Artigos de revistas: "Comets Dynamics"

1

Carusi, A., e G. B. Valsecchi. "Dynamics of Comets". Symposium - International Astronomical Union 152 (1992): 255–68. http://dx.doi.org/10.1017/s0074180900091245.

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The gravitational processes affecting the dynamics of comets are reviewed. At great distances from the Sun the motion of comets is primarily affected by the vertical component of the galactic field, as well as by encounters with stars and giant molecular clouds. When comets move in the region of the planets, encounters with these can strongly affect their motion. A good fraction of all periodic comets spend some time in temporary libration about mean motion resonances with Jupiter; some comets can be captured by this planet as temporary satellites. Finally, there is a small number of objects with orbital characteristics quite different from those of all other short-period comets.

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2

Zheng, J. Q., M. J. Valtonen, S. Mikkola e H. Rickman. "Dynamics and orbital evolution of Oort cloud comets". Symposium - International Astronomical Union 172 (1996): 209–12. http://dx.doi.org/10.1017/s007418090012738x.

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Investigators generally conjecture a steady flux of new comets from the Oort cloud through the inner Solar system. Due to gravitational perturbations by major planets these objects may escape, become long period comets (LPCs) if their orbital periods P are larger than 200yr or become short period comets (SPCs) when their period is less than 200yr. SPCs are further divided in two types: the Halley type comets (HT, for P > 20yr) and the Jupiter family comets (JF, for P < 20yr).

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Fernández, Julio A. "Dynamics of Comets: Recent Developments and New Challenges". Symposium - International Astronomical Union 160 (1994): 223–40. http://dx.doi.org/10.1017/s0074180900046568.

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There is a broad consensus that long-period comets come from a huge reservoir surrounding the solar system, as proposed originally by Oort. Yet, the classical picture of the Oort cloud has substantially changed during the last decade. In addition to passing stars, the tidal force of the galactic disk and giant molecular clouds have also been identified as major perturbers of the Oort cloud. In particular, the latter may be responsible for limiting the size of the stable Oort cloud to no more than ≈ 104AU, i.e. about one tenth of the classical Oort's radius.Most comets are injected into the planetary region by the quasi-steady action of the tidal force of the galactic disk. The concentration of aphelion points of dynamically young comets toward mid-galactic latitudes is a consequence of its dominant influence. The frequency of comet passages into the inner planetary region could experience significant fluctuations with time as the Oort cloud meets random strong perturbers. The observed ordered pattern of most comet aphelia, associated with the galactic structure, argues against a recent strong perturbation of the Oort cloud.The origin of the Jupiter family has become another point of intense debate. Jupiter family comets may come from a transneptunian comet belt -the Kuiper belt- from where they can reach the planetary region through chaotic motion. The Kuiper belt has become accessible to large telescopes, as shown by the recent discoveries of 1992QB1 and 1993FW, possibly belt members. The major challenge will be to explore the region usually inaccessible to external perturbers that goes from ~30 AU to a few thousand AU. A significant mass may have been locked there from the beginnings of the solar system, giving rise to an inner core that feeds the outer or classical Oort cloud. Our aim will be to briefly discuss some of the topics summarized here.

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4

Neslušan, L. "Dynamics of comets in the collapsing protosolar nebula". International Astronomical Union Colloquium 173 (1999): 45–50. http://dx.doi.org/10.1017/s0252921100031225.

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AbstractComets are created in the cool, dense regions of interstellar clouds. These macroscopic bodies take place in the collapse of protostar cloud as mechanically moving bodies in contrast to the gas and miscroscopic dust holding the laws of hydrodynamics. In the presented contribution, there is given an evidence concerning the Solar system comets: if the velocity distribution of comets before the collapse was similar to that in the Oort cloud at the present, then the comets remained at large cloud-centric distances. Hence, the comets in the solar Oort cloud represent a relict of the nebular stage of the Solar system.

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5

Borinskaya, Sofya, Katrina B. Velle, Kenneth G. Campellone, Arthur Talman, Diego Alvarez, Hervé Agaisse, Yi I. Wu, Leslie M. Loew e Bruce J. Mayer. "Integration of linear and dendritic actin nucleation in Nck-induced actin comets". Molecular Biology of the Cell 27, n.º 2 (15 de janeiro de 2016): 247–59. http://dx.doi.org/10.1091/mbc.e14-11-1555.

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The Nck adaptor protein recruits cytosolic effectors such as N-WASP that induce localized actin polymerization. Experimental aggregation of Nck SH3 domains at the membrane induces actin comet tails—dynamic, elongated filamentous actin structures similar to those that drive the movement of microbial pathogens such as vaccinia virus. Here we show that experimental manipulation of the balance between unbranched/branched nucleation altered the morphology and dynamics of Nck-induced actin comets. Inhibition of linear, formin-based nucleation with the small-molecule inhibitor SMIFH2 or overexpression of the formin FH1 domain resulted in formation of predominantly circular-shaped actin structures with low mobility (actin blobs). These results indicate that formin-based linear actin polymerization is critical for the formation and maintenance of Nck-dependent actin comet tails. Consistent with this, aggregation of an exclusively branched nucleation-promoting factor (the VCA domain of N-WASP), with density and turnover similar to those of N-WASP in Nck comets, did not reconstitute dynamic, elongated actin comets. Furthermore, enhancement of branched Arp2/3-mediated nucleation by N-WASP overexpression caused loss of the typical actin comet tail shape induced by Nck aggregation. Thus the ratio of linear to dendritic nucleation activity may serve to distinguish the properties of actin structures induced by various viral and bacterial pathogens.

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S. Najm, Rasha, Salman Z. Khalaf e Khaleel I. Abood. "X-Ray Emission Spectroscopy Analysis for Near-Asteroid Belt of Atmospheric of the Comets". Iraqi Journal of Physics 20, n.º 3 (1 de setembro de 2022): 86–97. http://dx.doi.org/10.30723/ijp.v20i3.1006.

