New Horizons flies past MU69 After flying past Pluto in 2015, the New Horizons spacecraft shifted course to encounter (486958) 2014 MU69, a much smaller body about 30 kilometers in diameter. MU69 is part of the Kuiper Belt, a collection of small icy bodies orbiting in the outer Solar System. Stern et al. present the initial results from the New Horizons flyby of MU69 on 1 January 2019. MU69 consists of two lobes that appear to have merged at low speed, producing a contact binary. This type of Kuiper Belt object is mostly undisturbed since the formation of the Solar System and so will preserve clues about that process. Science, this issue p. eaaw9771 Initial results from the New Horizons flyby of Kuiper Belt object (486958) 2014 MU69, a contact binary, are presented. INTRODUCTION The Kuiper Belt is a broad, torus-shaped region in the outer Solar System beyond Neptune’s orbit. It contains primordial planetary building blocks and dwarf planets. NASA’s New Horizons spacecraft conducted a flyby of Pluto and its system of moons on 14 July 2015. New Horizons then continued farther into the Kuiper Belt, adjusting its trajectory to fly close to the small Kuiper Belt object (486958) 2014 MU69 (henceforth MU69; also informally known as Ultima Thule). Stellar occultation observations in 2017 showed that MU69 was ~25 to 35 km in diameter, and therefore smaller than the diameter of Pluto (2375 km) by a factor of ~100 and less massive than Pluto by a factor of ~106. MU69 is located about 1.6 billion kilometers farther from the Sun than Pluto was at the time of the New Horizons flyby. MU69’s orbit indicates that it is a “cold classical” Kuiper Belt object, thought to be the least dynamically evolved population in the Solar System. A major goal of flying past this target is to investigate accretion processes in the outer Solar System and how those processes led to the formation of the planets. Because no small Kuiper Belt object had previously been explored by spacecraft, we also sought to provide a close-up look at such a body’s geology and composition, and to search for satellites, rings, and evidence of present or past atmosphere. We report initial scientific results and interpretations from that flyby. RATIONALE The New Horizons spacecraft completed its MU69 flyby on 1 January 2019, with a closest approach distance of 3538 km—less than one-third of its closest distance to Pluto. During the high-speed flyby, made at 14.4 km s−1, the spacecraft collected ~50 gigabits of high-resolution imaging, compositional spectroscopy, temperature measurements, and other data on this Kuiper Belt object. We analyzed the initial returned flyby data from the seven scientific instruments carried on the spacecraft: the Ralph multicolor/panchromatic camera and mapping infrared composition spectrometer; the Long Range Reconnaissance Imager (LORRI) long–focal length panchromatic visible imager; the Alice extreme/far ultraviolet mapping spectrograph; the Radio Experiment (REX); the Solar Wind Around Pluto (SWAP) solar wind detector; the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) high-energy charged particle spectrometer; and the Venetia Burney Student Dust Counter (VBSDC), a dust impact detector. RESULTS Imaging of MU69 showed it to be a bilobed, contact binary. MU69’s two lobes appear to have formed close to one another, becoming an orbiting pair that subsequently underwent coupled tidal and orbital evolution to merge into the contact binary we observe today. The object rotates on its axis every 15.92 hours; its rotation pole is inclined approximately 98° to the plane of its heliocentric orbit. Its entire surface has a low visible-wavelength reflectivity (albedo) but displays brighter and darker regions across its surface, ranging from 5 to 12% reflectivity. The brightest observed regions are the “neck” of MU69, where the two lobes are joined, and two discrete bright spots inside the largest crater-like feature on the object’s surface. Although MU69’s albedo varies substantially across its surface, it is uniformly red in color, with only minor observed color variations. This coloration likely represents a refractory residue from ices and organic molecules processed by ultraviolet light and cosmic rays. Spectra of the surface revealed tentative absorption band detections due to water ice and methanol. The geology of MU69 consists of numerous distinct units but shows only a small number of craters, providing evidence that there is a deficit of Kuiper Belt objects smaller than ~1 km in diameter, and that there is a comparatively low collision rate in its Kuiper Belt environment compared to what would be expected in a collisional equilibrium population. A three-dimensional shape model derived from the images shows MU69 is not simply elongated but also flattened. The larger lobe was found to be lenticular, with dimensions of approximately 22 × 20 × 7 km (uncertainty <0.6 × 1 × 2 km), whereas the smaller lobe is less lenticular, with dimensions of approximately 14 × 14 × 10 km (uncertainty <0.4 × 0.7 × 3 km). No evidence of satellites, rings, or an extant atmosphere was found around MU69. CONCLUSION Both MU69’s binarity and unusual shape may be common among similarly sized Kuiper Belt objects. The observation that its two lobes are discrete, have retained their basic shapes, and do not display prominent deformation or other geological features indicative of an energetic or disruptive collision indicates that MU69 is the product of a gentle merger of two independently formed bodies. Image of MU69 from New Horizons LORRI observations taken 1 January 2019. 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.

