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Dynamical balance arguments that involve the capture of long-period comets from the Oort cloud imply that there should be ∼ 1000 times more Halley-type objects than are actually observed. If the active comets rapidly become dormant, with albedos comparable to those of known cometary surfaces, hundreds of such bodies should by now have been detected whereas in fact only a few have been found. If, on the other hand, they disintegrate to dust, we show here that the debris would create a bright, near-spherical zodiacal cloud and ∼ 15‐30 strong annual meteor showers, also contrary to observation. Here we demonstrate that the surfaces of inactive comets, if composed of loose, fluffy organic material like cometary meteoroids, develop reflectivities that are vanishingly small in visible light. The near-Earth objects may therefore be dominated by a population of fast, multi-kilometre bodies too dark to be seen with current near-Earth object surveys. Deflection strategies that assume decades or centuries of warning before impact are inapplicable to this hazard.

We investigate the effect that a close stellar encounter would have on the growing scattered disk and Oort cloud. Such an encounter has been suggested by Melita et al. as the cause of the Kuiper belt's outer edge. Thus, we restrict our study to encounters that could have caused such a structure. We find that we probably can rule out all such encounters that occurred either at or subsequent to 10 million years after the Oort cloud started to form. In our simulations, these encounters either produce an extended scattered disk that is too populous to be consistent with observations or produce an Oort cloud that is too anemic.

In this paper, a source of cometary outbursts is examined. As a potential cause of some outbursts, impacts of a comet with meteoroids are considered. The probabilities and consequences of such events have been investigated in relation to the case of the Comet 29P/Schwassmann–Wachmann 1 outbursts. This paper focuses on two problems. The first one concerns the changes in brightness of the comet after the impact of a meteoroid into its nucleus. It is shown that impacts of comets with even metre-sized solid impactors give an immediate considerable jump in their brightness consistent with that observed during real outbursts. The second problem is to estimate how real such events are. It is concluded that the probability of impacts with metre-sized projectiles in relation to the Comet 29P/Schwassmann–Wachmann 1 is very low – practically negligible. The possibility of self-bombardment of this comet by pieces of cometary material ejected earlier from its nucleus was also discussed. It is shown that the probability of such events is also hardly probable. That is why the main conclusion of this paper is that the impact mechanism could not trigger the outburst activity of the considered comet.

For an Oort cloud comet to be seen as a new comet, its perihelion must be moved from a point exterior to the loss cylinder boundary to a point interior to observable limits in a single orbit. The galactic tide can do this continuously, in a non-impulsive manner. Near-parabolic comets, with specific angular momentum H∝ q , will most easily be made observable. Therefore, to reduce the perihelion distance H must decrease. Since weakly perturbed comets are, in general, more numerous than strongly perturbed comets, we can anticipate that new comets made observable by a weak tidal torque will more likely be first observed when their slowly changing perihelion distances are approaching their minimum osculating values under the action of the tide, rather than receding from their minimum values. That is, defining ΔHtide as the vector change due to the galactic tidal torque during the prior orbit, and Hobs as the observed vector, the sign S≡Sign(Hobs·ΔHtide) will more likely be −1 than +1 if a weak galactic tidal perturbation indeed dominates in making comets observable. Using comet data of the highest quality class (1A) for new comets (a>10,000 AU), we find that 49 comets have S=−1 and 22 have S=+1. The binomial probability that as many or more would exhibit this characteristic if in fact S=∓1 were equally likely is only 0.0009. This characteristic also persists in other long-period comet populations, lending support to the notion that they are dominated by comets recently arrived from the outer Oort cloud. The preponderance of S=−1 also correlates with weakly perturbed (i.e., smaller semimajor axis) new comets in a statistically significant manner. This is strong evidence that the data are of sufficiently high quality and sufficiently free of observational selection effects to detect this unique imprint of the tide.

The source of long-period comets can be numerically modeled by means of Monte Carlo simulation of the Oort cloud dynamics. Tracing a comet motion under the galactic per- turbations over a long time interval requires taking into account of planetary perturbations. A method for the approximate treating of the planetary perturbations in such simulations is de- scribed. Some peculiarities found in planetary action on long-period simulated cometary sample are also discussed.

The contribution of high-eccentricity trans-Neptunian objects to the observed populations of both short-period and long-period comets is estimated. About $10^{10}$ objects with a radius $R>0.7$ km in orbits with perihelion distances $28 AU and semimajor axes $60 AU are the main source of Jupiter-family comets. If the population of high-eccentricity trans-Neptunian objects formed about 4.5 Gyr ago, the mean near-parabolic flux produced by these objects is on the order of 0.3-1.0 AU $^{-1}$ yr $^{-1}$ for comets with $R>0.7$ km. To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

After the last apparition of comet Halley in 1985-86, a number of other comets were observed in polarimetry with IHW-continuum filters. From the in situ dust measurements of Halley, dust size distribution functions were obtained (Lamy et al. 1987, AA Mazets et al. 1986, Nature, 321, 276), which were later used by several authors to interpret polarisation data of Halley. In the present work, polarimetric data of various comets have been analyzed, using Mie theory and assuming that the composition of dust particles does not differ from comet to comet (Delsemme 1987, ESA SP-278, 19). The individual grain size distribution functions so obtained for various comets suggest different values for the relative abundance of coarser grains. Introducing a "relative abundance of coarser grains" index g, we study any possible dependence of g on the dynamical age of a comet, where the dynamical age can be defined in terms of some of the orbital parameters of the comet. For the four non-periodic comets available for our analysis, we find a clear empirical relation g = −2.5.q 2/3 emerging from our work. This relation strengthens the concept that comets whose grains are processed more by the solar radiation (these comets may be dynamically older) contain a relatively smaller number of finer grains. The case for other periodic comets is also discussed here. Since the work has been carried out using Mie scattering theory meant for perfect compact spheres, it is also suggested to repeat the calculations with more realistic porous grains in a follow-up paper.