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According to Chandra Survey Observatory Near-Asteroid Belt Comets, the solar wind's contact with the comet produces a variety of spectral characteristics. The study of X-ray spectra produced by charge exchange is presented here. The spectrum of a comet can reveal a lot about its composition. This study has concentrated on the elemental abundance in six different comets, including 17P/Holmes, C/1999T1, C/2013A1, 9p/Temple1, and 103p/Hartley2 (NEAT). Numerous aspects of the comet's dynamics allow it to behave in a unique manner as it gets closer to the Near-Asteroid Belt. These characteristics are being examined, and some studies are still ongoing. The computations allow us to observe, for instance, how the composition of a comet's upper atmosphere affects how much gas it produces. For several comet morphologies, both linear and nonlinear, bow shock, contact surface, and stagnation point are investigated in relation to gas production rate. Our results shed light on the complex interactions between cometary ions and the solar wind. An increase in gas production rate was shown to be significantly correlated with sharp drops in average molecular weight.

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7

Mysen, Eirik. "Rotational dynamics of subsolar sublimating triaxial comets". Planetary and Space Science 52, n.º 10 (agosto de 2004): 897–907. http://dx.doi.org/10.1016/j.pss.2004.04.001.

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8

Emel'yanenko, V. V. "Dynamics of periodic comets and meteor streams". Celestial Mechanics and Dynamical Astronomy 54, n.º 1-3 (1992): 91–110. http://dx.doi.org/10.1007/bf00049546.

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9

Haghighipour, N., T. I. Maindl, C. Schäfer, R. Speith e R. Dvorak. "Triggering Comet-Like Activity of Main Belt Comets". Proceedings of the International Astronomical Union 10, S318 (agosto de 2015): 135–41. http://dx.doi.org/10.1017/s1743921315009680.

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AbstractMain Belt Comets (MBCs) have attracted a great deal of interest since their identification as a new class of bodies by Hsieh and Jewitt in 2006. Much of this interest is due to the implication that MBC activity is driven by the sublimation of volatile material (presumed to be water-ice) presenting these bodies as probable candidates for the delivery of a significant fraction of Earth's water. Results of the studies of the dynamics of MBCs suggest that these objects might have formed in-situ as the remnants of the break-up of large icy asteroids. Simulations also show that collisions among MBCs and small objects could have played an important role in triggering the cometary activity of these bodies. Such collisions might have exposed sub-surface water-ice which sublimated and created thin atmospheres and tails around MBCs. In order to drive the effort of understanding the nature of the activation of MBCs, we have investigated these collision processes by simulating the impacts in detail using a smooth particle hydrodynamics (SPH) approach that includes material strength and fracture models. We have carried out simulations for a range of impact velocities and angles, allowing m-sized impactors to erode enough of an MBC's surface to expose volatiles and trigger its activation. Impact velocities were varied between 0.5 km/s and 5.3 km/s, and the projectile radius was chosen to be 1 m. As expected, we observe significantly different crater depths depending on the impact energy, impact angle, and MBC's material strength. Results show that for all values of impact velocity and angle, crater depths are only a few meters, implying that if the activity of MBCs is due to the sublimation of water-ice, ice has to exist in no deeper than a few meters from the surface. We present details of our simulations and discuss the implications of their results.

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10

Fernández, J. A., e T. Gallardo. "From the Oort cloud to Halley-type comets". International Astronomical Union Colloquium 173 (1999): 327–38. http://dx.doi.org/10.1017/s0252921100031638.

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AbstractThe Oort cloud probably is the source of Halley-type (HT) comets and perhaps of some Jupiter-family (JF) comets. The process of capture of Oort cloud comets into HT comets by planetary perturbations and its efficiency are very important problems in comet ary dynamics. A small fraction of comets coming from the Oort cloud − of about 10−2− are found to become HT comets (orbital periods < 200 yr). The steady-state population of HT comets is a complex function of the influx rate of new comets, the probability of capture and their physical lifetimes. From the discovery rate of active HT comets, their total population can be estimated to be of a few hundreds for perihelion distancesq <2 AU. Randomly-oriented LP comets captured into short-period orbits (orbital periods < 20 yr) show dynamical properties that do not match the observed properties of JF comets, in particular the distribution of their orbital inclinations, so Oort cloud comets can be ruled out as a suitable source for most JF comets. The scope of this presentation is to review the capture process of new comets into HT and short-period orbits, including the possibility that some of them may become sungrazers during their dynamical evolution.

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11

Fernández, Julio Angel, e John C. Brandt. "Comets: Nature, Dynamics, Origin, and Their Cosmogonical Relevance". Physics Today 59, n.º 9 (2006): 60. http://dx.doi.org/10.1063/1.2364249.

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12

Haghighipour, Nader. "Dynamics, Origin, and Activation of Main Belt Comets". Proceedings of the International Astronomical Union 5, S263 (agosto de 2009): 207–14. http://dx.doi.org/10.1017/s1743921310001766.

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AbstractThe discovery of Main Belt Comets (MBCs) has raised many questions regarding the origin and activation mechanism of these objects. Results of a study of the dynamics of these bodies suggest that MBCs were formed in-situ as the remnants of the break-up of large icy asteroids. Simulations show that similar to the asteroids in the main belt, MBCs with orbital eccentricities smaller than 0.2 and inclinations lower than 25° have stable orbits implying that many MBCs with initially larger eccentricities and inclinations might have been scattered to other regions of the asteroid belt. Among scattered MBCs, approximately 20% reach the region of terrestrial planets where they might have contributed to the accumulation of water on Earth. Simulations also show that collisions among MBCs and small objects could have played an important role in triggering the cometary activity of these bodies. Such collisions might have exposed sub-surface water ice which sublimated and created thin atmospheres and tails around MBCs. This paper discusses the results of numerical studies of the dynamics of MBCs and their implications for the origin of these objects. The results of a large numerical modeling of the collisions of m-sized bodies with km-sized asteroids in the outer part of the asteroid belt are also presented and the viability of the collision-triggering activation scenario is discussed.

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13

Eggers, S., H. U. Keller, P. Kroupa e W. J. Markiewicz. "Origin and dynamics of comets and star formation". Planetary and Space Science 45, n.º 9 (setembro de 1997): 1099–104. http://dx.doi.org/10.1016/s0032-0633(97)00065-2.

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14

Gabryszewski, R. "Numerical modelling of Kuiper belt objects’ dynamics – limitations". International Astronomical Union Colloquium 173 (1999): 315–20. http://dx.doi.org/10.1017/s0252921100031614.

