Formation of Kuiper-belt binaries by dynamical friction and three-body encounters

The Kuiper belt is a disk of icy bodies that orbit the Sun beyond Neptune; the largest known members are Pluto and its companion Charon. A few per cent of Kuiper-belt bodies have recently been found to be binaries with wide separations and mass ratios of the order of unity. Collisions were too infrequent to account for the observed number of binaries, implying that these binaries formed through collisionless interactions mediated by gravity. These interactions are likely to have been most effective during the period of runaway accretion, early in the Solar System's history. Here we show that a transient binary forms when two large bodies penetrate one another's Hill sphere (the region where their mutual forces are larger than the tidal force of the Sun). The loss of energy needed to stabilize the binary orbit can then occur either through dynamical friction from surrounding small bodies, or through the gravitational scattering of a third large body. Our estimates slightly favour the former mechanism. We predict that five per cent of Kuiper-belt objects are binaries with apparent separations greater than 0.2 arcsec, and that most are in tighter binaries or systems of higher multiplicity.     

冥王星的奇特轨道性质

被称为第九大行星的冥王星具有与其它八大行星不同的高倾角、高偏心率的共振锁定轨道，而在新发现的外太阳系的Kuiper带中许多天体却具有与之相似的轨道性质，因此冥王星更像是一颗Kuiper带天体而不是大行星．     

The trans-neptunian object UB313 is larger than Pluto

The most distant known object in the Solar System, 2003 UB313 (97 au from the Sun), was recently discovered near its aphelion. Its high eccentricity and inclination to the ecliptic plane, along with its perihelion near the orbit of Neptune, identify it as a member of the 'scattered disk'. This disk of bodies probably originates in the Kuiper belt objects, which orbit near the ecliptic plane in circular orbits between 30 and 50 au, and may include Pluto as a member. The optical brightness of 2003 UB313, if adjusted to Pluto's distance, is greater than that of Pluto, which suggested that it might be larger than Pluto. The actual size, however, could not be determined from the optical measurements because the surface reflectivity (albedo) was unknown. Here we report observations of the thermal emission of 2003 UB313 at a wavelength of 1.2 mm, which in combination with the measured optical brightness leads to a diameter of 3,000 +/- 300 +/- 100 km. Here the first error reflects measurement uncertainties, while the second derives from the unknown object orientation. This makes 2003 UB313 the largest known trans-neptunian object, even larger than Pluto (2,300 km). The albedo is 0.60 +/- 0.10 +/- 0.05, which is strikingly similar to that of Pluto, suggesting that the methane seen in the optical spectrum causes a highly reflective icy surface.     

Discovery of two new satellites of Pluto

Pluto's first known satellite, Charon, was discovered in 1978. It has a diameter (approximately 1,200 km) about half that of Pluto, which makes it larger, relative to its primary, than any other moon in the Solar System. Previous searches for other satellites around Pluto have been unsuccessful, but they were not sensitive to objects less, similar150 km in diameter and there are no fundamental reasons why Pluto should not have more satellites. Here we report the discovery of two additional moons around Pluto, provisionally designated S/2005 P 1 (hereafter P1) and S/2005 P 2 (hereafter P2), which makes Pluto the first Kuiper belt object known to have multiple satellites. These new satellites are much smaller than Charon, with estimates of P1's diameter ranging from 60 km to 165 km, depending on the surface reflectivity; P2 is about 20 per cent smaller than P1. Although definitive orbits cannot be derived, both new satellites appear to be moving in circular orbits in the same orbital plane as Charon, with orbital periods of approximately 38 days (P1) and approximately 25 days (P2).     

A giant impact origin for Pluto's small moons and satellite multiplicity in the Kuiper belt

The two newly discovered satellites of Pluto (P1 and P2) have masses that are small compared to both Pluto and Charon-that is, between 5 x 10(-4) and 1 x 10(-5) of Pluto's mass, and between 5 x 10(-3) and 1 x 10(-4) of Charon's mass. This discovery, combined with the constraints on the absence of more distant satellites of Pluto, reveal that Pluto and its moons comprise an unusual, highly compact, quadruple system. These facts naturally raise the question of how this puzzling satellite system came to be. Here we show that P1 and P2's proximity to Pluto and Charon, the fact that P1 and P2 are on near-circular orbits in the same plane as Pluto's large satellite Charon, along with their apparent locations in or near high-order mean-motion resonances, all probably result from their being constructed from collisional ejecta that originated from the Pluto-Charon formation event. We also argue that dust-ice rings of variable optical depths form sporadically in the Pluto system, and that rich satellite systems may be found--perhaps frequently--around other large Kuiper belt objects.     

