First results of the exploration of new horizons of 2014 MU69, a small object of the Kuiper belt

General properties

MU₆₉ (Fig. 1) has a bilobed shape with two discrete lobes, of unequal size. This form shows that it is a binary contact, that is, a pair of once-separated objects that are now physically in contact with each other. These two lobes are in contact at an annular interface with above-average surface reflectance, which we call MU.₆₉The neck. Larger and smaller lobes of MU₆₉ were informally designated "Ultima" and "Thule", respectively; the official names will be attributed at a later date.

The bilobed nature of MU₆₉ recalls the known bilobed comets that have been imaged by a spacecraft (17–22). However, MU₆₉The residence of the Kuiper classic and cold belt clearly indicates that its bilobed shape must be paramount.₆₉ the only contact binary undeniably primordial until here explored by spaceships.

MU₆₉Both lobes are discrete, have retained their basic forms and do not have (at available resolution) large compression fractures, deformations or other geological features indicative of energetic or violent fusion. All available evidence indicates that MU₆₉ is rather the product of a collision or a smooth fusion of two independently formed bodies, possibly touching (or slower than) their mutual gravitational attraction velocity, which we estimate to be several meters per second (on the basis of plausible densities, see below). ).

The bilobal shape observed is incompatible with recent formation resulting from the collision of two distinct heliocentric CCKBOs, since the relative impact velocities characteristic of CCKBOs are currently around 300 m s.^-1 (23). Such a collision would have strongly deformed or destroyed the two components of the contact binary. We conclude that the lobes probably formed and merged in a soft dynamic environment, such as in a collapse of particle cloud located at the beginning of the solar system's history.[forexample([Eg([parexemple([eg(24, 25)], further indicating that MU₆₉ he himself is primordial. The similarity of the albedos and the colors of the two lobes (discussed below) is further evidence of this formation hypothesis.

Images taken during the approach phase show that the rotation period of the MU₆₉ is 15.92 ± 0.02 hours; this period is in line with the other rotation periods of the CCKBO (9, 26, 27). MU₆₉It was found that the rotational pole indicated an approximately straight rise = 311 °, a declination = -25 °, corresponding to an obliquity of 98 ° with respect to MU₆₉The heliocentric orbital plane; that's, MU₆₉ is currently turning almost facing the sun. This pole position and the fact that the approach vector of the spacecraft was only about 36 ° from the direction of the KBO rotation axis resulted in a very small amplitude light curve, which was detected shortly before overflight despite many weeks of flying before. New Horizons observations. Because the effects of asymmetric reradiation (ie, YORP or Yarkovsky-O'Keefe-Radzievskii-Paddack) are ineffective on the bodies the size of MU₆₉ away from the sun (28, 29), and because (as described below) the visible evidence of crater formation by impact on₆₉ is modest, the period of spin and the obliquity of MU₆₉ are unlikely to have changed significantly since the binary merger.

We simultaneously adapted the shape of MU₆₉ and its leading position using a downstream modeling process. For each LORRI and MVIC fast return image in which MU₆₉ has been resolved, this process has rendered the shape with the appropriate illumination and orientation using the photometric parameters described below, converted this rendered image with the appropriate spot extension function (for LORRI or MVIC, as appropriate) to simulate the angular resolution and the actual image streaks, then numerically compare the result to all resolved images of MU₆₉ which had been returned. The shape model has been defined parametrically, separately for each lobe, using the "octantoid" formalism (30) (see methods). Our best fit model (Fig. 2) has overall dimensions of approximately 35 × 20 × 10 km, with estimated uncertainties less than 1 × 1 × 3 km. The thickness (ie the third dimension) is the least constrained because the spacecraft approach was at a very negative (i.e., south) latitude on the object, if little of the positive half (that is, north) of MU₆₉ was directly observed.

From this shape model, the Ultima lobe proved to be lenticular, measuring approximately 22 × 20 × 7 km (uncertainty <0.6 × 1 × 2 km), whereas the Thule lobe is more equidimensional, about 14 × 14 × 10 km (uncertainty <0.4 × 0.7 × 3 km). The model volumes of Ultima and Thule are 1400 km³ (± <600 km³) and 1050 km³ (± <400 km³), respectively. The centers of the lobes are 16 km apart. The main axes of the lobes are approximately parallel to each other (probably a misalignment <7 °), which strongly suggests that the lobes are locked before contact (see below).