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AbstractThe investigation of KBOs’ dynamics is based on numerical orbital integrations on extremly long time scales due to orbital evolution of particles. The evolution of KBOs to JFCs needs a time-span of order of 109years. Such a long time of integration affects errors. So the question arises what is the boundary of an integration time to distinguish the physical solution from numerical noise and what it depends on. This paper presents numerical integrations of less than 150 massless test particles in the model of the Solar System which consists of 4 giant planets and the central mass. For each test particle computations were repeated at least twice on different computers and using two different methods of integration. The results show that an increase of errors in a solution depends on the eccentricity and the inclination of an orbit. The estimated maximum time-span of integration is of the order of 10 million years for highly elliptic orbits (e 0.6) and up to 125 million years for quasi-circular orbits (for particular model of the Solar System with orbits of massless objects outside Neptune's orbit). After long time-span of integration (120-130 Myrs) the solution can be completely chaotic. It cannot be stated unequivocally that this is one of the possible particle's paths or that this is just a numerical noise. So a different way of studying KBOs’ and SP comets’ dynamical evolution is needed. The integration of equations of motion between particular phases of objects which are considered as comets in different phases of their lives (KBOs − Centaurs − Comets − possibly extinct Comets) could be the new way of studying the dynamical evolution of SP comets.

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15

Antonov, V. A. "Problems of the dynamics and the origin of comets". Astronomical School’s Report 1, n.º 2 (2000): 56–62. http://dx.doi.org/10.18372/2411-6602.01.2056.

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Shoyoqubov, Shoayub, Shohrukh Shoyoqubov e Alisher Ibrohimov. "Dynamics and rate of complex ions formation in comets". Advances in Space Research 56, n.º 11 (dezembro de 2015): 2449–54. http://dx.doi.org/10.1016/j.asr.2015.10.011.

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17

Yeomans, D. K. "Cometary Orbital Dynamics and Astrometry". International Astronomical Union Colloquium 116, n.º 1 (1989): 3–17. http://dx.doi.org/10.1017/s0252921100109613.

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AbstractComets are the only large solar system bodies where nongravitational forces directly affect their dynamic motions. Their approach to within a few AU of the Sun initiates the vaporization of nucleus ices, and the resulting rocket-like effects either add to or subtract from the comet’s orbital energy; the sign of the energy change depends upon the comet’s rotation direction and its spin pole orientation. The cometary outgassing phenomena have generally been modeled by assuming a rapidly rotating nucleus of water ice that outgasses symmetrically with respect to perihelion. Although this nongravitational acceleration model has been quite successful in providing accurate orbits and ephemerides, several comets exhibit water production rates and visual light curves that are noticeably asymmetric with respect to perihelion. New asymmetric models are being developed that attempt to represent more closely the cometary outgassing phenomena. For the same comet, derived nongravitational parameters can differ widely, depending upon which model is used to fit the astrometric data. The uncertainties in the data and in the nongravitational acceleration model prevent realistic extrapolations of these objects’ motion beyond a few hundred years, particularly if close planetary encounters are involved. Accurate orbits, ephemerides and efforts to model the nongravitational effects ultimately depend upon the quality of the astrometric data. Using a combination of long-focus telescopes, charge coupled device (CCD) detectors, microdensitometer reductions and modern star catalogs, cometary astrometric data can be generated that are accurate to the sub arcsecond level. While occultation, spacecraft, and radar observations can provide powerful astrometric data when available, it is still the ground-based optical observations that must provide the vast majority of data for cometary astrometry in the foreseeable future.

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18

Mustyatsa, V. V., A. V. Kostarev, A. V. Tvorogova, F. I. Ataullakhanov, N. B. Gudimchuk e I. A. Vorobjev. "Fine structure and dynamics of EB3 binding zones on microtubules in fibroblast cells". Molecular Biology of the Cell 30, n.º 17 (agosto de 2019): 2105–14. http://dx.doi.org/10.1091/mbc.e18-11-0723.

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End-binding (EB) proteins associate with the growing tips of microtubules (MTs)and modulate their dynamics directly and indirectly, by recruiting essential factors to fine-tune MTs for their many essential roles in cells. Previously EB proteins have been shown to recognize a stabilizing GTP/GDP-Pi cap at the tip of growing MTs, but information about additional EB-binding zones on MTs has been limited. In this work, we studied fluorescence intensity profiles of one of the three mammalian EB-proteins, EB3, fused with red fluorescent protein (RFP). The distribution of EB3 on MTs in mouse fibroblasts frequently deviated from single exponential decay and exhibited secondary peaks. Those secondary peaks, which we refer to as EB3-islands, were detected on 56% comets of growing MTs and were encountered once per 44 s of EB3-RFP comet growth time with about 5 s half-lifetime. The majority of EB3-islands in the vicinity of MT tips was stationary and originated from EB3 comets moving with the growing MT tips. Computational modeling of the decoration of dynamic MT tips by EB3 suggested that the EB3-islands could not be explained simply by a stochastic first-order GTP hydrolysis/phosphate release. We speculate that additional protein factors contribute to EB3 residence time on MTs in cells, likely affecting MT dynamics.

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Fernández, Julio A., e Andrea Sosa. "Active bodies in the near-Earth region: The tenuous boundary between comets and asteroids". Proceedings of the International Astronomical Union 10, S318 (agosto de 2015): 142–43. http://dx.doi.org/10.1017/s1743921315008480.

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AbstractWe analyze the dynamics and activity observed in bodies approaching the Earth (perihelion distancesq< 1.3 au) in short-period orbits (P< 20 yr), which essentially are near-Earth Jupiter Family Comets (NEJFCs) and near-Earth asteroids (NEAs). In the general definition, comets are “active”, i.e. they show some coma, while asteroids are “inactive”, i.e. they only show a bare nucleus. Besides their activity, NEJFCs are distinguished from NEAs by their dynamical evolution: NEJFCs move on unstable orbits subject to frequent close encounters with Jupiter, whereas NEA orbits are much more stable and tend to avoid close encounters with Jupiter. However, some JFCs are found to move on stable, asteroidal-type orbits, so the question arises if these objects are asteroids that have become active, perhaps upon approach to the Sun. In this sense they may be regarded as the counterparts of the main-belt comets (Hsieh & Jewitt 2006). On the other hand, some seemingly inert NEAs move on unstable, comet-type orbits, so the question about what is a comet and what is an asteroid has become increasingly complex.

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Nemeth, Z. "The Dynamics of the Magnetic-field-free Cavity around Comets". Astrophysical Journal 891, n.º 2 (11 de março de 2020): 112. http://dx.doi.org/10.3847/1538-4357/ab6e69.