用密度波理论描述旋涡星系的演化方向和太阳系的八大行星模式

基于密度波理论的非线性方程,用定性分析理论可导出旋涡星系的演化方向和由观测可得到的演化条件,根据最新的观测结果,作者定量论证了太阳系的八大行星模式：它是对称的四个类地行星和小行星带;四个类木行星和柯伊伯带(包括冥王星)。     

Stellar encounters as the origin of distant Solar System objects in highly eccentric orbits

The Kuiper belt extends from the orbit of Neptune at 30 au to an abrupt outer edge about 50 au from the Sun. Beyond the edge is a sparse population of objects with large orbital eccentricities. Neptune shapes the dynamics of most Kuiper belt objects, but the recently discovered planet 2003 VB12 (Sedna) has an eccentric orbit with a perihelion distance of 70 au, far beyond Neptune's gravitational influence. Although influences from passing stars could have created the Kuiper belt's outer edge and could have scattered objects into large, eccentric orbits, no model currently explains the properties of Sedna. Here we show that a passing star probably scattered Sedna from the Kuiper belt into its observed orbit. The likelihood that a planet at 60-80 au can be scattered into Sedna's orbit is about 50 per cent; this estimate depends critically on the geometry of the fly-by. Even more interesting is the approximately 10 per cent chance that Sedna was captured from the outer disk of the passing star. Most captures have very high inclination orbits; detection of such objects would confirm the presence of extrasolar planets in our own Solar System.     

Saturn's moon Phoebe as a captured body from the outer Solar System

The orbital properties of Phoebe, one of Saturn's irregular moons, suggest that it was captured by the ringed planet's gravitational field rather than formed in situ. Phoebe's generally dark surface shows evidence of water ice, but otherwise the surface most closely resembles that of C-type asteroids and small outer Solar System bodies such as Chiron and Pholus that are thought to have originated in the Kuiper belt. A close fly-by of Phoebe by the Cassini-Huygens spacecraft on 11 June 2004 (19 days before the spacecraft entered orbit around Saturn) provided an opportunity to test the hypothesis that this moon did not form in situ during Saturn's formation, but is instead a product of the larger protoplanetary disk or 'solar nebula'. Here we derive the rock-to-ice ratio of Phoebe using its density combined with newly measured oxygen and carbon abundances in the solar photosphere. Phoebe's composition is close to that derived for other solar nebula bodies such as Triton and Pluto, but is very different from that of the regular satellites of Saturn, supporting Phoebe's origin as a captured body from the outer Solar System.     

Charon's size and an upper limit on its atmosphere from a stellar occultation

Pluto and its satellite, Charon (discovered in 1978; ref. 1), appear to form a double planet, rather than a hierarchical planet/satellite couple. Charon is about half Pluto's size and about one-eighth its mass. The precise radii of Pluto and Charon have remained uncertain, leading to large uncertainties on their densities. Although stellar occultations by Charon are in principle a powerful way of measuring its size, they are rare, as the satellite subtends less than 0.3 microradians (0.06 arcsec) on the sky. One occultation (in 1980) yielded a lower limit of 600 km for the satellite's radius, which was later refined to 601.5 km (ref. 4). Here we report observations from a multi-station stellar occultation by Charon, which we use to derive a radius, R(C) = 603.6 +/- 1.4 km (1sigma), and a density of rho = 1.71 +/- 0.08 g cm(-3). This occultation also provides upper limits of 110 and 15 (3sigma) nanobar for an atmosphere around Charon, assuming respectively a pure nitrogen or pure methane atmosphere.     