The seemingly lenticular shape of the Ultima lobe is unlike any other solar system body in known heliocentric orbit, but recalls some of Saturn's small circular satellites, such as Atlas and Pan, which have equatorial ridges of fine-grained materials (31). The origin of this flattened form of Ultima is indeterminate; Possible explanations include accretion from a thin layer of particles (that is, flattened) during the collapse of a cloud of pebbles (24), the frontal collision of two bodies of very similar size in a narrow speed range (32), or deformation resulting from rapid rotation or tidal forces prior to its merger with Thule. These possibilities are discussed below.

Because no MU satellite₆₉ have been detected (see below), MU₆₉The density is limited. Cometary nuclei are the bodies of the solar system likely to look more like the UM₆₉; there is usually a low density (<10³ kg m^-3) and high porosity (> 50%) (21). The density of the nucleus of the comet most precisely determined is that of 67P / Churyumov-Gerasimenko, at 533 ± 6 kg m^-3 (33), but we do not know how much the density of the comet has evolved in relation to its primordial value. Adopting the shape model of Figure 2, a lower limit on MU₆₉Density of ~ 280 kg m^-3 can be derived assuming that MU₆₉The two lobes are only marginally bound by their own gravity and there is no tensile strength in the neck. Alternatively, assuming a characteristic cometary density of 500 kg m^-3 (21) gives a dissolution turnover rate for MU₆₉ ~ 12 hours. At this density, if the two lobes had fused from a mutual circular orbit, it would have required a substantial loss of kinetic momentum to reach their current 15.92 hour period. As mutual gravity would exceed centrifugal acceleration, densities greater than about 280 kg m^-3 also implies that the neck region is currently in compression.

We calculated the gravitational and rotational potential on the MU surface₆₉ assuming the shape model of Figure 2, a uniform density of 500 kg m^-3, and its rotation period of 15.92 hours. The surface acceleration is between 0.5 and 1 mm s^-2 (and is nowhere negative). Local acceleration slopes (ie gradients of gravitational and rotational potential on the surface) are small, except in the neck, where they can exceed 35 °. The Ultima and Thule equators are high gravitational and rotational potentials.

Surface reflectivity, color and composition

Figure 3A shows a visible wavelength IF (reflected contour divided by the incident flux) contour map from LORRI's close observation 04 (CA04) (solar phase angle of 13 °) and its IF histogram. At this solar phase angle, the modal IF of MU₆₉ is 0.078. the IF MU distributions₆₉Both lobes have the same mode with a measurement error of ± 0.05 IF. IF variations ranging from about 0.02 to about 0.12 are observed on both lobes (including their termination regions). However, away from the terminator (that is, the surface boundary between day and night where light effects are biased), the minimum IF is ~ 0.05. This occurs on a part of the Thule Great Depression wall, which we informally call Maryland (the surface features of the Ultima Thule carry the informal name of American states that have made a major contribution to New Horizons); see Fig. 1. The maximum IF ~ 0.12 occurs at both bright spots in Maryland and on the neck region. the IF The distribution of the Ultima lobe has a sharper peak than that of Thule (Fig. 3B).

The brightest material on Ultima and Thule is mainly divided into three types of surface manifestations: (i) almost circular spots that become more numerous with a decreasing size (ranging from a few kilometers to the resolution limit of several tens of meters) ); (ii) curvilinear and quasi-linear features narrow in relation to their length; and (iii) large, more visible areas on the Thule lobe. The more binary surface units than the average on the binary also fall into types (i) and (iii), but not (ii).

It is unclear how these three types of relatively clear terrain came into being and how they can be different. However, the initial stereographic analysis of New Horizons images using established techniques (34, 35) shows that many of the bright areas seen on the MU₆₉ are located either in topographic depressions (eg, neck, Maryland light points and soils of troughs and valleys), either at the base or at the points of inflection of slopes. Therefore, one possible explanation is that bright, fine-grained materials have been transported downstream of these sites, in which case the higher luminosity may be due to a substantially smaller particle size (36). However, other interpretations, including compositional or thermal effects, spatial erosion and cold trapping are also possible.

