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Images of the dwarf planet’s far side are revealing possible signs of liquid water, mysterious shards of ice and new theories for the frigid world’s birth.

Pluto is thought to possess a present-day ocean beneath a thick ice shell. It has generally been assumed that Pluto accreted from cold material and then later developed its ocean due to warming from radioactive decay; in this ‘cold start’ scenario, the ice shell would have experienced early compression and more recent extension. Here we compare thermal model simulations with geological observations from the New Horizons mission to suggest that Pluto was instead relatively hot when it formed, with an early subsurface ocean. Such a ‘hot start’ Pluto produces an early, rapid phase of extension, followed by a more prolonged extensional phase, which totals ~0.5% linear strain over the last 3.5 Gyr. The amount of second-phase extension is consistent with that inferred from extensional faults on Pluto; we suggest that an enigmatic ridge–trough system recently identified on Pluto is indicative of early extensional tectonics. A hot initial start can be achieved with the gravitational energy released during accretion if the final stage of Pluto’s accretion is rapid (

Acquiring and processing astrometric measurements of a spacecraft’s target using on-board images, generically referred to as optical navigation, is an integral function of the orbit determination and navigation of NASA’s New Horizons spacecraft. Since New Horizons’ reconnaissance of the Pluto system in July 2015, many preparations have been completed to further enhance the optical navigation system and prepare for the reconnaissance of New Horizons’ next target, Kuiper Belt Object (486958) 2014 MU69 (unofficially nicknamed Ultima Thule). Due to its low relative brightness compared to most planetary exploration targets, Ultima Thule presents several unique challenges to the optical navigation system. The optical navigation system design, imaging schedule, and technical algorithms that were developed and tailored to these challenges are explored in detail. Additionally, several operational readiness tests, simulation methods, and test results are presented and analyzed to assess the optical navigation system performance and implications to flight operations. Lastly, a first look at Ultima as viewed from the New Horizons LORRI imager is presented.

Many icy Solar System bodies possess subsurface oceans. On Pluto, Sputnik Planitia’s location near the equator suggests the presence of a subsurface ocean and a locally thinned ice shell. To maintain an ocean, Pluto needs to retain heat inside. On the other hand, to maintain large variations in its thickness, Pluto’s ice shell needs to be cold. Here we show, by thermal evolution and viscous relaxation calculations, that the presence of a thin layer of clathrate hydrates (gas hydrates) at the base of the ice shell can explain both the long-term survival of the ocean and the maintenance of shell thickness contrasts. Clathrate hydrates act as a thermal insulator, preventing the ocean from completely freezing while keeping the ice shell cold and immobile. The most likely clathrate guest gas is methane, derived from precursor bodies and/or cracking of organic materials in the hot rocky core. Nitrogen molecules initially contained and/or produced later in the core would probably not be trapped as clathrate hydrates, instead supplying the nitrogen-rich surface and atmosphere. The formation of a thin clathrate hydrate layer cap to a subsurface ocean may be an important generic mechanism to maintain long-lived subsurface oceans in relatively large but minimally heated icy satellites and Kuiper belt objects. Pluto’s subsurface ocean and thickness variation in its ice shell may be maintained by a layer of methane clathrates forming an insulating cap to the ocean, according to calculations of thermal evolution and viscous relaxation.

This brief review will discuss the current knowledge on the origin and evolution of the nitrogen atmospheres of the icy bodies in the solar system, particularly of Titan, Triton and Pluto. An important tool to analyse and understand the origin and evolution of these atmospheres can be found in the different isotopic signatures of their atmospheric constituents. The $^{14}$N/$^{15}$N ratio of the N$_2$-dominated atmospheres of these bodies serve as a footprint of the building blocks from which Titan, Triton and Pluto originated and of the diverse fractionation processes that shaped these atmospheres over their entire evolution. Together with other measured isotopic and elemental ratios such as $^{12}$C/$^{13}$C or Ar/N these atmospheres can give important insights into the history of the icy bodies in the solar system, the diverse processes that affect their N$_2$-dominated atmospheres, and the therewith connected solar activity evolution. Titan's gaseous envelope most likely originated from ammonia ices with possible contributions from refractory organics. Its isotopic signatures can yet be seen in the - compared to Earth - comparatively heavy $^{14}$N/$^{15}$N ratio of 167.7, even though this value slightly evolved over its history due to atmospheric escape and photodissociation of N$_2$. The origin and evolution of Pluto's and Triton's tenuous nitrogen atmospheres remain unclear, even though it might be likely that their atmospheres originated from the protosolar nebula or from comets. An in-situ space mission to Triton such as the recently proposed Trident mission, and/or to the ice giants would be a crucial cornerstone for a better understanding of the origin and evolution of the icy bodies in the outer solar system and their atmospheres in general., Comment: 60 pages, 6 figures. This is a preprint of an article published in Space Science Reviews. The final authenticated version can be found online at : https://doi.org/10.1007/s11214-020-00752-0