We estimated the gravitational influence of giant molecular clouds passing near the Solar system on the orbital evolution of Oort cloud comets. We performed a comparative analysis of the accuracies of the following two methods of allowance for the perturbations from giant molecular clouds: the impulse approximation and numerical integration. The impulse approximation yields fairly accurate estimates of the change in the energy of Oort cloud comets and the probability of their ejection under the influence of a molecular cloud if the path of the Solar system does not cross its boundary and if the molecular cloud may be treated as a point perturbing mass. The comet survival probability in the Oort cloud depends significantly on the internal structure of the perturbing molecular cloud and the impact parameter of the encounter. The most massive injection of comets into the planetary region and their ejection from the Oort cloud take place if the Solar system passes through a giant molecular cloud composed of several high-mass condensations. In this case, most of the comets injected into the planetary region were initially comets of the inner Oort cloud (a ≤ 10–4 AU) with high orbital eccentricities.

We analyze the conditions for the formation and time evolution of peripheral comet structures of solar-type planetary systems. In the Solar system, these include the Kuiper belt, the Oort cloud, the comet spear, and the Galactic comet ring that marks the Galactic orbit of the Sun. We consider the role of the viscosity of a protoplanetary gas–dust disk, major planets, field stars, globular clusters, giant molecular clouds, and the Galactic gravitational field in the formation of these peripheral structures marked by comets and asteroids. We give a list of the closest past and future passages of neighboring stars through the solar Oort cloud that perturb the motion of its comets and, thus, contribute to the enhancement of its cometary activity, on the one hand, and to the replenishment of the solar comet spear with new members, on the other hand.

The possibility of impacts and their results in relation to the cometary outbursts between comets and other small bodies in the solar system has been investigated. Taking into consideration certain physical features of cometary nuclei and impacting bodies, the probability of impacts of small bodies moving in the main asteroid belt with hypothetical comets which represent three types: Jupiter family comets, Halley family comets and long period comets has been computed. The probability of impacts between comets and meteoroids at large heliocentric distances has also been estimated. Potential consequences of these events in relation to outbursts of the cometary brightness have been discussed. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Charging characteristics of dust grains are analyzed, and typical parameters of dust plasmas in comets are computed. Discussion is made for Comet Halley and Comet G-Z, and some main regularities of the equilibrium potential of dust grains are found. Generally speaking, these regularities can also be applied to the plasmas of other comets.

We present our current understandings of small bodies and dust grains located in the outer Solar System. Small icy bodies – Edgeworth-Kuiper Belt objects (EKBOs) and Oort Cloud objects orbit the Sun at distances from Neptune’s orbit outward to 10 4 –10 5 AU. Both EKBOs and Oort Cloud objects are believed to be remnants of planetesimals formed in the proto-planetary disk. They provide a possible source for icy bodies that enter the inner Solar System and are observed as comets. A possible scenario for the formation and dynamical evolution of icy objects under the influence of gas drag forces and gravitational scattering by protoplanets is briefly discussed. The outer Solar System plays the role of a corridor for interstellar matter entering into the Solar System. Further dust grains existing beyond Neptune’s orbit are produced as ejecta of icy dust particles from the EKBOs due to the impact of interstellar dust grains. Their expected amount and lifetimes are examined. Compared to the extension of the region of planetesimals around the Sun, the region of influence of the solar wind extends to relatively small distances of the order of several hundred AU. But both complexes are coupled through the presence of interstellar dust that depends on the extension and the physical parameters of the heliosphere. The existence of a stronger solar wind in the early stages of the Solar System indicates that the heliosphere in a distant past might have been 10–100 times larger than the current one which possibly influenced the evolution of the planetary system. 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.

The physical structure and morphology of the interstellar medium that surrounds our solar system directly effects the heliosphere and the interplanetary environment. High resolution ultraviolet absorption spectra of nearby stars and the intervening interstellar medium, observed by the Hubble Space Telescope, provide important information about the chemical abundance, ionization, temperature, kinematics, density, morphology, and turbulent structures of the local interstellar medium. Fortunately, nearly all observations of nearby stars contain useful local interstellar medium absorption lines. The number of useful observations is large enough that we can start analyzing the local interstellar medium as a three-dimensional object, as opposed to focusing on individual sightlines. We present the results of high resolution observations of nearby gas obtained by the Hubble Space Telescope. Our focus will be on the kinematic, temperature, and turbulent velocity structures in the Local Interstellar Cloud and other nearby clouds. Understanding the physical characteristics of these structures is necessary if we are to discuss the morphology of the local interstellar medium, its evolution, origin, and impact on the heliosphere and our solar system. 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.

The orbit of Comet C/2002C1 (Ikeya–Zhang) has a similarity to that of Comet C/1661C1 (Hevelius), and the numerical integration of the motion of C/2002C1 backward shows a possible linkage of those two comets. Thus, 153P/Ikeya–Zhang was designated a periodic comet. Historical records of comets in 877 and 1273 are also identified with Comet 153P/Ikeya–Zhang. The integrated orbital elements during 77 and 2362, and historical records of the comet are also presented and discussed.