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21

Fouchard, M., H. Rickman, Ch Froeschlé e G. B. Valsecchi. "Planetary perturbations for Oort Cloud comets. I. Distributions and dynamics". Icarus 222, n.º 1 (janeiro de 2013): 20–31. http://dx.doi.org/10.1016/j.icarus.2012.10.027.

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Fouchard, M., Ch Froeschle e G. B. Valsecchi. "Is the dynamics of Jupiter family comets amenable to Monte Carlo modelling?" Monthly Notices of the Royal Astronomical Society 344, n.º 4 (outubro de 2003): 1283–95. http://dx.doi.org/10.1046/j.1365-8711.2003.06904.x.

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23

Mysen, E. "The dynamics of globally active triaxial comets, with applications to asteroid rotation". Monthly Notices of the Royal Astronomical Society 381, n.º 1 (17 de setembro de 2007): 301–7. http://dx.doi.org/10.1111/j.1365-2966.2007.12266.x.

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24

Zheng, Hui-nan, e Shui Wang. "A numerical study of the dynamics of the plasma tail of comets". Chinese Astronomy and Astrophysics 20, n.º 1 (janeiro de 1996): 106–14. http://dx.doi.org/10.1016/0275-1062(96)00015-x.

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Mandt, K. E., A. Eriksson, A. Beth, M. Galand e E. Vigren. "Influence of collisions on ion dynamics in the inner comae of four comets". Astronomy & Astrophysics 630 (20 de setembro de 2019): A48. http://dx.doi.org/10.1051/0004-6361/201834828.

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Context. Collisions between cometary neutrals in the inner coma of a comet and cometary ions that have been picked up into the solar wind flow and return to the coma lead to the formation of a broad inner boundary known as a collisionopause. This boundary is produced by a combination of charge transfer and chemical reactions, both of which are important at the location of the collisionopause boundary. Four spacecraft measured ion densities and velocities in the inner region of comets, exploring the part of the coma where an ion-neutral collisionopause boundary is expected to form. Aims. The aims are to determine the dominant physics behind the formation of the ion-neutral collisionopause and to evaluate where this boundary has been observed by spacecraft. Methods. We evaluated observations from three spacecraft at four different comets to determine if a collisionopause boundary was observed based on the reported ion velocities. We compared the measured location of the ion-neutral collisionopause with measurements of the collision cross sections to evaluate whether chemistry or charge exchange are more important at the location where the collisionopause is observed. Results. Based on measurements of the cross sections for charge transfer and for chemical reactions, the boundary observed by Rosetta appears to be the location where chemistry becomes the more probable result of a collision between H2O and H2O+ than charge exchange. Comparisons with ion observations made by Deep Space 1 at 19P/Borrelly and Giotto at 1P/Halley and 26P/Grigg-Skjellerup show that similar boundaries were observed at 19P/Borrelly and 1P/Halley. The ion composition measurements made by Giotto at Halley confirm that chemistry becomes more important inside of this boundary and that electron-ion dissociative recombination is a driver for the reported ion pileup boundary.

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Rampino, Michael R., e Richard B. Stothers. "Mass Extinctions, Comet Impacts, and The Galaxy". Highlights of Astronomy 11, n.º 1 (1998): 246–51. http://dx.doi.org/10.1017/s1539299600020670.

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AbstractThe hypothesis relating mass extinctions of life on Earth to impacts of comets whose flux is partly modulated by the dynamics of the Milky Way Galaxy contains a number of postulates that can be tested by geologic evidence and statistical analyses. In an increasing number of cases, geologic evidence for impact (widespread impact debris and/or large impact craters) is found at times of mass extinction events, and the record of dated impact craters has been found to show a significant correlation with mass extinctions. Statistical analyses suggest that mass extinction events exhibit a periodic component of about 26 to 30 Myr, and periodicities of 30± 0.5 Myr and 35 ±2 Myr have been extracted from sets of well-dated impact craters. The evidence is consistent with periodic or quasi-periodic showers of impactors, probably Oort Cloud comets, with an approximately 30-Myr cycle. The best explanation for these proposed quasi-periodic comet showers involves the Sun’s vertical oscillation through the galactic disk, which may have a similar cycle time between crossings of the galactic plane.

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Emel’yanenko, V. V. "Dynamics and Origin of Comets: New Problems Appeared after the Rosetta Space Mission". Solar System Research 52, n.º 5 (setembro de 2018): 382–91. http://dx.doi.org/10.1134/s0038094618050039.

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Paučo, R., e J. Klačka. "Sedna and the cloud of comets surrounding the solar system in Milgromian dynamics". Astronomy & Astrophysics 589 (14 de abril de 2016): A63. http://dx.doi.org/10.1051/0004-6361/201527713.

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Froeschlé, C. "Mappings in Astrodynamics". Symposium - International Astronomical Union 152 (1992): 375–90. http://dx.doi.org/10.1017/s0074180900091415.

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We review mappings mainly devised for the study of the dynamics of comets and asteroids. An attempt of a typology according to the method used to devise the mapping and to its deterministic or stochastic character is made.

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Engrand, Cécile, Jean Duprat, Noémie Bardin, Emmanuel Dartois, Hugues Leroux, Eric Quirico, Karim Benzerara et al. "The asteroid-comet continuum from laboratory and space analyses of comet samples and micrometeorites". Proceedings of the International Astronomical Union 11, A29A (agosto de 2015): 253–56. http://dx.doi.org/10.1017/s1743921316002994.

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AbstractComets are probably the best archives of the nascent solar system, 4.5 Gyr ago, and their compositions reveal crucial clues on the structure and dynamics of the early protoplanetary disk. Anhydrous minerals (olivine and pyroxene) have been identified in cometary dust for a few decades. Surprisingly, samples from comet Wild2 returned by the Stardust mission in 2006 also contain high temperature mineral assemblages like chondrules and refractory inclusions, which are typical components of primitive meteorites (carbonaceous chondrites - CCs). A few Stardust samples have also preserved some organic matter of comet Wild 2 that share some similarities with CCs. Interplanetary dust falling on Earth originate from comets and asteroids in proportions to be further constrained. These cosmic dust particles mostly show similarities with CCs, which in turn only represent a few percent of meteorites recovered on Earth. At least two (rare) families of cosmic dust particles have shown strong evidences for a cometary origin: the chondritic porous interplanetary dust particles (CP-IDPs) collected in the terrestrial stratosphere by NASA, and the ultracarbonaceous Antarctic Micrometeorites (UCAMMs) collected from polar snow and ice by French and Japanese teams. Analyses of dust particles from the Jupiter family comet 67P/Churyumov-Gerasimenko by the dust analyzers on Rosetta orbiter (COSIMA, GIADA, MIDAS) suggest a relationship to interplanetary dust/micrometeorites. A growing number of evidences highlights the existence of a continuum between asteroids and comets, already in the early history of the solar system.