A Pluto-like radius and a high albedo for the dwarf planet Eris from an occultation

The dwarf planet Eris is a trans-Neptunian object with an orbital eccentricity of 0.44, an inclination of 44 degrees and a surface composition very similar to that of Pluto. It resides at present at 95.7 astronomical units (1?AU is the Earth-Sun distance) from Earth, near its aphelion and more than three times farther than Pluto. Owing to this great distance, measuring its size or detecting a putative atmosphere is difficult. Here we report the observation of a multi-chord stellar occultation by Eris on 6 November 2010 UT. The event is consistent with a spherical shape for Eris, with radius 1,163?±?6?kilometres, density 2.52?±?0.05 grams per cm(3) and a high visible geometric albedo, Pv = 0.96(+0.09)(-0.04). No nitrogen, argon or methane atmospheres are detected with surface pressure larger than ～1?nanobar, about 10,000 times more tenuous than Pluto's present atmosphere. As Pluto's radius is estimated to be between 1,150 and 1,200 kilometres, Eris appears as a Pluto twin, with a bright surface possibly caused by a collapsed atmosphere, owing to its cold environment. We anticipate that this atmosphere may periodically sublimate as Eris approaches its perihelion, at 37.8 astronomical units from the Sun.     

Crystalline water ice on the Kuiper belt object (50000) Quaoar

The Kuiper belt is a disk-like structure consisting of solid bodies orbiting the Sun beyond Neptune. It is the source of the short-period comets and the likely repository of the Solar System's most primitive materials. Surface temperatures in the belt are low ( approximately 50 K), suggesting that ices trapped at formation should have been preserved over the age of the Solar System. Unfortunately, most Kuiper belt objects are too faint for meaningful compositional study, even with the largest available telescopes. Water ice has been reported in a handful of objects, but most appear spectrally featureless. Here we report near-infrared observations of the large Kuiper belt object (50000) Quaoar, which reveal the presence of crystalline water ice and ammonia hydrate. Crystallinity indicates that the ice has been heated to at least 110 K. Both ammonia hydrate and crystalline water ice should be destroyed by energetic particle irradiation on a timescale of about 10(7) yr. We conclude that Quaoar has been recently resurfaced, either by impact exposure of previously buried (shielded) ices or by cryovolcanic outgassing, or by a combination of these processes.     

Neptune's capture of its moon Triton in a binary-planet gravitational encounter

Triton is Neptune's principal satellite and is by far the largest retrograde satellite in the Solar System (its mass is approximately 40 per cent greater than that of Pluto). Its inclined and circular orbit lies between a group of small inner prograde satellites and a number of exterior irregular satellites with both prograde and retrograde orbits. This unusual configuration has led to the belief that Triton originally orbited the Sun before being captured in orbit around Neptune. Existing models for its capture, however, all have significant bottlenecks that make their effectiveness doubtful. Here we report that a three-body gravitational encounter between a binary system (of approximately 10(3)-kilometre-sized bodies) and Neptune is a far more likely explanation for Triton's capture. Our model predicts that Triton was once a member of a binary with a range of plausible characteristics, including ones similar to the Pluto-Charon pair.     

小波分析在视频检测中的应用

针对复杂背景下机器视觉系统中运动目标的检测,提出一种基于小波分析的运动目标检测方法。首先,利用双Haar小波软阈值法,去除视频图像的噪声。然后,建立了一种基于小波变换的运动目标检测算法,直接在二进小波变换域提取运动区域,从而检测出运动目标。实验结果表明,本文方法可以有效地从图像序列中检测出完整的运动目标,而且在平滑噪声的同时还可以保护图像边缘细节不受损失,效果要好于传统帧差法。     

The formation of Kuiper-belt binaries through exchange reactions

Recent observations have revealed that an unexpectedly high fraction--a few per cent--of the trans-Neptunian objects (TNOs) that inhabit the Kuiper belt are binaries. The components have roughly equal masses, with very eccentric orbits that are wider than a hundred times the radius of the primary. Standard theories of binary asteroid formation tend to produce close binaries with circular orbits, so two models have been proposed to explain the unique characteristics of the TNOs. Both models, however, require extreme assumptions regarding the size distribution of the TNOs. Here we report a mechanism that is capable of producing binary TNOs with the observed properties during the early stages of their formation and growth. The only required assumption is that the TNOs were initially formed through gravitational instabilities in the protoplanetary dust disk. The basis of the mechanism is an exchange reaction in which a binary whose primary component is much more massive than the secondary interacts with a third body, whose mass is comparable to that of the primary. The low-mass secondary component is ejected and replaced by the third body in a wide but eccentric orbit.     