The neck function in the Ultima-Thule fusion zone may not have the same origin as other light regions. Possibilities include (i) surface processes such as the accumulation of fine particles, as discussed above; (ii) processes related to lobe fusion, such as the extrusion of pre-existing glossy surface material on a lobe during impact; (iii) thermal ice extrusion after impact due to changes in thermal properties or conditions in the melting zone after contact; and (iv) evolutionary processes such as preferential effects of space alteration or thermal effects created by the geometry surrounding the neck.

Analysis of the approach and departure images allowed us to characterize the solar phase curve of MU₆₉ at phase angles up to 153 °, which is considerably higher than possible for terrestrial observations of this object, which are limited to ~ 2 ° (Fig. 4). The derived phase coefficient, or slope, is β = 0.038 ± 0.014 magnitudes per degree, between 1.3 ° and 32.5 °. This slope is consistent with that measured for other small bodies of the weak albedo solar system, including comet nuclei.[forexample([Eg([parexemple([eg(37)]. Application of a Hapke photometric model (36) to the complete phase curve gives the nominal photometric properties of MU₆₉Surface: single diffusion albedo single_o = 0.24, average topographic slope angle θ = 33 ° and single-particle phase function parameters using a McGuire-Hapke formalism (37) of b = 0.32 and c = 0.75. These results give MU₆₉'S visible (0.55 μm Vband) geometric albedo (i.e. the albedo at 0 ° solar phase) of p_V = 0.165 ± 0.01. This is a typical value of CCKBOs, whose geometric albedos range from 0.09 to 0.23, with an average of 0.15 (38). MU₆₉The phase integral is q = 0.37 ± 0.16, giving a spherical albedo (Bond) of 0.061 ± 0.026.

MVIC color images reveal a reddish reflectance slope of overall mean visible wavelength of 31.1 ± 0.5% per 100 nm, calculated using MVIC's blue, red and near-infrared filters. (Fig. 5, A to C), where the uncertainty quoted is as follows: statistics only. This color is consistent with that of other CCKBOs (39–41). Only subtle differences in color (and spectral, see below) (and spectral differences, see below) are detected between the two lobes of MU.₆₉despite their distinct forms and appearances. Both lobes are clearly solved one by one at the resolution of the color data. KBO's remote observations with satellites show nearly equal colors of the bodies in orbit, interpreted as resulting from the co-accretion of a locally homogeneous part of the nebula (42).

La couleur régionale la plus claire et les signatures spectrales à travers la MU₆₉La surface de la peau apparaît (i) au cou entre les deux lobes et (ii) aux points lumineux des lobes de Thule dans le Maryland, présentant des pentes spectrales respectives de 28,2 ± 0,2% par 100 nm et de 30,8 ± 0,2% par 100 nm . Au moins deux endroits sur le lobe Ultima montrent également moins de rouge que sa couleur moyenne. L’analyse en composantes principales montre que 97% de la variance dans les données de couleur MVIC sont attribuables à l’ombrage et à l’albédo, alors que les contrastes de bruit d’image et de couleur vraie ne représentent que 3%. Les différences de couleur subtiles observées pourraient indiquer des différences de composition, bien que les différences de taille des particules, de porosité, etc., puissent également produire des différences de pentes spectrales.

Les observations spectrales LEISA (Fig. 5D; voir méthodes) montrent que₆₉ est plus brillant dans le proche infrarouge que dans le spectre visible, ce qui montre que la pente rouge observée avec MVIC s&#39;étend dans l&#39;infrarouge. De 1,2 à 2,5 µm, les valeurs observées IF après étalonnage radiométrique, de 0,15 à 0,2. Le colorant responsable de la rougeur de MU₆₉ et d&#39;autres CCKBO pourraient être des macromolécules organiques complexes de type tholine, produites à partir d&#39;espèces plus simples par rupture radiolytique et photolytique de liaisons conduisant à une recombinaison en molécules progressivement plus lourdes[parexemple([eg([parexemple([eg(43, 44)]. L&#39;altération spatiale des silicates peut également produire une coloration rouge, mais aucune trace directe de silicates n&#39;est visible sur MU.₆₉. L’analyse en composantes principales montre que plus de 90% de la variance dans les données LEISA est également due à l’ombrage et à l’albédo, la variance relativement faible étant imputable à la variabilité spectrale régionale.