Anderson et al. [Phys. Rev. D 65 (2002) 082004] recently reported new evidence that both Pioneer 10 and 11 are experiencing nearly the same unmodeled anomalous acceleration directed toward the Sun. Numerous mechanisms, both internal and external to the spacecraft, have been proposed to explain this unmodeled acceleration. If we assume that the cause of the anomalous acceleration is (1) external to the spacecraft, (2) isotropic, and (3) acts on bodies of cometary mass, then it would imply that new comets are more tightly bound to the Sun than previously believed. Here we show that the implied higher binding energy is incompatible with the established evidence that the galactic tide is dominant in making Oort cloud comets observable. We conclude that one or more of these assumptions must be invalid.

Comets are remnants from the time when the outer planets formed, approximately 4-4.5 billion years ago. They have been in storage since then in the Oort cloud and Kuiper belt-distant regions that are so cold and sparsely populated that it was long thought that comets approaching the Sun were pristine samples from the time of Solar System formation. It is now recognized, however, that a variety of subtle but important evolutionary mechanisms operate on comets during their long storage, so they can no longer be regarded as wholly pristine.

Accepted: 2002-11-25, 資料番号: SA1004068000

Our extensive orbital calculations show that the motion of comet C/1965 S1 (Ikeya-Seki), a major member of the Kreutz sungrazing system, can be derived from the motion of its "sister" comet C/1882 R1 (Great September Comet) on the assumption that the two objects are fragments of a common parent that split in the year 1106, at the time when a very bright comet appeared near the Sun according to a number of historical records. Specifically, the orbit of Ikeya-Seki derived from astrometric positions in 1965-1966 is matched with remarkably high accuracy, well within the errors of observation, by (1) integrating the motion of comet C/1882 R1 back to 1106; (2) launching from the parent a fragment some 18 days after perihelion, 0.75 AU from the Sun, with a relative velocity of about 7 m s-1 nearly in the antisolar direction; and (3) integrating the fragment's motion forward to 1965. We find that having the break-up closer to the Sun or before perihelion yields grossly inferior solutions. We conclude that the fragmentation event itself was not tidal in nature, but appears to have been due to rotational, and possibly other, forces acting on the parent comet, afflicted with cracks and fissures caused by the Sun's tidal forces a few weeks earlier. We note that the derived separation velocity is in the range established for nontidal fragmentation of minor sungrazers at large heliocentric distances and that there are obvious similarities with the behavior of comet D/1993 F2 (Shoemaker-Levy 9) following its close encounter with Jupiter in 1992 July.

This is the first of a series of papers presenting an attempt to reproduce the mechanisms acting currently on the Oort cloud of comets and producing the observed long-period comets. In this first part we give a short review of papers on the stellar perturbations on the cometary cloud and on the observed long-period comet perihelia/aphelia directions distribution. We present in detail the effects of a single stellar passage through or near the cometary cloud by means of a Monte Carlo simulation based on the improved impulse approximation. The simulations are performed for two different steady-state numerical models of the Oort cloud and for different parameters of the stellar passage. Among others, we calculate the probabilities of producing an observable comet as well as ejecting it from the cloud, we characterize cometary shower resulting from the close stellar passage and discuss anisotropies in the observable sample of comets, produced by a simulated single stellar perturbation.

Current models of the formation of comets in the Solar System assume that the most likely formation site for these pristine objects is in the Uranus-Neptune zone or just beyond with subsequent dynamical ejection by the growing protoplanets to distant orbits to form the Oort cloud. However, the composition of the recently disintegrated comet C/1999 S4 LINEAR suggests that it was most likely formed in the Jupiter-Saturn region (Mumma et al. 2001b; Kawakita et al. 2001). In this paper we argue that cometesimals could easily appear inside exterior resonances resulting from gas drag, between the orbits of Jupiter and Saturn and beyond Saturn, during the formation of these giant planets. Then, the trapped material could evolve into actual comets by means of collisional coagulation, followed by gravitational instability of a layer of macroscopic bodies or two-body accretion. Properties of these objects would be rather different from those found in classical comets formed beyond Neptune as a result of different physicochemical environment. Our results also suggest that temporarily stable cometary belts may have existed in the Jupiter-Saturn region. We also discuss the implications of this scenario on the existence of a bimodal Centaur population.

ABSTRA C T We test different possibilities for the origin of short-period comets captured from the Oort Cloud. We use an efficient Monte Carlo simulation method that takes into account nongravitational forces, Galactic perturbations, observational selection effects, physical evolution and tidal splittings of comets. We confirm previous results and conclude that the Jupiter family comets cannot originate in the spherically distributed Oort Cloud, since there is no physically possible model of how these comets can be captured from the Oort Cloud flux and produce the observed inclination and Tisserand constant distributions. The extended model of the Oort Cloud predicted by the planetesimal theory consisting of a non-randomly distributed inner core and a classical Oort Cloud also cannot explain the observed distributions of Jupiter family comets. The number of comets captured from the outer region of the Solar system are too high compared with the observations if the inclination distribution of Jupiter family comets is matched with the observed distribution. It is very likely that the Halley-type comets are captured mainly from the classical Oort Cloud, since the distributions in inclination and Tisserand value can be fitted to the observed distributions with very high confidence. Also the expected number of comets is in agreement with the observations when physical evolution of the comets is included. However, the solution is not unique, and other more complicated models can also explain the observed properties of Halley-type comets. The existence of Jupiter family comets can be explained only if they are captured from the extended disc of comets with semimajor axes of the comets a , 5000 au. The original flattened distribution of comets is conserved as the cometary orbits evolve from the outer Solar system era to the observed region.