Predicting the large-scale eruptions from the solar corona and their propagation through interplanetary space remains an outstanding challenge in solar- and helio-physics research. In this article, we describe three dimensional magnetohydrodynamic simulations of the inner heliosphere leading up to and including the extreme interplanetary coronal mass ejection (ICME) of 23 July 2012, developed using the code PLUTO. The simulations are driven using the output of coronal models for Carrington rotations 2125 and 2126 and, given the uncertainties in the initial conditions, are able to reproduce an event of comparable magnitude to the 23 July ICME, with similar velocity and density profiles at 1 au. The launch-time of this event is then varied with regards to an initial 19 July ICME and the effects of solar wind preconditioning are found to be significant for an event of this magnitude and to decrease over a time-window consistent with the ballistic refilling of the depleted heliospheric sector. These results indicate that the 23 July ICME was mostly unaffected by events prior, but would have travelled even faster had it erupted closer in time to the 19 July event where it would have experienced even lower drag forces. We discuss this systematic study of solar wind preconditioning in the context of space weather forecasting., 17 pages, 5 figures, 1 table. Solar Physics accepted 26 August 2020

Numerous Solar System atmospheres possess photochemically generated hazes, including the characteristic organic hazes of Titan and Pluto. Haze particles substantially impact atmospheric temperature structures and may provide organic material to the surface of a world, potentially affecting its habitability. Observations of exoplanet atmospheres suggest the presence of aerosols, especially in cooler (

In July 2015, NASA’s New Horizons mission performed a flyby of Pluto, revealing details about the geology, surface composition and atmospheres of this world and its moons that are unobtainable from Earth. With a resolution as small as 80 metres per pixel, New Horizons’ images identified a large number of surface features, including a large basin filled with glacial ices that appear to be undergoing convection. Maps of surface composition show latitudinal banding, with non-volatile material dominating the equatorial region and volatile ices at mid- and polar latitudes. This pattern is driven by the seasonal cycle of solar insolation. New Horizons’ atmospheric investigation found the temperature of Pluto’s upper atmosphere to be much cooler than previously modelled. Images of forward-scattered sunlight revealed numerous haze layers extending up to 200 km from the surface. These discoveries have transformed our understanding of icy worlds in the outer Solar System, demonstrating that even at great distances from the Sun, worlds can have active geologic processes. This Review addresses our current understanding of the Pluto system and places it in context with previous investigations. The New Horizons spacecraft performed a flyby of Pluto and its system in July 2015, providing more than 50 Gb of high-resolution images and data that transformed our view and understanding of the dwarf planet. This Review summarizes its main discoveries.

The location of Sputnik Planitia on Pluto is shown to result from volatiles sequestered within the basin forcing the reorientation of the dwarf planet, as supported by the planet-wide fault network. James Keane et al. report that the location of Sputnik Planitia, the approximately 1,000-kilometre-diameter topographic basin that dominates the surface of Pluto, is a natural consequence of the infill of volatile ices within the basin and the resulting reorientation (true polar wander) of the dwarf planet. As Sputnik Planitia slowly loaded with volatiles, the pull of tidal torques from Charon would have resulted in the eventual alignment of the basin with the Pluto–Charon tidal axis. Sputnik Planitia probably formed northwest of its present location, and was loaded with volatiles over million-year timescales as a result of volatile transport cycles. This is one of four papers on the geology of Sputnik Planitia in this issue of Nature. In News & Views, Amy Barr puts these latest contributions into context. Pluto is an astoundingly diverse, geologically dynamic world. The dominant feature is Sputnik Planitia—a tear-drop-shaped topographic depression approximately 1,000 kilometres in diameter possibly representing an ancient impact basin1,2. The interior of Sputnik Planitia is characterized by a smooth, craterless plain three to four kilometres beneath the surrounding rugged uplands, and represents the surface of a massive unit of actively convecting volatile ices (N2, CH4 and CO) several kilometres thick1,2,3,4,5. This large feature is very near the Pluto–Charon tidal axis. Here we report that the location of Sputnik Planitia is the natural consequence of the sequestration of volatile ices within the basin and the resulting reorientation (true polar wander) of Pluto. Loading of volatile ices within a basin the size of Sputnik Planitia can substantially alter Pluto’s inertia tensor, resulting in a reorientation of the dwarf planet of around 60 degrees with respect to the rotational and tidal axes. The combination of this reorientation, loading and global expansion due to the freezing of a possible subsurface ocean generates stresses within the planet’s lithosphere, resulting in a global network of extensional faults that closely replicate the observed fault networks on Pluto. Sputnik Planitia probably formed northwest of its present location, and was loaded with volatiles over million-year timescales as a result of volatile transport cycles on Pluto6,7. Pluto’s past, present and future orientation is controlled by feedbacks between volatile sublimation and condensation, changing insolation conditions and Pluto’s interior structure.