Comme le montre la figure 5D, il existe des similitudes clés en termes de pente de couleur entre le spectre de MU₆₉ et ceux de la KBO (55638) 2002 VE₉₅ (45) et les KBO 5145 en fuite, Pholus[parexemple([eg([parexemple([eg(46)]. De plus, ces objets présentent tous une bande d’absorption proche de 2,3 µm, attribuée provisoirement au méthanol (CH₃OH) ou peut-être des molécules organiques plus complexes dont la masse est intermédiaire entre les glaces moléculaires simples et les tholins (47). Des caractéristiques spectrales similaires sont également apparentes sur la grande région équatoriale rouge foncé de Pluton appelée officieusement Cthulhu.[parexemple([eg([parexemple([eg(48)], suggérant des similitudes dans la matière première et les procédés chimiques pouvant fonctionner ici et sur l&#39;UM₆₉.

Large absorption spectrale sur MU₆₉ près de 1,5 et 2,0 μm indiquent la présence de H₂O glace. Cependant, la faible profondeur de ces caractéristiques suggère que la glace d&#39;eau pourrait avoir une abondance relativement faible dans l&#39;UM.₆₉La surface la plus haute, du moins par rapport aux satellites planétaires riches en glace d’eau, et même à Pholus et à Cthulhu où H₂La glace est plus clairement détectée. À cet égard, les espèces opaques telles que les composés organiques complexes sont connues pour leur capacité à masquer la signature spectrale de H₂O ice dans le proche-IR (49). Aucune signature spectrale non ambiguë de silicates ou de glaces volatiles telles que celles observées sur Pluton (CO, N₂, NH₃ou CH₄) (4), ont été détectés pour MU₆₉, mais la plupart de ces espèces supervolatiles ne sont pas attendues à MU₆₉ en raison de la fuite de telles glaces de cet objet, entraînée thermiquement, au fil du temps. Une identification moléculaire n&#39;a pas encore été attribuée à une absorption apparente à 1,8 µm.

Considérations thermiques

Températures sur MU₆₉ sont déterminés par l’équilibre entre l’absorption de la lumière solaire et l’émission thermique dans l’espace. Pour l&#39;albédo à 6% estimé estimé ci-dessus et une émissivité, une absorption et une émission supposées à 90% en MU₆₉Distance moyenne du Soleil est équilibrée à 42 K. Pour les valeurs plausibles de la conductivité thermique dans les 10^–5 J m^-1 s^-1 K^-1 nous estimons les profondeurs cutanées caractéristiques auxquelles se propagent les ondes diurnes et saisonnières à environ 0,001 m et ~ 1 m, respectivement (voir les méthodes). Par conséquent, les variations de température diurnes et saisonnières n’affectent probablement que les quelques millimètres à quelques mètres₆₉De la surface, la température moyenne concerne donc la grande majorité des MU₆₉L’intérieur. À cette température de 42 K, les espèces volatiles congelées telles que le CO, le N₂et CH₄ ne pas être piégé dans des clathrates se sublimerait et s&#39;échapperait relativement rapidement par rapport à l&#39;âge du système solaire, mais le H amorphe₂La glace ne cristalliserait pas et pourrait donc survivre au cours de l&#39;âge du système solaire.

Près de la surface de MU₆₉, la température varie sur les échelles de temps saisonnières et diurnes. À la suite de MU₆₉L’excentricité orbitale étant faible, l’insolation ne diffère que de 17% au cours de son orbite. Cependant, l&#39;ensoleillement été / hiver varie considérablement sur l&#39;UM₆₉L’orbite de 293 ans est due à l’obliquité de son pôle à 98 °, ce qui a entraîné de longs jours et nuits polaires au cours desquels des régions reçoivent un rayonnement solaire continu, voire inexistant, pendant de nombreuses décennies. Around equinox, the 15.92-hour rotation period would also produce strong diurnal variations in insolation. Changing insolation generates thermal waves as heat is conducted into and out of the subsurface.

