S.A. Stern et al., Science 17 May 2019
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.
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.
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.
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.
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.
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.
The Kuiper Belt, a torus-shaped ensemble of objects in the outer Solar System beyond the orbit of Neptune, was discovered in 1992. This is the source region for Jupiter-family comets and contains primordial planetesimals and dwarf planets. The 2003 Planetary Decadal Survey ranked exploration of the Kuiper Belt at the top of funding priorities for NASA’s planetary program. The resultant NASA mission, New Horizons, flew through and explored the Pluto dwarf planet system in 2015. The spacecraft has since continued farther to explore Kuiper Belt objects (KBOs) and the Kuiper Belt radiation and dust environment.
The target selected for the subsequent New Horizons KBO flyby was (486958) 2014 MU69 (hereafter MU69, also informally referred to as Ultima Thule). This KBO was discovered in 2014 when the Hubble Space Telescope (HST) was being used to conduct a dedicated search for New Horizons KBO flyby targets. Before the arrival of New Horizons, the only definitive facts regarding MU69 were its orbit, its red color, its size of ~30 km, and its lack of detectable variations in its light curve or large, distant satellites.
MU69’s orbit has a semimajor axis a = 44.6 astronomical units (AU), with eccentricity e = 0.042 and inclination i = 2.45°, making it a member of the cold classical KBO (CCKBO) population (here, cold refers to low dynamical excitation, not surface temperature). CCKBOs are thought to be (i) distant relics formed from the Solar System’s original protoplanetary disk and (ii) more or less dynamically undisturbed bodies that therefore formed in situ ~4.5 billion years ago and have since remained at or close to their current, large heliocentric distances. Relative to other Kuiper Belt populations, CCKBOs have a more uniformly red color distribution, as well as a different size-frequency distribution (i.e., the population of objects as a function of object size) and higher average visible albedos than are typical in the Kuiper Belt. Additionally, many CCKBOs have satellites.
Because CCKBOs are dynamically undisturbed from their formation location, they have never been warmed above the ambient, radiative equilibrium temperatures of 30 to 60 K in the Kuiper Belt. MU69’s small equivalent spherical diameter of ~19 km is insufficient to drive internal evolution long after its formation. Therefore, small CCKBOs like MU69 are expected to be primordial planetesimals, preserving information on the physical, chemical, and accretional conditions in the outer solar nebula and the processes of planetesimal formation.
New Horizons flew closest to MU69 at 05:33:22.4 (±0.2 s, 1σ) universal time (UT) on 1 January 2019. The closest approach distance of 3538.5 ± 0.2 (1σ) km was targeted to the celestial north of MU69’s center; its relative speed past MU69 was 14.43 km s–1. The asymptotic approach direction of the trajectory was approximately in the ecliptic plane at an angle of 11.6° from the direction to the Sun. The flyby’s observation planning details have been summarized elsewhere. This report of initial flyby results is based on the ~10% of all collected flyby data that had been sent to Earth before 1 March 2019; full data transmission is expected to complete in mid-2020.
New Horizons carries a suite of seven scientific instruments; all were used in the flyby of MU69. These instruments are (i) Ralph, which consists of the Multispectral Visible Imaging Camera (MVIC), a multicolor/panchromatic mapper, and the Linear Etalon Imaging Spectral Array (LEISA), an infrared (IR) composition mapping spectrometer; (ii) the Long Range Reconnaissance Imager (LORRI), a long–focal length panchromatic visible camera; (iii) the Alice extreme/far ultraviolet mapping spectrograph; (iv) a Radio Experiment (REX) to measure surface brightness temperatures and X-band radar reflectivity; (v) the Solar Wind Around Pluto (SWAP) charged-particle solar wind spectrometer; (vi) the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) MeV charged-particle spectrometer; and (vii) the Venetia Burney Student Dust Counter (VBSDC), a dust impact detector.