The discovery of x-ray emission from comet Hyakutake was surprising given that comets are known to be cold. Observations by x-ray satellites such as the Röntgen Satellite (ROSAT) indicate that x-rays are produced by almost all comets. Theoretical and observational work has demonstrated that charge-exchange collisions of highly charged solar wind ions with cometary neutral species can explain this emission. X-ray observations of comets and other solar system objects may be used to determine the structure and dynamics of the solar wind.

Single passage of a star through the Oort cloud yields anisotropic distribution of observable long period comets. This conclusion emerges from our extensive Monte Carlo simulations of the directional distribution of long-period comets induced by a stellar passage through the Oort cloud. For a broad range of simulation parameters we obtained the asymmetric distribution of comets as result of a single stellar passage. A direct result of our simulation is a guide-line for studies of the anisotropies in the observed long-period comet sample and searching for fingerprints of a recent stellar passage. We obtained an isotropic distribution, as simulated by Weissman ([CITE]) only for a very peculiar choice of input parameters. The accuracy of our calculations was verified by consistency of the impulse approximation and direct orbit integration results.

The small solid particles in the space between the stars provide the surfaces for the production of many simple and complex molecules. Processes involving the effects of ultraviolet irradiation of the thin (hundredth micron) mantles are shown to produce a wide range of molecules and ions also seen in comets. Some of the more complex ones inferred from laboratory experiments are expected to play an important role in the origin of life. An outline of the chemical evolution of interstellar dust as observed and as studied in the laboratory is presented. Observations of comets are shown to provide substantial evidence for their being fluffy aggregates of interstellar dust as it was in the protosolar nebula, i.e. the interstellar cloud which collapsed to form the solar system. The theory that comets may have brought the progenitors of life to the earth is summarized.

For a few months around perihelion, the central part of the Hale-Bopp hydrogen cloud has been optically thick to the solar Lyα radiation, and has significantly reduced the solar flux available for the resonance glow of interstellar hydrogen beyond the comet. This shadowing effect on the interstellar gas is the first ever observed comet shadow. It is modeled and compared with SWAN observations. Shadow modelling will help to constrain the comet water production and radiative transfer effects in the interstellar ionisation cavity.

Aims. This paper is devoted to a comparison between observations and simulations of the so-called Oort spike formed by the “new” observable long-period comets. Methods. The synthetic distributions of observable comets come from the propagation of a huge sample of objects during the age of the solar system that were initially in a proto-Oort cloud, which was flattened around the ecliptic and had perihelia in the region of Uranus and Neptune. For the known new long-period comets, two samples were used, one that is assumed to be complete, and the comets of the other exclusively come from the Warsaw catalog of comets. The original orbital energy of the comets in this catalog is more reliable. Results. Considering comets with a perihelion distance smaller than 4 AU, for which one of our samples of known comets can be assumed to be complete, the comparison shows small but significant differences in the orbital energy distribution and in the proportion of retrograde comets. When we extend the limiting perihelion distance to 10 AU, the observed samples are obviously strongly incomplete. The synthetic distribution showsthat the number of observable comets per year and per perihelion distance unit is ∝q1.09 for q< 4 AU and ∝q2.13 for 6 6 AU comes from comets that were already within the Jupiter-Saturn barrier (q< 15 AU) at their previous perihelion passage (which we call creepers and Kaib and Quinn creepers), with original semi-major axes generally smaller than 20 000 AU. Conclusions. To explain the small but significant differences between our synthetic sample and the known comets for a perihelion distance smaller than 4 AU, different hypotheses are proposed: a still erroneous value of the original orbital energy in the observed sample, a higher density of low-mass stars in the actual solar neighborhood, a ninth planet, and obviously the initial population of objects from which the synthetic distributions are derived.

The cratering record on the Earth and Moon shows that our planet has been exposed to high velocity impacts for much or all of its existence. Some of these craters were produced by the impact of long period comets (LPCs). These probably originated in the Oort cloud, and were put into their present orbits through gravitational perturbations arising from the Galactic tide and stellar encounters, both of which are modulated by the solar motion about the Galaxy. Here we construct dynamicalmodels of these mechanisms in order to predict the time-varying impact rate of LPCs and the angular distribution of their perihelia (which is observed to be non-uniform). Comparing the predictions of these dynamical models with other models, we conclude that cometary impacts induced by the solar motion contribute only a small fraction of terrestrial impact craters over the past 250 Myr. Over this time scale the apparent cratering rate is dominated by a secular increase towards the present, which might be the result of the disruption of a large asteroid. Our dynamical models, together with the solar apex motion, predict a non-uniform angular distribution of the perihelia, without needing to invoke the existence of a massive body in the outer Oort cloud. Our results are reasonably robust to changes in the parameters of the Galaxy model, Oort cloud, and stellar encounters., 22 pages, 8 tables, 20 figures; accepted for publication by MNRAS