The precession of the perihelion of Mercury’s orbit for 100 years in the gravitational field of the Sun and the planets has been numerically modeled within the framework of a generalized law of universal gravitation with refined data on the parameters of the orbits of the planets (including the asteroid belt and Pluto), and also the gravitational constant and with a smaller iteration step (0.0002 s). The calculations were performed with enhanced computational accuracy. It has been shown that the average precession of Mercury’s orbit in 100 years within the framework of the generalized law of universal gravitation comprises ~565.3''. This is less than the observed shift of the perihelion by about 8.8''. The observed shift of the perihelion, as is well known, comprises ~574.1''. It is not ruled out that inside Mercury’s orbit some unknown undetected object (or several such objects) of small size remains to be found.

NASA’s New Horizons mission measures the distances of two stars from the outer reaches of the Solar System. NASA’s New Horizons mission measures the distances of two stars from the outer reaches of the Solar System.

This review of Pluto laboratory research presents some of the recent advancements and motivations in our understanding enabled by experimental simulations, the need for experiments to facilitate models, and predictions for future laboratory work. The spacecraft New Horizons at Pluto has given a large amount of scientific data already rising to preliminary results, spanning from the geology to the atmosphere. Different ice mixtures have now been detected, with the main components being nitrogen, methane, and carbon monoxide. Varying geology and atmospheric hazes, however, gives us several questions that need to be addressed to further our understanding. Our review summarizes the complexity of Pluto, the motivations and importance of laboratory simulations critical to understanding the low temperature and pressure environments of icy bodies such as Pluto, and the variability of instrumentation, challenges for research, and how simulations and modeling are complimentary.

A growing body of evidence suggests the operation of life and life processes in comets as well in larger icy bodies in the solar system including Enceladus. Attempts to interpret such data without invoking active biology are beginning to look weak and flawed. The emerging new paradigm is that life is a cosmic phenomenon as proposed by Hoyle and Wickramasinghe (Lifecloud: the Origin of Life in the Galaxy, 1978) and first supported by astronomical spectroscopy (Wickramasinghe and Allen, Nature 287:518, 1980; Allen and Wickramasinghe, Nature 294:239, 1981; Wickramasinghe and Allen, Nature 323:44, 1986). Comets are the transporters and amplifiers of microbial life throughout the Universe and are also, according to this point of view, the carriers of viruses that contribute to the continued evolution of life. Comets brought life to Earth 4.2 billion years ago and they continue to do so. Space extrapolations of comets, Enceladus and possibly Pluto supports this point of view. Impacts of asteroids and comets on the Earth as well as on other planetary bodies leads to the ejection of life-bearing dust and rocks and a mixing of microbiota on a planetary scale and on an even wider galactic scale. It appears inevitable that the entire galaxy will be a single connected biosphere.

This year, NASA's Dawn and New Horizons rendezvoused with Ceres and Pluto, respectively. These worlds, despite their modest sizes, have much to teach us about the accretion of the Solar System and its dynamical evolution.