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新视野号探索 2014 MU69 的初步结果，这是一个小型柯伊伯带天体

新视野号飞过 MU69 在 2015 年飞过冥王星后，新视野号航天器改变航线，遇到 (486958) 2014 MU69，这是一个直径约 30 公里的小得多的天体。MU69 是柯伊伯带的一部分，柯伊伯带是在外太阳系中运行的小型冰天体的集合。斯特恩等人。展示了 2019 年 1 月 1 日新视野号 MU69 飞越的初步结果。 MU69 由两个似乎以低速合并的瓣组成，产生了一个接触双星。这种类型的柯伊伯带天体自太阳系形成以来几乎没有受到干扰，因此将保留有关该过程的线索。科学，这个问题 p。eaaw9771 介绍了新视野号飞越柯伊伯带天体 (486958) 2014 MU69（一个接触双星）的初步结果。简介 柯伊伯带是一条宽阔的、海王星轨道外太阳系外环面形状的区域。它包含原始行星构建块和矮行星。美国宇航局的新视野号航天器于 2015 年 7 月 14 日对冥王星及其卫星系统进行了一次飞越。然后新视野号继续深入柯伊伯带，调整其轨迹以接近柯伊伯带小天体 (486958) 2014 MU69（以下称为 MU69；也非正式地称为 Ultima Thule）。2017 年的恒星掩星观测表明，MU69 的直径约为 25 至 35 公里，因此比冥王星的直径（2375 公里）小约 100 倍，质量比冥王星小约 106 倍。MU69 距离太阳约 16 亿公里，比新视野号飞掠时冥王星的距离还要远。MU69的轨道表明它是一个“冷经典”柯伊伯带天体，被认为是太阳系中进化最少的种群。飞越这个目标的一个主要目标是研究外太阳系的吸积过程以及这些过程如何导致行星的形成。由于航天器之前没有探索过柯伊伯带小天体，我们还试图近距离观察这样一个天体的地质和组成，并寻找卫星、环以及现在或过去大气的证据。我们报告了那次飞越的初步科学结果和解释。基本原理 新视野号航天器于 2019 年 1 月 1 日完成了 MU69 的飞越，最近的接近距离为 3538 公里——不到它与冥王星最近距离的三分之一。在以 14.4 km s−1 进行的高速飞越期间，航天器收集了约 50 Gb 的高分辨率图像，这个柯伊伯带天体的成分光谱、温度测量和其他数据。我们分析了航天器上携带的七种科学仪器的初始返回飞越数据：拉尔夫多色/全色相机和测绘红外成分光谱仪；远程侦察成像仪 (LORRI) 长焦距全色可见光成像仪；爱丽丝极/远紫外映射光谱仪；无线电实验（REX）；冥王星周围的太阳风（SWAP）太阳风探测器；冥王星高能粒子光谱仪科学调查（PEPSSI）高能带电粒子光谱仪；和 Venetia Burney 学生粉尘计数器 (VBSDC)，一种粉尘撞击探测器。结果 MU69 的成像显示它是一个双叶接触双星。MU69 的两个裂片似乎彼此靠近形成，成为一个轨道对，随后经历了耦合的潮汐和轨道演化，以合并到我们今天观察到的接触双星中。物体每 15.92 小时绕其轴旋转一次；它的自转极与其日心轨道平面倾斜大约 98°。它的整个表面具有较低的可见波长反射率（反照率），但在其表面上显示出更亮和更暗的区域，反射率在 5% 到 12% 之间。观察到的最亮区域是 MU69 的“颈部”，在那里连接了两个瓣，以及物体表面最大的类似陨石坑的特征内的两个离散亮点。尽管 MU69 的反照率在其表面上变化很大，但它的颜色是均匀的红色，仅观察到很小的颜色变化。这种颜色可能代表了冰和有机分子经紫外线和宇宙射线处理后的耐火残留物。由于水冰和甲醇，表面的光谱显示了初步的吸收带检测。MU69 的地质由许多不同的单元组成，但只显示了少量的陨石坑，提供证据表明柯伊伯带直径小于 1 公里的天体存在缺陷，并且柯伊伯带的碰撞率相对较低与碰撞平衡种群中预期的环境相比。来自图像的三维形状模型显示 MU69 不仅被拉长，而且还被压扁。发现较大的叶是透镜状的，尺寸约为 22 × 20 × 7 km（不确定度 <0.6 × 1 × 2 km），而较小的叶则透镜状较小，尺寸约为 14 × 14 × 10 km（不确定度 <0.4 × 0.7 × 3 km）。在 MU69 周围没有发现卫星、环或现存大气的证据。结论 MU69 的二元性和不寻常的形状可能在类似大小的柯伊伯带天体中很常见。观察到它的两个裂片是离散的，保留了它们的基本形状，并且没有表现出明显的变形或其他表明发生能量或破坏性碰撞的地质特征，这表明 MU69 是两个独立形成的天体温和合并的产物。来自新视野号 LORRI 观测的 MU69 图像于 2019 年 1 月 1 日拍摄。柯伊伯带是外太阳系的一个遥远区域。2019 年 1 月 1 日，新视野号飞船飞近 (486958) 2014 MU69，一个直径约 30 公里的冷经典柯伊伯带天体。这些物体从未被太阳充分加热，因此自形成以来保存完好。我们描述了这些遭遇观察的初步结果。MU69 是一个双叶接触双星，具有扁平的形状、离散的地质单元和明显的反照率异质性。然而，几乎没有表面颜色或成分异质性。没有检测到卫星、环或其他尘埃结构、气体昏迷或太阳风相互作用的证据。MU69 的起源似乎与卵石云坍塌一致，随后它的两个瓣低速合并。我们描述了这些遭遇观察的初步结果。MU69 是一个双叶接触双星，具有扁平的形状、离散的地质单元和明显的反照率异质性。然而，几乎没有表面颜色或成分异质性。没有检测到卫星、环或其他尘埃结构、气体昏迷或太阳风相互作用的证据。MU69 的起源似乎与卵石云坍塌一致，随后它的两个瓣低速合并。我们描述了这些遭遇观察的初步结果。MU69 是一个双叶接触双星，具有扁平的形状、离散的地质单元和明显的反照率异质性。然而，几乎没有表面颜色或成分异质性。没有检测到卫星、环或其他尘埃结构、气体昏迷或太阳风相互作用的证据。MU69 的起源似乎与卵石云坍塌一致，随后它的两个瓣低速合并。