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Demers, Patricia. "Tails of Cross-Channel Comets: From Acclaim to Obscurity". Renaissance and Reformation 43, n.º 2 (28 de setembro de 2020): 213–34. http://dx.doi.org/10.33137/rr.v43i2.34797.

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This article explores the diverse materialities of texts created by three female luminaries that expand our understanding of translation and transformation in early modern Europe. Lady Anne Cooke Bacon’s translation of Bishop Jewel’s Apologia was praised as the official text of the Elizabethan Settlement and printed without change for the edification of both English readers and Continental sceptics. Yet despite its centrality in the vitriolic controversy between Jewel and Louvain Romanist Thomas Harding, within a generation Bacon’s name disappeared. Bilingual calligrapher and miniaturist Esther Inglis prepared and presented stunning manuscript gift books, often including self-portraits, to patrons on both sides of the Channel. Her artisanal expertise emulated and often outdid the typographic variety of the printed text. Scholarly and lionized participant in the Neo-Latin Republic of Letters, Anna Maria van Schurman, whose landmark Dissertatio was translated as The Learned Maid, scandalized her conservative Calvinist supporters by embracing Labadism and praising its simple ways in her autobiography Eukleria. These three early modern women, distinct in temperament, time, and social status, are the subject of this exploration, which seeks to understand the dynamics and fluctuations of cross-Channel transmission and the role played by the Channel divide or bridge in creating a brief notoriety soon to be followed by obscurity.

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Yabushita, S. "On the effect of non-gravitational processes on the dynamics of nearly parabolic comets". Monthly Notices of the Royal Astronomical Society 283, n.º 1 (21 de outubro de 1996): 347–52. http://dx.doi.org/10.1093/mnras/283.1.347.

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Brandt, John C. "Comets Comets: Nature, Dynamics, Origin, and Their Cosmogonical Relevance , Julio Angel Fernández , Springer, Dordrecht, the Netherlands, 2005. $159.00 (383 pp.). ISBN 1-4020-3490-3". Physics Today 59, n.º 9 (setembro de 2006): 60. http://dx.doi.org/10.1063/1.2372432.

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Wysoczańska, Rita, Piotr A. Dybczyński e Magdalena Polińska. "A surprise in the updated list of stellar perturbers of long-period comet motion". Astronomy & Astrophysics 640 (agosto de 2020): A129. http://dx.doi.org/10.1051/0004-6361/202037876.

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Context. The second Gaia data release (Gaia DR2) provided us with the precise five-parameter astrometry for 1.3 billion of sources. As stars passing close to the Solar System are thought to influence the dynamical history of long-period comets, we update and extend the list of stars that could potentially perturb the motion of these comets. Aims. We announce a publicly available database containing an up-to-date list of stars and stellar systems potentially perturbing the motion of long-period comets. We add new objects and revise previously published lists. Special emphasis is placed on stellar systems. A discussion of mass estimation is included. Methods. Using the astrometry, preferably from Gaia DR2, augmented with data from other sources, we calculate nominal spatial positions and velocities for each star. To filter studied objects on the basis of their nominal minimum heliocentric distances we numerically integrate the motion of stars under the Galactic potential and their mutual interactions. Results. We announce the updated list of stellar perturbers of cometary motion, including the masses of perturbers along with the publicly available database interface. These data are ready to be used with the observed long-period comets orbits to study an individual influence of a whole sample of perturbers, or specific stars, on a dynamical past or future of a specific comet. New potential perturbers were added; there are 138 more than in the previously published sources. Conclusions. We demonstrate that a new set of prospective perturbers is an important tool in studies of cometary dynamics. The use of our data changes the results of the past and future cometary motion analysis. We point out a puzzling object in our list, star ALS 9243. The Gaia DR2 astrometry suggests a very close encounter of this star with the Sun; however, its astrophysical parameters result in a completely different current distance of ALS 9243 and its high mass.

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Nesvorný, David, David Vokrouhlický, Petr Pokorný e Diego Janches. "DYNAMICS OF DUST PARTICLES RELEASED FROM OORT CLOUD COMETS AND THEIR CONTRIBUTION TO RADAR METEORS". Astrophysical Journal 743, n.º 1 (21 de novembro de 2011): 37. http://dx.doi.org/10.1088/0004-637x/743/1/37.

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Emel’yanenko, V. V. "Structure and Dynamics of the Centaur Population: Constraints on the Origin of Short-Period Comets". Earth, Moon, and Planets 97, n.º 3-4 (1 de julho de 2006): 341–51. http://dx.doi.org/10.1007/s11038-006-9095-5.

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Thomas, Fabrice, e Alessandro Morbidelli. "The Kozai resonance in the outer solar system and the dynamics of long-period comets". Celestial Mechanics & Dynamical Astronomy 64, n.º 3 (1996): 209–29. http://dx.doi.org/10.1007/bf00728348.

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Gortsas, N., U. Motschmann, E. Kührt, J. Knollenberg, S. Simon e A. Boesswetter. "Mapping of coma anisotropies to plasma structures of weak comets: a 3-D hybrid simulation study". Annales Geophysicae 27, n.º 4 (2 de abril de 2009): 1555–72. http://dx.doi.org/10.5194/angeo-27-1555-2009.

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Abstract. The effects of coma anisotropies on the plasma environment of comets have been studied by means of a 3-D hybrid model which treats electrons as a massless, charge-neutralizing fluid, whereas ion dynamics are covered by a kinetic approach. From Earth-based observations as well as from in-situ spacecraft measurements the shape of the coma of many comets is ascertained to be anisotropic. However, most plasma simulation studies deploy a spherically symmetric activity pattern. In this paper anisotropy is studied by considering three different coma shape models. The first model is derived from the Haser model and is characterised by spherically symmetry. This reference model is then compared with two different neutral gas shape models: the dayside restricted model with no nightside activity and a cone shaped model with opening angle of π/2. In all models the integrated surface activity is kept constant. The simulations have been done for the Rosetta target comet 67P/Churyumov-Gerasimenko for two heliocentric distances, 1.30 AU and 3.25 AU. It is found that shock formation processes are modified as a result of increasing spatial confinement. Characteristic plasma structures of comets such as the bow shock, magnetic barrier region and the ion composition boundary exhibit a shift towards the sun. In addition, the cone shaped model leads to a strong increase of the mass-loaded region which in turn leads to a smooth deceleration of the solar wind flow and an increasing degree of mixture between the solar wind and cometary ion species. This creates an additional transport channel of the magnetic field from the magnetic barrier region away which in turn leads to a broadening of this region. In addition, it leads to an ion composition boundary which is only gradually developed.