To explain the position of the Sputnik Planitia basin on Pluto, the feature would need to have formed via impact and Pluto would need to have a subsurface ocean. The present location of Sputnik Planitia—the prominent deep icy basin on Pluto—is close to one of the longitudes of the dwarf planet's tidal axis. By analogy with other large basins in the Solar System, it is thought to be an impact feature. Although reorientation arising from tidal and rotational torques can explain the present-day location of the basin, it requires the feature to be a positive gravity anomaly, despite its negative topography. Francis Nimmo et al. argue that if Sputnik Planitia formed via impact and if Pluto possesses a subsurface ocean, a positive gravity anomaly would naturally result because of shell thinning and ocean uplift, followed by later modest nitrogen deposition. This is one of four papers on the geology of Sputnik Planitia in this issue of Nature. In News & Views, Amy Barr puts these latest contributions into context. The deep nitrogen-covered basin on Pluto, informally named Sputnik Planitia, is located very close to the longitude of Pluto’s tidal axis1 and may be an impact feature2, by analogy with other large basins in the Solar System3,4. Reorientation5,6,7 of Sputnik Planitia arising from tidal and rotational torques can explain the basin’s present-day location, but requires the feature to be a positive gravity anomaly7, despite its negative topography. Here we argue that if Sputnik Planitia did indeed form as a result of an impact and if Pluto possesses a subsurface ocean, the required positive gravity anomaly would naturally result because of shell thinning and ocean uplift, followed by later modest nitrogen deposition. Without a subsurface ocean, a positive gravity anomaly requires an implausibly thick nitrogen layer (exceeding 40 kilometres). To prolong the lifetime of such a subsurface ocean to the present day8 and to maintain ocean uplift, a rigid, conductive water-ice shell is required. Because nitrogen deposition is latitude-dependent9, nitrogen loading and reorientation may have exhibited complex feedbacks7.

A parameterized convection model and observations of the puzzling polygons of the Sputnik Planum region of Pluto are used to compute the Rayleigh number of its nitrogen ice and show that it is vigorously convecting, kilometres thick and about a million years old. NASA's New Horizons spacecraft has revealed fascinating details of the surface of Pluto, including a vast ice-filled basin known as Sputnik Planum, which is central to Pluto's geological activity. Much of the surface of Sputnik Planum, consisting mostly of nitrogen ice, is divided into irregular polygons that are tens of kilometres in diameter and whose centres rise tens of metres above their sides. Two papers in this issue of Nature analyse New Horizons images of this polygonal terrain. Both conclude that it is continually being resurfaced by convection, but arrive at contrasting models for the process. Alexander Trowbridge et al. report a parameterized convection model in which the nitrogen ice is vigorously convecting, ten or more kilometres thick and about a million years old. William McKinnon et al. — from the New Horizons team — show that 'sluggish lid' convective overturn in a several-kilometre-thick layer of solid nitrogen can explain both the presence of the cells and their great width. Pluto’s surface is surprisingly young and geologically active1. One of its youngest terrains is the near-equatorial region informally named Sputnik Planum, which is a topographic basin filled by nitrogen (N2) ice mixed with minor amounts of CH4 and CO ices1. Nearly the entire surface of the region is divided into irregular polygons about 20–30 kilometres in diameter, whose centres rise tens of metres above their sides. The edges of this region exhibit bulk flow features without polygons1. Both thermal contraction and convection have been proposed to explain this terrain1, but polygons formed from thermal contraction (analogous to ice-wedges or mud-crack networks)2,3 of N2 are inconsistent with the observations on Pluto of non-brittle deformation within the N2-ice sheet. Here we report a parameterized convection model to compute the Rayleigh number of the N2 ice and show that it is vigorously convecting, making Rayleigh–Benard convection the most likely explanation for these polygons. The diameter of Sputnik Planum’s polygons and the dimensions of the ‘floating mountains’ (the hills of of water ice along the edges of the polygons) suggest that its N2 ice is about ten kilometres thick. The estimated convection velocity of 1.5 centimetres a year indicates a surface age of only around a million years.

The Hill stability of the nine known triple asteroid systems in the solar system has been investigated in a framework of the three body system. In this paper, the Sun and triple-asteroid system are treated as a four body system to analyze the influence of the Sun on the Hill stability of the triple subsystem. First, the relationship of the total energy and the angular momentum between the four body system and the triple subsystem is derived. It is found that the total energy of this 1–3 configuration four body system is the sum of the energy of the triple subsystem and the energy of a two-body system composed of the Sun and the mass center of the subsystem; so is the angular momentum. Then, the Hill stability of the triple subsystem is reinvestigated using a previous criterion in the four body problem (Gong and Liu in Mon. Not. R. Astron. Soc. 462:547–553, 2016) and the results are compared to those in the three body problem. Among the nine known triple-asteroid systems, 1995 CC and 1999 TC are Hill stable for both models; the others are stable in the three body model while not stable in the four body model. In addition, the exploration of Pluto by New Horizons has attracted great attention in recent years, the Sun-Pluto-Charon-Hydra four body system is investigated in the paper, and it is found that the system is Hill stable.