We expect summer surface temperatures to approach a maximum instantaneous equilibrium temperature of ~60 K, whereas winter temperatures (as on the unilluminated face of MU₆₉ during the New Horizons flyby) are much lower, down to the seasonal skin depth. Analysis of the REX radiometry data at the 4.2-cm X-band radio wavelength indicates a brightness temperature in the 20 to 35 K range on the winter (night) side of MU₆₉. Thermal emission at centimeter wavelengths emerges from a range of depths, potentially up to many tens of wavelengths below the surface, so the warmer subsurface is an important source of flux. For typical icy satellite emissivity at centimeter wavelengths (50, 51), the observed REX brightness temperature range appears to be consistent with the expected low thermal inertia values estimated for other KBOs (52).

Geophysical and geological properties

We measured the limb profile of MU₆₉ using previously published techniques (53). These limb profiles (Fig. 6, A and B) were derived from the CA04 LORRI (characteristic 140 m per pixel) and CA06 MVIC (characteristic 130 m per pixel) observations and were compared with best-fitting ellipses for the projected shape of each lobe. The elliptical axes remove the long-wavelength signal and are not necessarily representative of the 3D body shape (i.e., they are not measured with respect to the shape model in Fig. 2).

On the larger lobe Ultima, the limb topography shows total (i.e., minimum to maximum deviation) relief of ~1 km, whereas the relief on the smaller lobe Thule is more muted at ~0.5 km. Assuming a density ρ = 500 kg m^–3, a surface gravity g = 0.001 m s^–2, and topography of vertical scale h = 0.5 to 1 km implies stresses on the order of ρgh/3, or ~100 Pa. Such modest stresses can be supported by friction. Excessively steep regions (>35°) near MU₆₉’s neck likely require additional internal strength to remain stable. Cohesion of several hundreds of pascals would be capable of supporting these slopes. Such strengths are thought to be normal for comet-like bodies (22) and so are plausible for KBOs of MU₆₉’s size as well.

A geomorphological map of MU₆₉ is shown in Fig. 6C. The two lobes have somewhat different surface geology. Thule’s surface is dominated by Maryland, a depression of probable impact origin (unit labeled lc; see Fig. 6C for key to these abbreviations), ~7 km in diameter (see above). Stereographic analysis shows that the depth of Maryland is <2 km; this depth is consistent with the observed limb topography variations. Maryland’s interior shows no unambiguous signs of horizontal layering but does contain two prominent bright spots of similar size and albedo. Two distinct, kilometer-scale, possible impact craters occur on Maryland’s rim crest. Separately, four distinct troughs appear near the terminator of Thule in unit um.

Apart from Maryland, the rest of Thule’s observed surface is characterized by broad (few kilometers wide), dark swaths (unit dm) that separate lighter-toned, mottled units (units pm, um, and rm). In some places, these dark swaths contain bright spots and a bright-floored, quasi-linear trough. The crenulated boundaries of unit dm may be a decrescence morphology, whereby this unit is partly bounded by scarps that have retreated. Unit dm may be a deposit of volatile ice with bounding scarps forming as a result of sublimation at its periphery, with the upper surface of the deposit being protected from sublimation by a dark, refractory mantle, perhaps derived from the deposit itself (54, 55). Portions of unit um that are proximal to Maryland may be ejecta from the crater, or related to ejecta, but this cannot be confirmed with the current analysis. A distinct, relatively bright region (unit rm) at the equatorial, distal end of Thule exhibits roughness at the scale of a few hundred meters; some of the features there appear to be pits, craters, or mounds.

A lightly cratered surface of MU₆₉ was predicted by a recent cratering model (23) indeed, few definitively impact-related scars are identified on MU₆₉. We have considered several hypotheses for the origin of the many pits seen near the terminator in Fig. 2A. These include structurally controlled collapse pits, outgassing pits, sublimation pits, and impact craters (56) they likely are not all created by the same process. Our assessment is that the chains of similarly sized pits are more likely to be formed by internal processes than by cratering, but the isolated pits that show approximately circular planform outlines, bowl-shaped interior depressions, and, in some cases, raised rims are more consistent with impact crater morphology. There are no obvious crater candidates that are intermediate in size between these ~1-km-diameter pits (unit sp) and Maryland (unit lc, ~7 km).