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Bauer, Henry. "Startling Discoveries and Contrarian Anomalies: Small Comets and Other Heresies". Journal of Scientific Exploration 36, n.º 1 (22 de maio de 2022): 167–76. http://dx.doi.org/10.31275/20222409.

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This book should be required reading for all scholars and students of Science and Technology Studies (STS), which encompasses the history and sociology of science and the interaction of science with society as a whole.1 Anomalists will find the discovery narrative engrossing and the whole book rewarding, well worth coping with the occasional technicalities. Lay readers should likewise appreciate Part 1 and will miss little of importance to them by scanning Part 2 more rapidly. Cosmic Rain is really several books in one. Most directly, it is a fascinating scientific detective story. At the same time, as Frank recognized (p. 4), it is an important case study in the history of science, illuminating most particularly the circumstances of scientific breakthroughs that are surprising and unforeseen. Frank’s experiences illustrate several general points about the manner in which science receives—or rather, resists—startling novelty. Furthermore, this book is a very detailed first-hand description of scientific activity, warts and all, that should enable non-scientists to begin to recognize that scientific activity is very much like other human activities: influenced by human behavior and human psychology, not only by the objective technical considerations. Louis Frank was a distinguished physicist at the University of Iowa whose specialty was plasma physics. In the early 1980s, he was puzzled by persistent dark spots in ultraviolet (UV) images of the outer reaches of the Earth taken from a satellite, the Dynamics Explorer, which carried several instruments that were Frank’s responsibility.

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Torres, S., M. X. Cai, A. G. A. Brown e S. P. Zwart. "Galactic tide and local stellar perturbations on the Oort cloud: creation of interstellar comets". Astronomy & Astrophysics 629 (setembro de 2019): A139. http://dx.doi.org/10.1051/0004-6361/201935330.

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Comets in the Oort cloud evolve under the influence of internal and external perturbations, such as giant planets, stellar passages, and the Galactic gravitational tidal field. We aim to study the dynamical evolution of the comets in the Oort cloud, accounting for the perturbation of the Galactic tidal field and passing stars. We base our study on three main approaches; analytic, observational, and numerical. We first construct an analytical model of stellar encounters. We find that individual perturbations do not modify the dynamics of the comets in the cloud unless very close (<0.5 pc) encounters occur. Using proper motions, parallaxes, and radial velocities from Gaia DR2 and combining them with the radial velocities from other surveys, we then construct an astrometric catalogue of the 14 659 stars that are within 50 pc of the Sun. For all these stars we calculate the time and distance of closest approach to the Sun. We find that the cumulative effect of relatively distant (≤1 pc) passing stars can perturb the comets in the Oort cloud. Finally, we study the dynamical evolution of the comets in the Oort cloud under the influence of multiple stellar encounters from stars that pass within 2.5 pc of the Sun and the Galactic tidal field over ±10 Myr. We use the Astrophysical Multipurpose Software Environment (AMUSE), and the GPU-accelerated direct N-body code ABIE. We considered two models for the Oort cloud, compact (a ≤ 0.25 pc) and extended (a ≤ 0.5 pc). We find that the cumulative effect of stellar encounters is the major perturber of the Oort cloud for a compact configuration while for the extended configuration the Galactic tidal field is the major perturber. In both cases the cumulative effect of distant stellar encounters together with the Galactic tidal field raises the semi-major axis of ~1.1% of the comets at the edge of the Oort cloud up to interstellar regions (a > 0.5 pc) over the 20 Myr period considered. This leads to the creation of transitional interstellar comets (TICs), which might become interstellar objects due to external perturbations. This raises the question of the formation, evolution, and current status of the Oort cloud as well as the existence of a “cloud” of objects in the interstellar space that might overlap with our Oort cloud, when considering that other planetary systems should undergo similar processes leading to the ejection of comets.

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Andreev, A. O., Yu A. Nefedyev e N. Yu Demina. "Development of an Isoline Modeling Method for Extended Small Celestial Objects". Uchenye Zapiski Kazanskogo Universiteta. Seriya Fiziko-Matematicheskie Nauki 165, n.º 2 (18 de janeiro de 2024): 105–14. http://dx.doi.org/10.26907/2541-7746.2023.2.105-114.

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This article presents a method for modeling extended small celestial (ESC) objects, which are mainly cometary systems. Special attention was given to the possibility of analyzing their structure and physical properties in line with the modern theories of the Solar System’s formation suggesting a rather complicated evolutionary dynamics. Modeling and investigating the structure of different extended celestial objects advance our understanding of the general evolutionary processes that have taken place in the Solar System because all its objects are evolutionarily related. The isoline modeling (IM) method was tested on the real comet data and proved effective in assessing the activity of the processes that occur as ESC objects move in space. The IM method is particularly useful for studying long-period comets that, in many cases, cross the perihelion only once within a foreseeable period of human existence.

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Nisticò, Giuseppe, Gaetano Zimbardo, Silvia Perri, Valery M. Nakariakov, Timothy J. Duckenfield e Miloslav Druckmüller. "Probing the Density Fine Structuring of the Solar Corona with Comet Lovejoy". Astrophysical Journal 938, n.º 1 (1 de outubro de 2022): 20. http://dx.doi.org/10.3847/1538-4357/ac8e62.

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Abstract The passage of sungrazing comets in the solar corona can be a powerful tool to probe the local plasma properties. Here, we carry out a study of the striae pattern appearing in the tail of sungrazing Comet Lovejoy, as observed by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO) during the inbound and outbound phases of the comet’s orbit. We consider the images in EUV in the 171 Å bandpass, where emission from oxygen ions O4+ and O5+ is found. The striae are described as due to a beam of ions injected along the local magnetic field, with the initial beam velocity decaying because of collisions. Also, ion collisional diffusion contributes to ion propagation. Both the collision time for velocity decay and the diffusion coefficient for spatial spreading depend on the ambient plasma density. A probabilistic description of the ion beam density along the magnetic field is developed, where the beam position is given by the velocity decay and the spreading of diffusing ions is described by a Gaussian probability distribution. Profiles of emission intensity along the magnetic field are computed and compared with the profiles along the striae observed by AIA, showing a good agreement for most considered striae. The inferred coronal densities are then compared with a hydrostatic model of the solar corona. The results confirm that the coronal density is strongly spatially structured.

