1,751 research outputs found
Main Belt Comets: Status Report of the Monitoring Large Programme at ESO
No full textA status report of the 4-years long ESO Large Program, aimed at monitoring and characterizing Main Belt Comets. The program is 50% completed. Some of the MBCs were observed active. In some cases, the dust is the aftermath of an impact
Predictions of the solar wind interaction with Comet Grigg-Skjellerup
No full textThe planned encounter of the Giotto spacecraft with comet Grigg-Skjellerup on 10th July 1992 promises to extend our knowledge of the solar wind interaction with comets substantially. While there have been spacecraft missions to comets before now, this mission is exploratory in the sense that the target comet is much older and there-fore it has a much lower gas production rate than comets Halley (by a factor approximately 200) or Giacobini-Zinner (factor approximately 10). Here we present theoretical predictions for the location of the bow shock and contact surface features, and compare similar predictions with the observed features at the previous encounters. We discuss the applicability of fluid-type theory which these models employ, in the case of strong and weak comets in the solar wind
Properties of mass-loading shocks, 2. Magnetohydrodynamics
No full textThe one-dimensional magnetohydrodynamics of shocked flows subjected to significant mass loading are considered. Recent observations at comets Giacobini-Zinner and Halley suggest that simple nonreacting MHD is an inappropriate description for active cometary bow shocks. The thickness of the observed cometary shock implies that mass loading represents an important dynamical process within the shock itself, thereby requiring that the Rankine-Hugoniot condition for the mass flux possess a source term. In a formal sense, this renders mass-loading shocks qualitatively similar to combustion shocks, except that mass loading induces the shocked flow to shear. Nevertheless, a large class of stable shocks exist, identified by means of the Lax conditions appropriate to MHD. Thus mass-loading shocks represent a new and interesting class of shocks, which, although found frequently in the solar system, both at the head of comets and, under suitable conditions, upsteam of weakly magnetized and nonmagnetized planets, has not been discussed in any detail. Owing to the shearing of the flow, mass-loading shocks can behave like switch-on shocks regardless of the magnitude of the plasma beta. Thus the behavior of the magnetic field in mass-loading shocks is significantly different from that occurring in nonreacting classical MHD shocks. It is demonstrated that there exist two types of mass-loading fronts for which no classical MHD analogue exists, these being the fast and slow compound mass-loading shocks. These shocks are characterized by an initial deceleration of the fluid flow to either the fast or the slow magnetosonic speed followed by an isentropic expansion to the final decelerated downstream state. Thus these transitions take the flow from a supersonic to a supersonic, although decelerated, downstream state, unlike shocks which occur in classical MHD or gasdynamics. It is possible that such structures have been observed during the Giotto-Halley encounter, and a brief discussion of the appropriate Halley parameters is therefore given, together with a short discussion of the determination of the shock normal from observations. A further interesting new form of mass-loading shock is the “slow-intermediate” shock, a stable shock which possesses many of the properties of intermediate MHD shocks yet which propagates like a slow mode MHD shock. An important property of mass-loading shocks is the large parameter regime (compared with classical MHD) which does not admit simple or stable transitions from a given upstream to a downstream state. This suggests that it is often necessary to construct compound structures consisting of shocks, slip waves, rarefactions, and fast and slow compound waves in order to connect given upstream and downstream states. Thus the Riemann problem is significantly different from that of classical MHD
Infrared observations of faint comets
No full textInfrared observations of the periodic comets Encke, Stephan-Oterma and Chernykh indicate that the dusty component in this class of comets is not radically different from the dusty component found in nonperiodic comets. The differences in the infrared behavior among these three comets suggest that a range of behaviors rather than a single behavior typifies the cometary activity. The range in albedo (0.02 to 0.10) of the dust calculated for the periodic comets is similar to the range in albedos seen among the asteroids
Small-body encounters using solar sail propulsion