On Ultima, eight similarly sized (~5 km scale) units of rolling topography (based on subtle albedo gradations and limb profiles) dominate its observed landscape (units my at mh). The albedo and texture of these units are generally similar to one another, although each contains brighter material to differing degrees. These units abut each other, typically bordered with distinct, curvilinear, and generally higher-reflectance boundary regions (particularly unit mh, which is ringed by a brighter annulus). In one instance (near the terminator), a trough and a short pit chain mark such a boundary, but the low solar incidence angles prevailing across much of Ultima make it unclear whether the boundaries are always associated with topographic features. Stereographic analysis indicates that most of these units have broadly positive relief, although the central unit mh is relatively flatter. It is also unclear whether these boundaries necessarily imply superposition relationships among the components. One unit, at the limb of Ultima (unit Maryland), appears to be more angular in stereography and is either more elevated or tilted with respect to the other components.

The apparent similarity in the size of Ultima’s units (my through mh) is likely a clue to their origin. Whether they are a relic of Ultima’s formation or a result of a later evolution is unclear. One formation-related hypothesis is that the individual units on Ultima are accretional subunits of smaller planetesimals that formed Ultima. This apparent assemblage of units on Ultima is consistent with observation of and proposals for the formation of layers on comets such as 9P/Tempel and 67P/Churyumov-Gerasimenko (20, 57). However, challenges to this hypothesis are the apparently (i.e., at the available resolution) nearly unimodal size of these units and their apparent absence from Thule, although the latter could be the result of resurfacing caused by the Maryland cratering event. Also, at the expected impact speeds during accretion, such as in a local particle cloud collapse resulting in a body like MU₆₉ (perhaps no more than a few meters per second, based on the mutual escape speed of the lobes), these accretional subunits would likely not be expected to merge into as compact a body as Ultima unless they were extremely weak (i.e., cohesionless and frictionless) at the time of their accretion (22).

Satellites and orbiting rings/dust search

Both satellites and rings have been detected around KBOs larger than MU₆₉ and around Centaurs (16, 58, 59), which have escaped the Kuiper Belt and now orbit among the giant planets. Although no KBO as small as MU₆₉ is known to have rings, satellites around KBOs are common, particularly in the CCKBO population (16).

We searched for satellites and rings of MU₆₉ using co-added stacks of LORRI images acquired during approach to MU₆₉ at exposure times of 10 to 30 s. High-resolution images taken near closest approach provide additional constraints on satellites close to MU₆₉. No satellites were detected in our data. Figure 7A shows the quantitative limits on satellites obtained from these data as a function of distance from MU₆₉.

Additionally, small particles orbiting MU₆₉ could form ring or other dust structures with unusual geometries (60) because MU₆₉’s very weak gravity is of similar magnitude to the solar radiation pressure. However, larger grains, which are less affected by solar radiation pressure, could form a conventional equatorial ring (60).

Approach images constrain any ring or dust assemblages located ≳250 km from MU₆₉ to have I/F ≲ 2 × 10^–7 (for a 10-km-wide ring) at a phase angle of 11° (Fig. 7B). This is equal to or less than the I/F of many faint outer rings of the giant planets, which have ring widths of >20 km (61, 62). A single downlinked high-phase (165°), forward-scattered observation also shows no evidence for rings or dust farther than ~400 km from MU₆₉ at I/F > 10^–4. The VBSDC dust counter experiment reported zero dust impact detections during the passage through the gravitational stability sphere of MU₆₉, consistent with a lack of extant rings or other dust assemblages.

Exosphere and heliospheric interaction searches

Because of MU₆₉’s small size, it is likely that highly volatile ices that might once have been present at its surface would have escaped to space long ago (6, 56). However, less volatile ices (e.g., methanol, acetylene, ethane, and hydrogen cyanide) could be retained over geological time scales, and irradiation of these species could result in reddening over time as longer-chain tholins are produced (63, 64). This implies a slow loss of hydrogen atoms to space as surface ices are converted into tholins. In addition to this escaping H flux, occasional large impacts could provide a source for a transient atmosphere.