We present Monte Carlo simulations of the dynamical history of the Oort cloud, where in addition to the main external perturbers (Galactic tides and stellar encounters) we include, as done in a companion paper (Fouchard, M., Rickman, H., Froeschle, Ch., Valsecchi, G.B. [2013b]. Icarus, in press), the planetary perturbations experienced each time the comets penetrate to within 50 AU of the Sun. Each simulation involves an initial sample of four million comets and extends over a maximum of 5 Gyr. For better understanding of the outcomes, we supplement the full dynamical model by others, where one or more of the effects are left out. We concentrate on the production of observable comets, reaching for the first time a perihelion within 5 AU of the Sun. We distinguish between four categories, depending on whether the comet jumps across, or creeps through, the Jupiter–Saturn barrier (perihelion distances between 5 and 15 AU), and whether the orbit leading to the observable perihelion is preceded by a major planetary perturbation or not. For reasons explained in the paper, we call the strongly perturbed comets “Kaib–Quinn comets”. We thus derive a synthetic picture of the Oort spike, from which we draw two main conclusions regarding the full dynamical model. One is that 2/3 of the observable comets are injected with the aid of a planetary perturbation at the previous perihelion passage, and about half of the observable comets are of the Kaib–Quinn type. The other is that the creepers dominate over the jumpers. Due to this fact, the spike peaks at only 31 000 AU, and the majority of new comets have semi-major axes less than this value. The creepers show a clear preference for retrograde orbits as a consequence of the need to avoid untimely, planetary ejection before becoming observable. Thus, the new comets should have a 60/40 preference for retrograde against prograde orbits in apparent conflict with observations. However, both these and other results depend on our model assumptions regarding the initial structure of the Oort cloud, which is isotropic in shape and has a relatively steep energy distribution. We also find that they depend on the details of the past history of external perturbations including GMC encounters, and we provide special discussions of those issues.

Context. With this work we start a systematic analysis of the distant activity of several long-period comets in order to investigate the evolution of activity throughout the solar system and explore differences between comets that pass their perihelion at far or very close distances from the Sun. Aims. We present observational data for eight long-period comets, observed for the first time beyond r = 5 AU. Three targets have been characterised on their inward orbital branch. The others have passed their perihelion at quite large heliocentric distances (rq from 4.5 to 7.5 AU). Methods. We analyse multicolour broadband images (V,R, and I filters) taken at the Telescopio Nazionale Galileo to characterise the dust coma of the comets and investigate their morphology, photometry, colours, and dust production. Results. The morphological analysis shows many differences among the sample, from the large twisted structure present in the coma of comet C/2005 L3 to the regular coma envelope of C/2010 R1. The colour of the dust coma of all the comets is redder than the Sun. The Afρ value (measured in a reference aperture of radius ρ = 104 km) ranges from 114 ± 2 (C/2005 S4) to 5091 ± 47 (C/2005 L3) cm, depicting a scenario of bodies from moderately to very active. This is confirmed by the first-order quantitative estimate of the dust mass-loss rate for the comets that was obtained from the photometric data: assuming a grain velocity of v = 20 m/s, the dust production rate is comparable with, or even significantly larger than, that measured for many short-period (“old”) comets at much smaller heliocentric distances.

Comets are surviving members of a formerly vast distribution of solid bodies that formed in the cold regions of the solar nebula. Cometary bodies escaped incorporation into planets and ejection from the solar system and most have been stored in two distant reservoirs, the Oort cloud and the Kuiper belt, for most of the age of the solar system. The past decade has seen great advances in the understanding of comets, but they still remain somewhat mysterious and enigmatic bodies from the fringes of the solar system. The distinction between comets and asteroids has become clouded with the discovery of active bodies in the asteroid belt and the finding that comets contain ‘common meteoritic solids.’

We present Monte Carlo simulations of the dynamical history of the Oort cloud, where in addition to the main external perturbers (Galactic tides and stellar encounters) we include, as done in a companion paper (Fouchard, M., Rickman, H., Froeschle, Ch., Valsecchi, G.B. [2013b] Icarus, in press), the planetary perturbations experienced each time the comets penetrate to within 50 AU of the Sun. Each simulation involves an initial sample of four million comets and extends over a maximum of 5 Gyr. For better understanding of the outcomes, we supplement the full dynamical model by others, where one or more of the effects are left out. In the companion paper we studied in detail how observable comets are injected from the Oort cloud, when account is taken of the planetary perturbations. In the present paper we concentrate on how the cloud may evolve in the long term and also on the production of decoupled comets, which evolve into semi-major axes less than 1000 AU. Concerning the long-term evolution, we find that the largest stellar perturbations that may statistically be expected during the age of the Solar System induce a large scale migration of comets within the cloud. Thus, comets leave the inner parts, but the losses from the outer parts are even larger, so at the end of our simulations the Oort cloud is more centrally condensed than at the beginning. The decoupled comets, which form a source of centaurs and Halley type comets (roughly in the proportions of 70% and 30%, respectively), are mainly produced by planetary perturbations, Jupiter and Saturn being the most efficient. This effect is dependent on synergies with the Galactic tide and stellar encounters, bringing the perihelia of Oort cloud comets into the planetary region. The star-planet synergy has a large contribution due to the strong encounters that produce major comet showers. However, outside these showers a large majority of decouplings may be attributed to the tide-planet synergy. (c) 2013 Elsevier Inc. All rights reserved.

Scientific understanding of comets has been accelerated by many recent missions to comets. The nuclei have sizes in the range 1–10 km and their surfaces are very dark and diverse; interiors are likely loose agglomerations of smaller icy cometesimals and have a bulk density of ≈0.5 g/cm3. Solar heating of the surface layers sublimates the ices to produce gas and liberate dust particles; these form the dust tail. The gases form the gas coma and ultimately the immense hydrogen cloud. Ionized molecules interact with the solar wind to produce the plasma tail. Chemically, comets are rather uniform and are primarily water and carbon dioxide ices plus dust. This composition is consistent with formation at temperatures near 30 K. Dynamical processes deliver these bodies to the inner solar system via the scattered disk and the Oort cloud. Eventually the ices are completely sublimated. The dusty remnants produce meteor showers.

