth has no detectable rings, and no satellites (larger than 180 meters diameter) within a radius of 8000 km, and 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

Lucy Mission to the Trojan Asteroids: Science Goals

The Lucy Mission is a NASA Discovery-class mission to send a highly capable and robust spacecraft to investigate seven primitive bodies near both the L4 and L5 Lagrange points with Jupiter: the Jupiter Trojan asteroids. These planetesimals from the outer planetary system have been preserved since early in solar system history. The Lucy mission will fly by and extensively study a diverse selection of Trojan asteroids, including all the recognized taxonomic classes, a collisional family member, and a near equal-mass binary. It will visit objects with diameters ranging from roughly 1 km to 100 km. The payload suite consists of a color camera and infrared imaging spectrometer, a high-resolution panchromatic imager, and a thermal infrared spectrometer. Additionally, two spacecraft subsystems will also contribute to the science investigations: the terminal tracking cameras will supplement imaging during closest approach and the telecommunication subsystem will be used to measure the mass of the Trojans. The science goals are derived from the 2013 Planetary Decadal Survey and include determining the surface composition, assessing the geology, determining the bulk properties, and searching for satellites and rings

Photometry and surface physical properties of comet 19P/Borrelly

The successful Deep Space 1 flyby of Comet P/19 Borrelly offers an unprecedented opportunity to perform disk-resolved photometry and photometric modeling of a comet's nucleus. The flyby occurred at a favorable ground-based apparition, enabling concomitant telescopic observations that provided both the "big picture" in time and space and observations at photometric viewing geometries not attained by the spacecraft. The solar phase angle of the encounter period changed from 87 to 52 degrees over a period of 1.5 hours; this range is ideal for determining the macroscopic roughness of the comet's surface. The microphysical texture of the surface is best determined by ground based observations at aphelion and near opposition. The combination of both disk resolved measurements from DS1 and disk integrated measurements from both DS1 and the ground permits a constrained set of photometric parameters to be derived. Preliminary analysis of the global geometric albedo yields a value at V wavelengths (550 nm) between 0.031 +/- 0.005 (for a Mathilde-type solar phase curve at phase angles less than 10 degrees) and 0.042 +/- 0.005 (for an average C-type phase curve). Albedo variegations of at least a factor of two exist on Borrelly's surface: Its light curve amplitude of nearly a magnitude may thus not be due to shape alone. The image resolution of 60 m allows mapping of albedo variegations in terms of active jet morphology. Funded by NASA

The morphology and surface processes of Comet 19/P Borrelly

The flyby of the nucleus of the Comet 19P/Borrelly by the Deep Space 1 spacecraft produced the best views to date of the surface of these interesting objects. It transformed Borrelly from an astronomical object shrouded in coma of gas and dust into a geological object with complex surface processes and a rich history of erosion and landform evolution. Based on analysis of the highest resolution images, stereo images, photometry, and albedo we have mapped four major morphological units and four terrain features. The morphological units are named dark spots, mottled terrain, mesas, and smooth terrain. The features are named ridges, troughs, pits, and hills. In strong contrast to asteroids, unambiguous impact craters were not observed on Borrelly's surface. Because of the relatively short period of this comet, surface erosion by volatile sublimation is, in geologic terms, a very active process. The formation and the morphologies of units and features appear to be driven by differential rates of sublimation erosion. Erosional rates across the comet are probably controlled by solar energy input and the location of the subsolar point during perihelion. Differences in energy input may produce different varieties of sublimation erosional landforms. The terrains on Borrelly suggest that solar energy input could map directly into erosional processes and landforms

Imaging Borrelly

The nucleus, coma, and dust jets of short-period Comet 19P/Borrelly were imaged from the Deep Space 1 spacecraft during its close flyby in September 2001. A prominent jet dominated the near-nucleus coma and emanated roughly normal to the long axis of nucleus from a broad central cavity. We show it to have remained fixed in position for more than 34 hr, much longer than the 26-hr rotation period. This confirms earlier suggestions that it is co-aligned with the rotation axis. From a combination of fitting the nucleus light curve from approach images and the nucleus' orientation from stereo images at encounter, we conclude that the sense of rotation is right-handed around the main jet vector. The inferred rotation pole is approximately perpendicular to the long axis of the nucleus, consistent with a simple rotational state. Lacking an existing IAU comet-specific convention but applying a convention provisionally adopted for asteroids, we label this the north pole. This places the sub-solar latitude at ~60° N at the time of the perihelion with the north pole in constant sunlight and thus receiving maximum average insolation

Observations of comet 19P/Borrelly from the Miniature Integrated Camera and Spectrometer (MICAS) aboard Deep Space 1

Images from the DS1 MICAS CCD camera reveal in three dimensions, the complex characteristics of Borrelly's nucleus, coma, and jets. The images acquired during the last 2 hours of the approach, as the nucleus became resolved and grew to roughly 150 pixels in length, provide stereo coverage of both the nucleus and inner coma over a wide range of phase angle and exposure time. The principal structure in the coma is a sunward-pointed collimated jet that is also visible in ground-based images. This jet is canted about 30 degrees off the sun line and appears to be roughly aligned with the local vertical at the surface from where it originates. Long-exposure images reveal details of the structure of the inner coma. They show the jet, visible at long range, to be composed of at least three discrete components whose locations evidently correspond to specific surface features. The elongated nucleus exhibits topographically distinct terrains and strong albedo variegations (of at least a factor of 2). The jets emanate from within the brighter smoother rolling plains. A consistent model is that the main jets are co-aligned with the rotation axis of the nucleus and issue from regions on the plains that are currently in constant sunlight. The other major terrain is a rough unit that is darker than the average, includes even darker isolated spots, and appears as a jumbled topography. Other surface features include parallel ridges, crater-like depressions, numerous narrow dark fracture-like features, and areas of mottled albedo. However no small fresh impact craters are evident attesting to a geologically young, actively evolving surface