Leslie and Eliot Young have spent their lives studying Pluto. Now they are gearing up for the biggest event of their careers.

Nature Astronomy 1, 0016 (2017); published 4 January 2017; corrected 13 January 2017. In the version of this News and Views originally published the sentence giving the penitent formation timescale was incorrect and should have read: ‘Moreover, in order to explain the current height and tri-modal orientation, they infer that these features must be a few tens of millions of years old.

Data from NASA’s New Horizons mission begin to reveal the stories behind the dwarf planet's complex geology.

The icy dwarf planet Makemake has projected axes of 1,430 ± 9 and 1,502 ± 45 km and a V-band geometric albedo larger than Pluto’s but smaller than Eris’s, with no global Pluto-like atmosphere. Makemake is thought to be the third-largest dwarf planet in our Solar System, a little smaller than Pluto and Eris, but until now knowledge of its size and albedo were only approximate. This paper reports the results of observations of the occultation of a faint star known as NOMAD 1181-0235723 by Makemake on 23 April 2011. The data confirm that Makemake is smaller than Pluto and Eris, with axes of 1,430±9 km and 1,502±45 km. Makemake's mean geometric albedo — the ratio of light reflected to light received — is intermediate between that of Pluto and that of Eris. All three are icy, making them among the most reflective objects in the Solar System. And the occultation light curves rule out the presence of a global Pluto-like atmosphere on Makemake, although the presence of dark terrain might imply the presence of a localized atmosphere. Pluto and Eris are icy dwarf planets with nearly identical sizes, comparable densities and similar surface compositions as revealed by spectroscopic studies1,2. Pluto possesses an atmosphere whereas Eris does not; the difference probably arises from their differing distances from the Sun, and explains their different albedos3. Makemake is another icy dwarf planet with a spectrum similar to Eris and Pluto4, and is currently at a distance to the Sun intermediate between the two. Although Makemake’s size (1,420 ± 60 km) and albedo are roughly known5,6, there has been no constraint on its density and there were expectations that it could have a Pluto-like atmosphere4,7,8. Here we report the results from a stellar occultation by Makemake on 2011 April 23. Our preferred solution that fits the occultation chords corresponds to a body with projected axes of 1,430 ± 9 km (1σ) and 1,502 ± 45 km, implying a V-band geometric albedo pV = 0.77 ± 0.03. This albedo is larger than that of Pluto, but smaller than that of Eris. The disappearances and reappearances of the star were abrupt, showing that Makemake has no global Pluto-like atmosphere at an upper limit of 4–12 nanobar (1σ) for the surface pressure, although a localized atmosphere is possible. A density of 1.7 ± 0.3 g cm−3 is inferred from the data.

Modelling suggests that Pluto's atmospheric temperature is regulated by haze, unlike the other planetary bodies in the Solar System. The finding has implications for our understanding of exoplanetary atmospheres. See Letter p.352 Pluto's atmospheric temperature has been observed to be colder than predicted by standard theory. One possible explanation is the presence of water vapour, but it would have to be many orders of magnitude out of equilibrium to generate such a cool atmosphere. Xi Zhang and collaborators show through modelling that Pluto's atmospheric temperature is regulated by haze particles, rather than gas molecules, making it unique in the Solar System. This leads to a clear prediction that Pluto should be brighter than previously thought at mid-infrared wavelengths. This prediction should be testable next year with the launch of the James Webb Space Telescope.

There has been a renewed interest in the recent years for the study of gravitational theories alternative to General Relativity (GR). This results from current observations that seem to question the role of General Relativity theory at large distances, i.e., galactic and cosmological ones. An example of such theories is that of Modified Newtonian Dynamics or (MOND). In an effort for a description of gravitation in a quantum framework Horava has recently proposed the Horava–Lifshitz gravity (HL). Our goal is to study the effect of radar signal delays in the vicinity of the sun for the Kehagias–Sfetsos space-time solution of the Horava–Lifshitz modified gravity. For that we use a spherically symmetric modified solar system space-time metric and we calculate the radar signal delay correction as a function of the non-dimensional HL parameter ψ. Next expressing ψ as a function of the planetary orbital elements we derive an expression for the Kehagias–Sfetsos radar time delay solution, which next calculate for all the planets in the solar system excluding Pluto. Finally, for the planets Mercury to Neptune, we obtain radar time delays in the range of \(531\times 10^{-9}~\mathrm{ps} \le t_{\mathit{KS}_{\mathit{MER}}} \le 5379.86~\mathrm{ps}\) for the planets Mercury to Neptune.