Comets are small icy bodies regarded as remnants of the time of planetary accretion when the Solar System was formed. The chapter discusses where comets originated, where they have spent most of their lives, and how they occasionally evolve through the planetary system and move close enough to the Sun to become the spectacular objects we sometimes see in the night sky. The majority of observational information we have about these objects comes from the short phase when they are close to the Sun. The rest of the story is gleaned by combining this information with computer-generated dynamical models of the Solar System. A discussion of the behavior of the orbits of comets is presented along with a classification scheme for comets. Finally, the cometary reservoirs are described, which are believed to exist in the Solar System, today.

The interaction of a comet with the solar wind undergoes various stages as the comet's activity varies along its orbit. For a comet like 67P/Churyumov Gerasimenko the target comet of ESA's Rosetta mission the various features include the formation of a Mach cone the bow shock and close to perihelion even a diamagnetic cavity. There are different approaches to simulate this complex interplay between the solar wind and the comet's extended neutral gas coma which include magnetohydrodynamics (MHD) and hybrid type models. The first treats the plasma as fluids (one fluid in basic single fluid MHD) and the latter treats the ions as individual particles under the influence of the local electric and magnetic fields. The electrons are treated as a charge neutralizing fluid in both cases. Given the different approaches both models yield different results in particular for a low production rate comet. In this paper we will show that these differences can be reduced when using a multifluid instead of a single fluid MHD model and increase the resolution of the Hybrid model. We will show that some major features obtained with a hybrid type approach like the gyration of the cometary heavy ions and the formation of the Mach cone can be partially reproduced with the multifluid type model. (C) 2014 Elsevier Inc. All rights reserved.

The Oort cloud is usually thought of as a collection of icy comets inhabiting the outer reaches of the Solar system, but this picture is incomplete. We use simulations of the formation of the Oort cloud to show that ~4% of the small bodies in the Oort cloud should have formed within 2.5 au of the Sun, and hence be ice-free rock-iron bodies. If we assume these Oort cloud asteroids have the same size distribution as their cometary counterparts, the Large Synoptic Survey Telescope should find roughly a dozen Oort cloud asteroids during ten years of operations. Measurement of the asteroid fraction within the Oort cloud can serve as an excellent test of the Solar system's formation and dynamical history. Oort cloud asteroids could be of particular concern as impact hazards as their high mass density, high impact velocity, and low visibility make them both hard to detect and hard to divert or destroy. However, they should be a rare class of object, and we estimate globally catastrophic collisions should only occur about once per billion years., Comment: Accepted for publication in Monthly Notices of the Royal Astronomical Society Main Journal

It is argued that Comet 72P/Denning–Fujikawa is an old and intermittently active comet evolving, at least observationally, towards a transitional minor planet status. We have studied the fate of hypothetical meteoroids ejected from the comet during its two known periods of activity (1881 and 1978). A complex history of orbital evolution is found. Meteoroids ejected in 1881 first become Earth-orbit-crossing in 1960, while meteoroids ejected in 1978 appear to hold stable, non-Earth-orbit-crossing orbits until at least 2110. If copious amounts of meteoroids were ejected in 1881 we find some indication that the Earth may encounter a populous, coherent subgroup, or ‘streamlet’, of them in 2009 and 2010, leading to the possibility of outburst activity in those years. We have investigated the possibility that the activity of Comet 72P/Denning–Fujikawa, over the past ∼200 years, has been governed by impacts suffered by the comet as it moves through the main-belt asteroid region. While encounters with centimetre-sized objects will take place each time the comet orbits the Sun, the likelihood of the comet encountering a large metre-sized asteroid is essentially zero on the time-scales considered. The outburst activity of the comet may be impact-modulated in the sense that small-object impacts might trigger the explosive release of gases trapped in subsurface cavities.

Time variation in impact probability is studied by assuming that the periodic flux of the Oort Cloud comets within 15 au arises from the motion of the Sun with respect to the Galactic mid-plane. The periodic flux clearly shows up in the impact rate of the captured Oort Cloud cometary population, with a phase shift caused by the orbital evolution. Depending on the assumed flux of comets and the size distribution of comets, the impact rate of the Oort Cloud comets of 1 km in diameter or greater is from 5 to 700 impacts Myr−1 on the Earth and from 0.5 to 70 impacts per 1000 yr on Jupiter. The relative fractions of impacts are 0.09, 0.11, 0.26 and 0.54 for long-period comets, Halley type comets, Jupiter family comets and near-Earth objects, respectively. For Jupiter, the corresponding fractions in the first three categories are 0.18, 0.31 and 0.51. If we consider physical fading of comet activity that is compatible with the observations, then the impact rates of active comets are two orders of magnitude smaller than the total impact rates by all kinds of comets and cometary asteroids of size 1 km or greater.

Observed visual magnitudes of the short-period comets 2P/Encke, 81P/Wild 2, 6P/d'Arrest and 73P/Schwassmann–Wachmann 3 and the long-period comet C/1984 V1 Levy–Rudenko are converted into water release rates. These H2O release rates are used to derive the nuclear surface areas, minimum nuclear radii and gas mass loss of these comets. Combined with estimated gas release rates, we make an uniform and systematic analysis of the observed continuum fluxes in the comae of these comets with the objective to find dust release rates, effective dust particle sizes and total mass release rates. The nuclei of Comets 2P/Encke, 6P/d'Arrest and 73P/Schwassmann–Wachmann 3, which has split into as many as four fragments, are potential targets for the flyby of the CONTOUR (COmet Nucleus TOUR) spacecraft, scheduled for launch in 2002 July, while Comet 81P/Wild 2 has been chosen as a target by the NASA spacecraft STARDUST, which was launched on 1999 February 7.