We searched for evidence of both a coma of escaping gas and charged particle emissions from MU₆₉. The searches included use of the Alice ultraviolet spectrograph to search for resonance line emission from a coma, as well as in situ searches for emitted MU₆₉ ions with SWAP and PEPSSI.

An Alice count rate spectrum is shown in Fig. 8A. This observation was made over a period of 300 s, ~90 min before closest approach, from a range r ~ 80,000 km. We fitted a model to the spectrum including background emissions from interplanetary hydrogen plus four nearby stars. No coma emissions from MU₆₉ were detected. At the brightest likely coma emission of the hydrogen 121.6-nm line, we find a 3σ upper limit source rate of <3 × 10²⁴ H atoms s^–1 released by MU₆₉. Scattering of sunlight by H atoms at this source rate would produce a detectable emission, assuming a distribution that falls off as r^–2 from MU₆₉.

No structured magnetic interaction between the solar wind and MU₆₉ was expected because, at a typical interplanetary magnetic field of 0.2 nT, the gyro-radius of a proton picked up by the solar wind is ~2 × 10⁴ km; this distance is larger than MU₆₉ by a factor of ~1000 and is also much larger than the flyby closest approach distance of ~3500 km. Shown in Fig. 8, B to E, are SWAP data taken during the time of the flyby. The bulk solar wind density and speed measured by SWAP near MU₆₉ are ~2000 protons m^–3 and ~425 km s^–1, respectively, for a solar wind flux of 8.5 × 10⁸ protons m^–2 s^–1. Variations are attributable to changes in spacecraft orientation relative to the solar wind, but there is no signature of any detected interaction of the solar wind with MU₆₉. Shown in Fig. 8, F to I, are PEPSSI data taken during the flyby, which also show no evidence of any MU₆₉-related signature. All PEPSSI variations in count rate are associated with spacecraft attitude changes, as with SWAP, and are consistent with an unperturbed interplanetary medium.

We can estimate the interaction that might be detected by SWAP or PEPSSI using the upper limit from Alice. For an outflow source rate of Q particles s^–1, the density at r is given by n ~ Q/4πr²v. For the Alice upper limit of Q ~ 3 × 10²⁴ H atoms s^–1, the density at closest approach range is ~2.4 × 10⁷ H atoms m^–3 (assuming the H atoms have a radial velocity v ~ 800 m s^–1, their thermal speed at 40 K). A fraction γ of those H atoms can become ionized and picked up by the solar wind to be detected by SWAP or PEPSSI. Then the count rate is R = εGnv_SWγ/4π, where ε is the detection efficiency near the solar wind speed, g is the instrument geometric factor, and v_SW is the solar wind speed. This indicates a PEPSSI count rate of R ~ 8000 γ counts s^–1. We estimate γ ~ 2 × 10^–7 from the fraction of H atoms that could be photoionized by sunlight during the travel time t ~ d_CA/v for H atoms to get from MU₆₉ to the closest approach distance of New Horizons (d_CA) the expected PEPSSI count rate of 1.6 × 10^–3 Hz would thus be smaller than the typical background count rate (~1 Hz) by a factor of ~600. A similar result is found for SWAP. Hence, the upper limit found with Alice data is more constraining than the SWAP and PEPSSI upper limits.

For comparison, we estimate the loss rate from photosputtering of water ice on MU₆₉ to be ~10¹⁹ H atoms s^–1, much less than our detection upper limit. This estimate is based on the combined direct solar and interplanetary medium H 121.6-nm flux at MU₆₉ of 1.6 × 10¹² photons m^–2 s^–1 and 4.0 × 10¹¹ photons m^–2 s^–1, respectively (65, 66), and a yield of 0.003 for H 121.6-nm sputtering of water ice (67–69). Projected and total surface areas for MU₆₉ of ~4.1 × 10⁸ m² and 1.7 × 10⁹ m², respectively, were used in this estimate. Over 4.5 billion years, the water ice lost by this process would reduce the size of MU₆₉ by a negligible ~0.01 m. Water ice is also eroded by solar wind ions (mainly protons), but this process is estimated to remove only about the same amount of material over time as the photosputtering (70).