In this work, the GOCE satellite orbit is described in the aspect of perturbations in the Keplerian osculating elements. The perturbations come from the Earth and ocean tides, the gravitation of the Moon, the gravitation of the Sun, the gravitation of planets and Pluto, and the relativity effects. These perturbations are computed for the 30-day interval with a sampling of 2 min. To obtain the simulated orbit, the Cowell numerical integration method of 8th order is used. The first part of the work contains the root mean square (RMS) values of aforementioned perturbations due to the specified forces. The perturbations were compared taking into account their RMS characteristics.

We describe the response of the Solar Wind Around Pluto (SWAP) instrument (McComas et al. in Space Sci. Rev. 140:261, 2008) to 1–40 amu ions in order to assess whether it can be used to determine plasma composition. Our goal is to enhance the scientific return on the SWAP plasma measurements obtained during the New Horizons traversal down Jupiter’s magnetotail in 2007. We present calibration data for the SWAP flight instrument and another largely flight-like SWAP sensor, dubbed “SWAP-II”. SWAP’s mass-dependent response was characterized by analyzing the count ratios from its two channel electron multipliers (CEMs). We observe significant differences in the instrument response between light (mass ≤ He) and heavy (mass > He) ions, especially for energies below ∼4 keV. We attribute these differences to the mass-dependent electron emission yield from SWAP’s ultra-thin (∼1 μg/cm2) carbon foil. Using these results, we develop a plasma composition analysis technique to statistically distinguish between light and heavy plasma ions measured by the instrument.

The Lyman Alpha Mapping Project (LAMP) is a far-ultraviolet (FUV) imaging spectrograph on NASA’s Lunar Reconnaissance Orbiter (LRO) mission. Its main objectives are to (i) identify and localize exposed water frost in permanently shadowed regions (PSRs), (ii) characterize landforms and albedos in PSRs, (iii) demonstrate the feasibility of using natural starlight and sky-glow illumination for future lunar surface mission applications, and (iv) characterize the lunar atmosphere and its variability. As a byproduct, LAMP will map a large fraction of the Moon at FUV wavelengths, allowing new studies of the microphysical and reflectance properties of the regolith. The LAMP FUV spectrograph will accomplish these objectives by measuring the signal reflected from the night-side lunar surface and in PSRs using both the interplanetary HI Lyman-α sky-glow and FUV starlight as light sources. Both these light sources provide fairly uniform, but faint, illumination. With the expected LAMP sensitivity, by the end of the primary 1-year LRO mission, the SNR for a Lyman-α albedo map should be >100 in polar regions >1 km2, providing useful FUV constraints to help characterize subtle compositional and structural features. The LAMP instrument is based on the flight-proven Alice series of spectrographs flying on the Rosetta comet mission and the New Horizons Pluto mission. A general description of the LAMP instrument and its initial ground calibration results are presented here.

An accurate development of the latest JPL’s numerical ephemeris of Pluto, DE421, to compact analytical series is done. Rectangular barycentric ICRF coordinates of Pluto from DE421 are approximated by compact Fourier series with a maximum error of 1.3 km over 1900–2050 (the entire time interval covered by the ephemeris). To calculate Pluto positions relative to the Sun, a development of rectangular heliocentric ICRF coordinates of the Solar System barycenter to Poisson series is additionally made. As a result, DE421 Pluto heliocentric positions by the new analytical series are represented to an accuracy of better than 5 km over 1900–2050.

From differential tracking techniques, required for appulse observations of KBOs with Laser Guide Star Adaptive Optics (LGSAO), to developing methods for collecting spectra at the precise moment of a predicted impact, each Solar System observation conducted on a large telescope presents a unique set of challenges. We present operational details and some key science results from our science program, adaptive optics observations of main belt asteroids and near earth objects; as well as the technical and operational details of several Keck Solar System observations conducted by other teams: the impact of Shoemaker-Levy 9 on Jupiter, volcanoes on Io, the Deep Impact mission to Comet 9P/Tempel 1, and recent observations of Pluto’s moons Nix and Hydra. For each of these observations, we draw from our Keck experience to predict what challenges may lie ahead when similar observations are conducted on next generation telescopes.