We investigate the first stage of the dynamical evolution of Oort cloud comets entering the planetary region for the first time. To this purpose, we integrate numerically the motions of a large number of fictitious comets pertaining to two samples, both w

We present results of some numerical studies on the dynamical history of a large number of observed long-period comets. In our investigations we include all comets with good quality orbits. We calculated barycentric, original orbital elements for comets at a distance of 250 AU from the Sun and then followed the motion of each comet (elliptic orbits only) backwards to its aphelion and then to its previous perihelion passage, including the galactic perturbations. Several previously published statistics are compared with the current sample of long-period comets having well-defined orbits. A distinction between dynamically "new" and "old" comets is proposed on the basis of their previous perihelion distance instead of the inverse of the original semimajor axis. We also demonstrated the importance of stellar perturbations and made attempts to find a correlation between past dynamics and various physical parameters of comets.

We have integrated the orbits of 27,700 test particles initially entering the planetary system from the Oort cloud in order to study the origin of Halley-type comets (HTCs). We included the gravitational influence of the Sun, giant planets, passing stars, and Galactic tides. We find that an isotropically distributed Oort cloud does not reproduce the observed orbital element distribution of the HTCs. In order to match the observations, the initial inclination distribution of the progenitors of the HTCs must be similar to the observed HTC inclination distribution. We can match the observations with an Oort cloud that consists of an isotropic outer cloud and a disklike massive inner cloud. These idealized two-component models have inner disks with median inclinations that range from 10° to 50°. This analysis represents the first link between observations and the structure of the inner Oort cloud.

The temporal distribution of the Kreutz group of sungrazing comets has been known to have an episodic character on timescales from weeks to tens of years. With the large number of minor members of this group being nowadays discovered in images taken with the Solar and Heliospheric Observatory coronagraphs, it has become apparent that the distribution of these faint comets is episodic on a much shorter timescale, with objects arriving in pairs during a small fraction of a day. It is shown that the rate of these pairs is much too high for a random sample. Their existence is readily explained as a result of secondary, low-velocity, nontidal fragmentation episodes, which occur virtually spontaneously at very large heliocentric distances and involve the products of near-perihelion splitting of progenitor fragments during their previous return to the Sun. In fact, the pairs are merely extreme manifestations of larger clusters of such subnuclei, with a complex hierarchy of fragments. Each cluster is an outcome of a sequence of nontidal fragmentation events, which begins—after the initial tidal breakup—at some point along the outbound leg of the orbit and then continues episodically to and past aphelion. A similar scenario of posttidal progressive disintegration was firmly established for comet Shoemaker-Levy 9, based on extensive observations of its secondary and tertiary nuclei during many months preceding the comet's collision with Jupiter. Also, there are similarities with the mechanism proposed recently for the formation of striations in the dust tail of comet Hale-Bopp, and a logical extension of this process is the evolution of comet dust trails.

Most of the biosphere was brought on the primitive Earth by an intense bombardment of comets. This included the atmosphere, the seawater and those volatile carbon compounds needed for the emergence of life. Comets were thrown into the inner Solar System by the strong perturbation induced by the growth of the giant planets' cores. The bulk of the Earth's bombardment came from those comets that accreted in Jupiter's zone, where the original deuterium enrichment had been diminished by steam coming from the hot, inner parts of the Solar System. This steam had condensed into icy chunks before their accretion into larger cometary nuclei. In contrast, comets that accreted in the zones of the outer giant planets kept their interstellar isotopic enrichments. Those comets contributed to the Earth's bombardment for a small amount only; they were mostly ejected into the Oort cloud and are the major source of the long-period comets observed today. The short-period comets, which come from the Kuiper Belt, should also have the same interstellar enrichment. The deuterium enrichment of seawater, accurately predicted by the previous scenario, has become one of the best telltales for the cometary origin of our biosphere. This cometary origin may have far-reaching cosmological consequences, in particular for the origin of life in other planetary systems.

Very little is known about how outgassing regions are distributed over the nucleus of comets. In periodic comets, active regions are believed to be few and of small extent. Since periodic comets are notorious for their lack of (small) solid particles that efficiently scatter sunlight, we try to find traces of the existence of production sites by examining the morphology of the gas coma. We use a new coma model in which results from hydrodynamics calculations describing the inner coma are used as limit conditions for a collisionless description of the outer coma. The production pattern of the parent species mainly depends on the extent and location of the production region(s) and on the rotational state of the nucleus. Analyzing 1980 observations of comet 2P/Encke, we find that free emission from a single, small, active region located near the subsolar point of a nonrotating nucleus is excluded. But such an active region on a rotating nucleus produces well the observed coma morphology. Our data then allow us to determine the orientation of the comet spin axis and the cometocentric latitude of the source. Emission from a few small production regions spread over the sunward part of a nonrotating nucleus or emission at a very low rate from a larger subsolar area could also fit the data. Although we do not find a unique solution to our problem, the excellent quality of our fits indicates that our approach, if used with enough care, can provide a new tool to investigate the properties of comet nuclei when the coma is far from spherically symmetric and, ultimately, to study the effects of the nongravitational force that is acting on comet nuclei.