Shape classification based on singular value decomposition transform

In this paper, a new shape classification system based on singular value decomposition (SVD) transform using nearest neighbour classifier was proposed. The gray scale image of the shape object was converted into a black and white image. The squared Euclidean distance transform on binary image was applied to extract the boundary image of the shape. SVD transform features were extracted from the the boundary of the object shapes. In this paper, the proposed classification system based on SVD transform feature...     

论楚文化对蛇的信仰和恐惧

中国的古代文化与蛇有着密切的关系，蛇崇拜有着十分丰富的文化涵义。蛇的象征符号功能往往足对立的、二元的，如：1．水神／旱神；2．男性／女性；3．地主和社神；4．冥神／邪祟；5．飞升／入地；6．重牛／死亡；7．智慧／阴险；8．祥瑞／灾异。蛇崇拜也是楚文化的重要内容之一，楚文化的文献记载和出土文物有很多蛇的资料。从四个方面阐述了楚文化对蛇的信仰和恐惧。楚文化中的蛇崇拜具有以下特色：1．楚人先祖与蛇有着密切的关系；2．蛇为冥神——保护神；3．蛇为邪祟——破坏者；4．蛇与鸟——死与生。蛇在楚文化系统中散发出独具魅力的风采，也让我们对楚先民的智慧产生了由衷的敬意。     

The formation of the Kuiper belt by the outward transport of bodies during Neptune's migration

The 'dynamically cold Kuiper belt' consists of objects on low-inclination orbits between approximately 40 and approximately 50 au from the Sun. It currently contains material totalling less than a tenth the mass of the Earth, which is surprisingly low because, according to accretion models, the objects would not have grown to their present size unless the cold Kuiper belt originally contained tens of Earth masses of solids. Although several mechanisms have been proposed to produce the observed mass depletion, they all have significant limitations. Here we show that the objects currently observed in the dynamically cold Kuiper belt were most probably formed within approximately 35 au and were subsequently pushed outward by Neptune's 1:2 mean motion resonance during its final phase of migration. Combining our mechanism with previous work, we conclude that the entire Kuiper belt formed closer to the Sun and was transported outward during the final stages of planet formation.     

A collisional family of icy objects in the Kuiper belt

The small bodies in the Solar System are thought to have been highly affected by collisions and erosion. In the asteroid belt, direct evidence of the effects of large collisions can be seen in the existence of separate families of asteroids--a family consists of many asteroids with similar orbits and, frequently, similar surface properties, with each family being the remnant of a single catastrophic impact. In the region beyond Neptune, in contrast, no collisionally created families have hitherto been found. The third largest known Kuiper belt object, 2003 EL61, however, is thought to have experienced a giant impact that created its multiple satellite system, stripped away much of an overlying ice mantle, and left it with a rapid rotation. Here we report the discovery of a family of Kuiper belt objects with surface properties and orbits that are nearly identical to those of 2003 EL61. This family appears to be fragments of the ejected ice mantle of 2003 EL61.     

Shape classification based on singular value decomposition transform

In this paper, a new shape classification system based on singular value decomposition (SVD) transform using nearest neighbour classifier was proposed. The gray scale image of the shape object was converted into a black and white image. The squared Euclidean distance transform on binary image was applied to extract the boundary image of the shape. SVD transform features were extracted from the the boundary of the object shapes. In this paper, the proposed classification system based on SVD transform feature extraction method was compared with classifier based on moment invariants using nearest neighbour classifier. The experimental results showed the advantage of our proposed classification system.     

基于纹理和直方图特征相融合的镜头边界检测方法

镜头切换检测是视频检索研究领域中一项基本的、重要的技术,对于数字视频镜头突变切换的检测,一般有模板匹配法、直方图法等基本算法,但这些算法在实际检测中通常达不到较高的检测精度。因此,提出了一种特征融合的镜头边界检测方法。HSV色彩直方图进行镜头边界检测是一种常用、有效的方法,该文通过小波变换得到视频帧的纹理特征,并将其与色彩直方图特征相融合,采用滑动窗口技术对视频镜头进行检测。实验表明,该方法能够有效地检测镜头突变,对镜头的渐变也能有较好地检测。