The New Horizons ALICE instrument is a lightweight (4.4 kg), low-power (4.4 Watt) imaging spectrograph aboard the New Horizons mission to Pluto/Charon and the Kuiper Belt. Its primary job is to determine the relative abundances of various species in Pluto's atmosphere. ALICE will also be used to search for an atmosphere around Pluto's moon, Charon, as well as the Kuiper Belt Objects (KBOs) that New Horizons hopes to fly by after Pluto-Charon, and it will make UV surface reflectivity measurements of all of these bodies as well. The instrument incorporates an off-axis telescope feeding a Rowland-circle spectrograph with a 520-1870 angstroms spectral passband, a spectral point spread function of 3-6 angstroms FWHM, and an instantaneous spatial field-of-view that is 6 degrees long. Different input apertures that feed the telescope allow for both airglow and solar occultation observations during the mission. The focal plane detector is an imaging microchannel plate (MCP) double delay-line detector with dual solar-blind opaque photocathodes (KBr and CsI) and a focal surface that matches the instrument's 15-cm diameter Rowland-circle. In what follows, we describe the instrument in greater detail, including descriptions of its ground calibration and initial in flight performance., Comment: 24 pages, 29 figures, 2 tables; To appear in a special volume of Space Science Reviews on the New Horizons mission

The New Horizons (NH) Radio Science Experiment, REX, is designed to determine the atmospheric state at the surface of Pluto and in the lowest few scale heights. Expected absolute accuracies in n, p, and T at the surface are 4⋅1019 m−3, 0.1 Pa, and 3 K, respectively, obtained by radio occultation of a 4.2 cm-λ signal transmitted from Earth at 10–30 kW and received at the NH spacecraft. The threshold for ionospheric observations is roughly 2⋅109 e− m−3. Radio occultation experiments are planned for both Pluto and Charon, but the level of accuracy for the neutral gas is expected to be useful at Pluto only. REX will also measure the nightside 4.2 cm-λ thermal emission from Pluto and Charon during the time NH is occulted. At Pluto, the thermal scan provides about five half-beams across the disk; at Charon, only disk integrated values can be obtained. A combination of two-way tracking and occultation signals will determine the Pluto system mass to about 0.01 percent, and improve the Pluto–Charon mass ratio. REX flight equipment augments the NH radio transceiver used for spacecraft communications and tracking. Implementation of REX required realization of a new CIC-SCIC signal processing algorithm; the REX hardware implementation requires 1.6 W, and has mass of 160 g in 520 cm3. Commissioning tests conducted after NH launch demonstrate that the REX system is operating as expected.

NASA's New Horizons (NH) Pluto-Kuiper belt (PKB) mission was launched on 19 January 2006 on a Jupiter Gravity Assist (JGA) trajectory toward the Pluto system for a 14 July 2015 closest approach; Jupiter closest approach occurred on 28 February 2007. It was competitively selected by NASA for development on 29 November 2001. New Horizons is the first mission to the Pluto system and the Kuiper belt; and will complete the reconnaissance of the classical planets. The ~400 kg spacecraft carries seven scientific instruments, including imagers, spectrometers, radio science, a plasma and particles suite, and a dust counter built by university students. NH will study the Pluto system over a 5-month period beginning in early 2015. Following Pluto, NH will go on to reconnoiter one or two 30-50 kilometer diameter Kuiper belt Objects (KBOs), if NASA approves an extended mission. If successful, NH will represent a watershed development in the scientific exploration of a new class of bodies in the solar system - dwarf planets, of worlds with exotic volatiles on their surfaces, of rapidly (possibly hydrodynamically) escaping atmospheres, and of giant impact derived satellite systems. It will also provide the first dust density measurements beyond 18 AU, cratering records that shed light on both the ancient and present-day KB impactor population down to tens of meters, and a key comparator to the puzzlingly active, former dwarf planet (now satellite of Neptune) called Triton, which is as large as Eris and Pluto., Comment: 18 pages, 4 figures, 2 tables; To appear in a special volume of Space Science Reviews on the New Horizons mission