Comet 15P/Finlay is unusual in that, contrary to ab initio expectations, it demonstrates no apparent linkage to any known meteor shower. Using data contained within the Electronic Atlas of Dynamical Evolutions of Short-Period Comets, we evaluate theoretical shower radiants for Comet 15P/Finlay, but find no evidence to link it to any meteoric anomalies in recorded antiquity. This result, however, must be tempered by the fact that any Comet 15P/ Finlay-derived meteoroids will have a low, 16kms 21 , encounter velocity with Earth's atmosphere. Typically, therefore, one would expect mostly faint meteors to be produced during an encounter with a Comet 15P/Finlay-derived meteoroid stream. We have conducted a D-criterion survey of meteoroid orbits derived from three southern hemisphere meteor radar surveys conducted during the 1960s, and again we find no evidence for any Comet 15P/Finlay-related activity. Numerical calculations following the orbital evolution of hypothetical meteoroids ejected from the comet, at each perihelion epoch since 1886

Approximately 25% of the 82 new class I Oort cloud comets have an anomalous distribution of orbital elements that can best be understood if there exists a bound perturber in the outer Oort cloud. Statistically signican t correlated anomalies include aphelia directions, energies, perihelion distances and signatures of the angular momentum change due to the Galaxy. The perturber, acting in concert with the galactic tide, causes these comets to enter the loss cylinder - an interval of Oort cloud comet perihelion distances in the planetary region which is emptied by interactions with Saturn and Jupiter. More concisely, the impulse serves to smear the loss cylinder boundary inward along the track of the perturber. Thus it is easier for the galactic tide to make these comets observable. A smaller number of comets are directly injected by the impulsive mechanism. We estimate that the perturber-comet interactions take place at a mean distance of 25000 AU. The putative brown dwarf would have a mass of 3 2MJupiter and an orbit whose normal direction is within 5 of the galactic midplane. This object would not have been detected in the IRAS database, but will be detectable in the next generation of planet/brown dwarf searches, including SIRTF. It is also possible that its radio emissions would make it distinguishable in sensitive radio telescopes such as the VLA.

A simplified analytical model of an evolutionary nebula is used to generate temperature–density radial profiles following the procedure elaborated by Dubrulle (Icarus106, 59, 1993). Each nebula disk is characterized by its initial mass MD, its initial radius RD, and the coefficient of turbulent viscosity α. We show that these parameters may be constrained by comparing temperature–density profiles to properly chosen physical and chemical Solar System data. Relatively weak constraints come from theories of formation of giant planets. The deuterium over hydrogen ratios observed in fossil water in primitive objects of the Solar System are much more constraining. For each model of the nebula, the temporal and radial evolution of the deuterium enrichment factor in water with respect to the protosolar abundance (in H2) is calculated for the first time by integrating the equation of diffusion and is compared to observed deuterium enrichments in LL3 meteorites, giant planets, and comets. Observations cannot be fitted when we assume that H2O is uniquely produced in the hot inner part of the nebula. The agreement with observed data requires highly enriched deuterium ices initially infalling from the presolar cloud onto the whole nebula discoid. In order to fit measurements of D/H in LL3 meteorites, MD, RD, and α must be between 0.03 and 0.3 M⊙, 8 and 28 AU, and 0.003 and 1.0, respectively. The source of the turbulence consistent with these α values is discussed. High viscosity disks are characterized by MHD turbulence, while the low viscosity disks (α between 0.003 and 0.01) are characterized by hydrodynamical turbulence. Magnetic fields in the selected nebulae are also calculated. Their lifetime is found to be equal to 104 years for α=0.1 and 105 years for α=0.003. Scenarios providing us with an interpretation of the high D/H ratio observed in comets are discussed. The first scenario in which comets coming from the Oort cloud were formed very rapidly in the Uranus–Neptune region of the turbulent nebula and expelled toward the Oort cloud prior to the complete formation of these planets implies some reprocessing in the nebula of the cometary matter coming from the presolar cloud. It may not be easy however to expel comets toward the Oort cloud quite early in the history of the Solar System. It might be, according to the second scenario, that both comets coming from the Oort cloud and comets of the Jupiter family were formed farther than Neptune in a nonturbulent region of the nebula. In such a case, comets might have conserved to a large extent the chemical signature of the interstellar medium. Possible tests of the scenarios are considered.

The infrared emission of various comets can be matched within the framework that all comets are made of aggregated interstellar dust. This is demonstrated by comparing results on Halley (a periodic comet), Borrelly (a Jupiter family short period comet), Hale-Bopp (a long period comet), and extra-solar comets in the β Pictoris disk. Attempts have been made to generalize the chemical composition of comet nuclei based on the observation of cometary dust and volatiles and the interstellar dust model. Finally, we deduce some of the expected dust and surface properties of comet Wirtanen from the interstellar dust model as applied to other comets.

The spectrum of HD 12895 (B8, V=8.2) was observed on 1997 April 3 as the nucleus of Comet Hale--Bopp passed at a minimum star--nucleus separation of 31 arcsec. Spectrograms were obtained at R=45 000 using the Keck I HIRES. No convincing enhancement of the equivalent width of any of the diffuse interstellar bands at 4726, 4779, 5780, 6269 and 6613 A was found during this event. This negative result is compatible with our previous lower resolution observations of Comet Halley in 1986. It is concluded that the carriers of these particular bands, if initially present in the nucleus of Hale--Bopp, either were not carried into the coma with the evaporating volatiles, or were modified or destroyed by the solar radiation field while in transit.