Implications for formation

The New Horizons flyby has revealed many properties of MU₆₉ but has also raised some puzzles. The latter include the origin of its two nonspherical and markedly different lobe shapes; the provenance of its brighter spots, zones, and linear/curvilinear features; the nature of the similarly sized surface units on Ultima; the degree to which the object is cratered; the origin of its bright neck; and how its two lobes formed and then merged to create the contact binary we observe.

Despite these puzzles, MU₆₉ has already provided information on the ancient accretion processes that operated in the distant protosolar nebula and the Kuiper Belt (71, 72). For example, MU₆₉ lends weight to model predictions that binaries in the Kuiper Belt may have formed in local, low- to medium-velocity accretion clouds as in pebble cloud gravitational collapse models (24). The lack of strong surface albedo, color, and composition heterogeneity between the two lobes supports this hypothesis, because in a local pebble cloud collapse, MU₆₉’s two lobes would form from a single source of material. However, it is also possible that material accreted throughout the cold classical Kuiper Belt may have been compositionally homogeneous to begin with.

The binary size ratios produced in existing pebble cloud collapse models (24), while focused on the formation of much more massive (100-km scale), co-orbiting binaries, are also consistent with the size ratio of the two lobes of MU₆₉ (size ratio ≈ 0.75). Such models also produce appropriately low merger speeds (24, 25) if they scale with the virial mass to much smaller clouds. However, MU₆₉’s pole is highly inclined to the ecliptic, which is not a common outcome in cloud collapse models, given that the cloud’s initial rotation state is set by heliocentric Keplerian shear. In contrast, turbulent particle concentration models, such as an overdense collection of swirling particles collapsing under the influence of aerodynamic drag in the protosolar nebula (73), have no preferred initial swirl (mean angular momentum) orientation.

Mechanisms fundamentally different from local pebble or particle cloud collapse have also been proposed for the formation of small-body binaries. However, some such mechanisms, such as YORP spin-up and fission (28, 29), apply only in the inner Solar System where thermal radiation forces are sufficiently strong. In the outer Solar System, binary systems may instead form via three-body exchange capture (74), but such mechanisms require heliocentric encounter velocities generally near or lower than the Hill speed[theheliocentricKeplerianshearspeedatthelimitoftheprimarybody’sgravitationalsphereofinfluence([theheliocentricKeplerianshearspeedatthelimitoftheprimarybody’sgravitationalsphereofinfluence([theheliocentricKeplerianshearspeedatthelimitoftheprimarybody’sgravitationalsphereofinfluence([theheliocentricKeplerianshearspeedatthelimitoftheprimarybody’sgravitationalsphereofinfluence(24)], which for MU₆₉ would have been an implausibly low ~1 cm s^–1; hence, such models are disfavored. We also do not favor this three-body formation mechanism because binaries formed by such three-body exchange would likely rotate either prograde or retrograde (24, 75), not highly obliquely like MU₆₉.

For MU₆₉’s two lobes to reach their current, merged spin state, they must have lost angular momentum if they initially formed as co-orbiting bodies. The lack of detected satellites of MU₆₉ may imply ancient angular momentum sink(s) via (i) the ejection of formerly co-orbiting smaller bodies by Ultima and Thule, (ii) gas drag, or both. This suggests that contact binaries may be rare in CCKBO systems with orbiting satellites. Another possibility, however, is that the lobes Ultima and Thule impacted one another multiple times, shedding mass along with angular momentum before making final contact. But the alignment of the principal axes of MU₆₉’s two lobes tends to disfavor this hypothesis. In contrast, tidal locking could quite plausibly have produced the principal axis alignment we observe, once the co-orbiting bodies were close enough and spin-orbit coupling was most effective (76). Gas drag could also have played a role in fostering the observed coplanar alignment of the Ultima and Thule lobes (Fig. 2). Post-merger impacts may have also somewhat affected the observed, final angular momentum state.