Pluto is a celestial object that had caught the imagination of increasingly ‘sky-aware’ people all around the world, perhaps due to the inspiring story of its discovery by a young amateur astronomer. It was celebrated as the ninth planet of the solar system for decades. A question mark hung over its nomenclature as a planet from the time of the discovery of its moon Charon, when a good estimate of its very small mass in comparison with other planets was obtained. The discovery of UB 313, a body in the outer regions of the solar system that was more massive than Pluto, magnified this question mark and compelled the International Astronomical Union to review the definition of the word ‘Planet’. A new and physical definition was given to this word and Pluto was categorized as a dwarf planet. The beauty of this physical definition of a planet is something that would definitely have been appreciated by Clyde Tombaugh — who once made this beautifully simple statement about Pluto, while its status was being questioned — “It is there, whatever it is”. Yes, whatever it is, Pluto is one celestial object that is somehow very endearing to everyone around the world and is also an object whose studies are compelling certain paradigm shifts in our understanding of the structure and evolution of the solar system.

International audience; Thanks to its sensitivity, spatial resolution and instantaneous uv-coverage, ALMA will permit many new studies related to the general topic of the couplings between chemistry and dynamics in planetary atmospheres. It will include: (1) three-dimensional mapping of composition, temperatures and winds in the atmospheres of Mars, Venus and Titan; (2) several aspects of Giant Planet composition and dynamics, such as the origin of oxygen, the evolution of Shoemaker Levy 9 products in Jupiter's atmosphere, and the deep atmosphere structure and meteorology; (3) the study of tenuous and distant atmospheres (Io, Enceladus, Pluto, Triton and other Kuiper Belt objects).

In the first mission to Pluto, the New Horizons spacecraft was launched on January 19, 2006, and flew by Jupiter on February 28, 2007, gaining a significant speed boost from Jupiter’s gravity assist. After a 9.5-year journey, the spacecraft will encounter Pluto on July 14, 2015, followed by an extended mission to the Kuiper Belt objects for the first time. The mission design for New Horizons went through more than five years of numerous revisions and updates, as various mission scenarios regarding routes to Pluto and launch opportunities were investigated in order to meet the New Horizons mission’s objectives, requirements, and goals. Great efforts have been made to optimize the mission design under various constraints in each of the key aspects, including launch window, interplanetary trajectory, Jupiter gravity-assist flyby, Pluto–Charon encounter with science measurement requirements, and extended mission to the Kuiper Belt and beyond. Favorable encounter geometry, flyby trajectory, and arrival time for the Pluto–Charon encounter were found in the baseline design to enable all of the desired science measurements for the mission. The New Horizons mission trajectory was designed as a ballistic flight from Earth to Pluto, and all energy and the associated orbit state required for arriving at Pluto at the desired time and encounter geometry were computed and specified in the launch targets. The spacecraft’s flight thus far has been extremely efficient, with the actual trajectory error correction ΔV being much less than the budgeted amount.

In order to satisfy the equivalence principle, any non-conventional mechanism proposed to gravitationally explain the Pioneer anomaly, in the form in which it is presently known from the so-far analyzed Pioneer 10/11 data, cannot leave out of consideration its impact on the motion of the planets of the Solar System as well, especially those orbiting in the regions in which the anomalous behavior of the Pioneer probes manifested itself. In this paper we, first, discuss the residuals of the right ascension \alpha and declination \delta of Uranus, Neptune and Pluto obtained by processing various data sets with different, well established dynamical theories (JPL DE, IAA EPM, VSOP). Second, we use the latest determinations of the perihelion secular advances of some planets in order to put on the test two gravitational mechanisms recently proposed to accommodate the Pioneer anomaly based on two models of modified gravity. Finally, we adopt the ranging data to Voyager 2 when it encountered Uranus and Neptune to perform a further, independent test of the hypothesis that a Pioneer-like acceleration can also affect the motion of the outer planets of the Solar System. The obtained answers are negative., Comment: Latex2e, 26 pages, 6 tables, 2 figure, 47 references. It is the merging of gr-qc/0608127, gr-qc/0608068, gr-qc/0608101 and gr-qc/0611081. Final version to appear in Foundations of Physics

First published findings from NASA’s New Horizons mission lay out the dwarf planet’s wildly varying terrain.

Data from New Horizons mission also reveal a hazy atmosphere that is growing colder and thinner.

Latest images from New Horizons mission show a patchwork terrain, and a surprisingly youthful Charon.

Nature Video’s guide to the highlights as the New Horizons spacecraft speeds past the icy dwarf planet.