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C de Jesus, Antônio Delson, Fabrício Pereira de Oliveira e Robson Lemos Rodovalho. "Kinetic impact of a space vehicle with asteroid on collision course with earth". Aeronautics and Aerospace Open Access Journal 7, n.º 3 (8 de agosto de 2023): 104–8. http://dx.doi.org/10.15406/aaoaj.2023.07.00177.

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NEO (Near Earth Objects) are near-Earth space objects (asteroids and comets), whose orbit approaches the planet at one third of the average Earth-Sun distance. NEO are potentially capable of hitting Earth in the next century, with the exception of comets that can do so by the gravitational effect of Jupiter, pushing them into the Solar System. Current estimates1 indicate about 940 near-Earth objects larger than 1 kilometer in diameter. The US Congress has set targets for NASA's search for NEO, with 90% of them with a diameter of up to 1 km or greater in 10 years. This was motivated by a recent discovery in 2009 of a NEO approximately 2 to 3 km in diameter. These objects could cause global devastation if they hit Earth. In this research the NEO-Earth collision dynamics is studied and a statistic of physical and technological parameters is established as a mitigation strategy.

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Sagdeev, R. Z., e G. M. Zaslavsky. "Stochasticity in the Kepler problem and a model of possible dynamics of comets in the Oort cloud". Il Nuovo Cimento B 97, n.º 2 (fevereiro de 1987): 119–30. http://dx.doi.org/10.1007/bf02888815.

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Lissauer, Jack J. "Dynamical effects of cometary bombardment of Saturn’s rings and moons". International Astronomical Union Colloquium 83 (1985): 181–82. http://dx.doi.org/10.1017/s0252921100083901.

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Extended AbstractSaturn’s ring particles and airless moons are exposed to a large flux of interplanetary debris, principally comets and comet dust. Collisions with this debris are responsible for both physical and dynamical changes in objects orbiting about Saturn. Physical changes include cratering of large bodies and catastrophic disruption of small bodies. Dynamical changes, which are analyzed in this paper, include orbital alteration (principally of ring particles) and changes in spin state (which are only important for moons, as ring particle spins are continually altered by mutual collisions).Saturn’s rings are rapidly being eroded by impacts of hypervelocity meteoroids in cometary orbits. Ejecta from these impacts will, in most cases, remain in orbit about Saturn and eventually be reaccreted by the rings, possibly at a different radial location. The resulting mass transport has been suggested as the cause of some of the features observed in Saturn’s rings (see Durisen 1984 for a review). Previous attempts to model this transport have used numerical simulations which have not included effects of angular momentum transport coincident with this mass transport. I have developed an analytic model for ballistic mass transport in Saturn’s rings. The model includes the effects of angular momentum advection and shows that the net material movement due to the combined effects of mass and angular momentum transport is roughly half that calculated when angular momentum advection is ignored. See Lissauer (1984) for further details.All of Saturn’s mid-sized moons are rotating synchronously with their orbital period; thus, the same hemisphere of these moons always faces the planet, and the same point is always at the center of the satellite’s leading hemisphere (the apex). The satellites orbit Saturn with velocities ranging from 14 km/sec for Mimas to 8.5 km/sec for Rhea and 3.3 km/sec for Iapetus. These speeds are a significant fraction of the encounter velocities between comets and the Saturn system (~ 10−25 km/sec); thus, due to a type of “windshield effect” (more raindrops hit the windshield of a moving car than hit the rear window), more comets will collide with the moons’ leading hemispheres than with their trailing hemispheres; also, higher relative velocities between comets and the moons will lead to larger craters for impacts by comets of a given mass on the leading hemispheres. The combination of these effects suggests that regions of the satellites’ surfaces near the apex should be much more heavily cratered than regions near the antapex (Shoemaker and Wolfe 1982). Such a major cratering asymmetry has not been observed (Plescia and Boyce 1983). A similar situation exists for Jupiter’s moons Ganymede and Callisto (Passey and Shoemaker 1982).McKinnon (1981) suggested that stochastic reorientation of the moons by impact-induced “spinup” during the establishment of the cratering record tended to equilibrate the crater densities between hemispheres. I have re-examined the dynamics of this problem, and I conclude that most impacts large enough to have caused “spinup” would have catastrophically disrupted the moons in question (Lissauer 1985).

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Efimov, Vladimir P., Jun Zhang e Xin Xiang. "CLIP-170 Homologue and NUDE Play Overlapping Roles in NUDF Localization in Aspergillus nidulans". Molecular Biology of the Cell 17, n.º 4 (abril de 2006): 2021–34. http://dx.doi.org/10.1091/mbc.e05-11-1084.

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Proteins in the cytoplasmic dynein pathway accumulate at the microtubule plus end, giving the appearance of comets when observed in live cells. The targeting mechanism for NUDF (LIS1/Pac1) of Aspergillus nidulans, a key component of the dynein pathway, has not been clear. Previous studies have demonstrated physical interactions of NUDF/LIS1/Pac1 with both NUDE/NUDEL/Ndl1 and CLIP-170/Bik1. Here, we have identified the A. nidulans CLIP-170 homologue, CLIPA. The clipA deletion did not cause an obvious nuclear distribution phenotype but affected cytoplasmic microtubules in an unexpected manner. Although more microtubules failed to undergo long-range growth toward the hyphal tip at 32°C, those that reached the hyphal tip were less likely to undergo catastrophe. Thus, in addition to acting as a growth-promoting factor, CLIPA also promotes microtubule dynamics. In the absence of CLIPA, green fluorescent protein-labeled cytoplasmic dynein heavy chain, p150Glued dynactin, and NUDF were all seen as plus-end comets at 32°C. However, under the same conditions, deletion of both clipA and nudE almost completely abolished NUDF comets, although nudE deletion itself did not cause a dramatic change in NUDF localization. Based on these results, we suggest that CLIPA and NUDE both recruit NUDF to the microtubule plus end. The plus-end localization of CLIPA itself seems to be regulated by different mechanisms under different physiological conditions. Although the KipA kinesin (Kip2/Tea2 homologue) did not affect plus-end localization of CLIPA at 32°C, it was required for enhancing plus-end accumulation of CLIPA at an elevated temperature (42°C).

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Ramirez-Rios, Sacnicte, Eric Denarier, Eléa Prezel, Angélique Vinit, Virginie Stoppin-Mellet, François Devred, Pascale Barbier et al. "Tau antagonizes end-binding protein tracking at microtubule ends through a phosphorylation-dependent mechanism". Molecular Biology of the Cell 27, n.º 19 (outubro de 2016): 2924–34. http://dx.doi.org/10.1091/mbc.e16-01-0029.

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Proper regulation of microtubule dynamics is essential for cell functions and involves various microtubule-associated proteins (MAPs). Among them, end-binding proteins (EBs) accumulate at microtubule plus ends, whereas structural MAPs bind along the microtubule lattice. Recent data indicate that the structural MAP tau modulates EB subcellular localization in neurons. However, the molecular determinants of EB/tau interaction remain unknown, as is the effect of this interplay on microtubule dynamics. Here we investigate the mechanisms governing EB/tau interaction in cell-free systems and cellular models. We find that tau inhibits EB tracking at microtubule ends. Tau and EBs form a complex via the C-terminal region of EBs and the microtubule-binding sites of tau. These two domains are required for the inhibitory activity of tau on EB localization to microtubule ends. Moreover, the phosphomimetic mutation S262E within tau microtubule-binding sites impairs EB/tau interaction and prevents the inhibitory effect of tau on EB comets. We further show that microtubule dynamic parameters vary, depending on the combined activities of EBs and tau proteins. Overall our results demonstrate that tau directly antagonizes EB function through a phosphorylation-dependent mechanism. This study highlights a novel role for tau in EB regulation, which might be impaired in neurodegenerative disorders.

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Belyaev, N. A. "The role op the researches of E.I. Kazimirchak-Polonskaya on the dynamical evolution of short-period comets". International Astronomical Union Colloquium 83 (1985): 237–41. http://dx.doi.org/10.1017/s0252921100083950.

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In 1961 Kazimirchak-Polonskaya (1961a, 1961b) published comprehensive reviews of all investigations on the dynamics of cometary orbits as well as on close approaches of the short-period comets with Jupiter for the time span covering 1770–1960 and for the first time put forward the basic problems of cometary astronomy from the standpoint of celestial mechanics. Chebotarev (1971) pointed out that “these reviews, supplemented with extensive references, can serve as a valuable manual for all researchers of cometary motions”. In 1967 Kazimirchak-Polonskaya (1967a) developed the advanced problems in a definitive form. The corresponding member of the USSR Academy of Science M.F. Subbotin (at that time the Head of the Institute) characterized the above mentioned works as “the general plan for cometary studies in the important branch of cometary astronomy”.

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Kuznetsov, I. A., A. V. Zakharov, L. M. Zelenyi, S. I. Popel, T. I. Morozova, I. A. Shashkova, G. G. Dolnikov et al. "Dust Particles in Space: Opportunities for Experimental Research". Астрономический журнал 100, n.º 1 (1 de janeiro de 2023): 41–69. http://dx.doi.org/10.31857/s0004629923010115.

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Space dust and dusty (complex) plasma are one of the most common manifestations of matter in space. Non-atmospheric bodies of the Solar System, such as the Moon, asteroids, comets, some satellites of the planets, are directly affected by external factors of outer space—solar electromagnetic radiation, interplanetary plasma flows, cosmic rays, micrometeors. Under the influence of these factors, regolith is formed on the surface of bodies during geological epochs. Under the influence of impacts of high-speed micrometeors, dust particles of regolith scatter at different speeds. Most of them return to the surface, but some form dust clouds or lose their gravitational connection with the parent body. Under the action of solar radiation, the surface acquires an electric charge, and dust particles under certain conditions can break away from the regolith surface and levitate. Observational evidence of such dynamic phenomena has been recorded on the Moon and on some asteroids. The study of the physical processes responsible for the activation of dust particles and their dynamics is of great interest for fundamental science and practical purposes. The article discusses the main processes occurring under the influence of outer space factors on regolith, as a result of which dust particles move and a near-surface plasma-dust exosphere is formed. Unresolved issues are discussed. Methods and means of laboratory modeling in studying the activation and dynamics of dust particles are considered.

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Poppe, A. R. "The contribution of Centaur-emitted dust to the interplanetary dust distribution". Monthly Notices of the Royal Astronomical Society 490, n.º 2 (9 de outubro de 2019): 2421–29. http://dx.doi.org/10.1093/mnras/stz2800.

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ABSTRACT Interplanetary dust grains originate from a variety of source bodies, including comets, asteroids, and Edgeworth–Kuiper belt objects. Centaurs, generally defined as those objects with orbits that cross the outer planets, have occasionally been observed to exhibit cometary-like outgassing at distances beyond Jupiter, implying that they may be an important source of dust grains in the outer Solar system. Here, we use an interplanetary dust grain dynamics model to study the behaviour and equilibrium distribution of Centaur-emitted interplanetary dust grains. We focus on the five Centaurs with the highest current mass-loss rates: 29P/Schwassmann-Wachmann 1, 166P/2001 T4, 174P/Echeclus, C/2001 M10, and P/2004 A1, which together comprise 98 per cent of the current mass loss from all Centaurs. Our simulations show that Centaur-emitted dust grains with radii s < 2 μm have median lifetimes consistent with Poynting–Robertson (P–R) drag lifetimes, while grains with radii s > 2 μm have median lifetimes much shorter than their P–R drag lifetimes, suggesting that dynamical interactions with the outer planets are effective in scattering larger grains, in analogy to the relatively short lifetimes of Centaurs themselves. Equilibrium density distributions of grains emitted from specific Centaurs show a variety of structure including local maxima in the outer Solar system and azimuthal asymmetries, depending on the orbital elements of the parent Centaur. Finally, we compare the total Centaur interplanetary dust density to dust produced from Edgeworth–Kuiper belt objects, Jupiter-family comets, and Oort cloud comets, and conclude that Centaur-emitted dust may be an important component between 5 and 15 au, contributing approximately 25 per cent of the local interplanetary dust density at Saturn.

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