No full textCometary Rendezvous and Flybys have large V requirements, which impose almost unattainable, and sometimes prohibitive, demands on the propellant budget of conventional, chemical propulsion. Ion Propulsion is a viable alternative, but as the number and difficulty of target objectives increases then the potential of this technology becomes rapidly less attractive. Solar sails exhibit an extremely high effective specific impulse over long mission durations. No propellant is required so that large changes in V could be realised without necessitating the introduction of complex gravity assists, which prolong mission duration and restrict launch opportunities. The endurance of the structures and materials are thus the only limiting factors dictating the number and range of bodies with which the solar-sail propelled vehicle can encounter throughout its lifetime. In this paper we have analysed a number of high-energy, small-body mission scenarios using a parameterised approach to sail control representation. The sail cone and clock angle histories were characterised by linear interpolation across a set of discrete nodes. The optimal control problem was thus transcribed to a Non-Linear Programming problem to select the optimal controls at the nodes that minimised the transfer time while enforcing the cartesian end-point boundary constraints (6 states for rendezvous, 3 for flypast). The Fortran77 optimisation package NPSOL 5.0 was used for this purpose with the variational equations of motion formulated in modified equinoctial orbital elements and integrated using a variable-order, adaptive step-size Adams-Moulton-Bashforth method. We present optimal rendezvous trajectories to Short-Period Comets such as 46P/Wirtanen in 484 days with a sail characteristic acceleration of 1.9 mms-2, and with 2P/Encke in 574 days with a characteristic acceleration of 1.0 mms-2. An analysis using high-performance sails has been conducted to permit fast flyby intercepts of newly discovered Long Period Comets (LPCs). Previous examples adopted were C/1995 O1/Hale- Bopp, C/1995 Y1/Hyakutake, C/1999 T1/McNaught-Hartley, C/1999 F1/Catalina, C/1999 N2/Lynn and C/1999 H1/Lee, to demonstrate the feasibility of a late launch to quickly intercept a new LPC using a solar sail. Since the time between discovery of a new LPC such as Hale-Bopp and perihelion passage was less then 2 years, this then leaves a very short time-span for orbit determination, preparation, planning and operational phases. Preliminary mission analysis shows that a Hale-Bopp perihelion flypast could have been achieved, with a sail characteristic acceleration of 5.0 mms-2, by launching just 209 days before comet perihelion passage. With a characteristic acceleration of 2.0 mms-2 Hale-Bopp could also have been intercepted at its descending node by launching 270 days before nodal descent. The sail could then have returned to rendezvous with the Earth 261 days later, giving a minimum total mission turn-around time of 531 days. An alternative, dual flyby scenario has been investigated, to continue on to C/1997 D1/Mueller, after which solar system escape was reached and arrival at Heliopause would occur in 12 years. Solar Electric Propulsion has been adopted as the primary propulsion system for the DAWN dual asteroid rendezvous mission scheduled for launch in 2006. The objective of this mission is to rendezvous with inner main-belt asteroids, Vesta and Ceres. We have also investigated solar sail adaptation to this mission, for the same launch date and 11 month orbiter stay-times. We have extended the mission objectives to two further asteroids, Lucina and Lutetia, with the aim of demonstrating a Mainbelt Asteroid Survey scenario
Assessment of early-modern observations of comets and supernovae: Focus on pre-telescopic European astrometric and physical data
No full textThe two-century period prior to the publication of Newton's Principia (first edition 1687; third edition 1726) was most important in terms of the radical changes that occurred in the observation, perception, and understanding of celestial objects that in turn spurred Newton to deduce his laws of gravitation and motion. Surprisingly, much of the available observational data embedded in contemporary texts from that two-century period has remained unused by modern astronomers, and this thesis (a) describes large amounts of data that were found and reanalyzed during the course of this Ph.D. research project, (b) places these data and their resulting analyses in context with the astronomy of the early-modern era, and (c) shows how modern astronomers and historians benefit from such information. The emphasis is placed here on west-European observations, as observations made elsewhere (eastern Europe, Asia) were isolated (not communicated for convenient rapid use by contemporary astronomers elsewhere) and did not develop or employ the level of precision that was utilized by western European astronomers through the extensive discussions that developed from correspondence and publication in Europe
Ground-based photometry of comets in the spectral interval 3000 to 3500 angstrom
No full textAbundances and production rates of CN, C3, and C2 for fifteen comets were determined and the variation of these parameters with helicentric distance for two comets were monitored. While these measurements provide much information about the similarities and differences among comets, only a small fraction of the total material in any of the comets observed was sampled
Alkali phenoxides in comets
No full textContext. Potassium was first detected in spectra of the sungrazer comet C/1965 S1 Ikeya-Seki at the heliocentric distance rh = 0.15 au and, 48 years later, in comets C/2011 L4 PanSTARRS and C/2012 S1 ISON at rh = 0.46 au. The alkali tail photoionization model provides a Na/K ratio close to the solar value in comets C/1965 S1 and C/2011 L4. No lithium was detected in any comet: the lower limit of the Na/Li ratio was almost one order of magnitude greater than the solar ratio.
Aims. Here we searched for the emissions of the alkali NaI, KI, and LiI in Comets C/2020 F3 NEOWISE and C/2024 G3 ATLAS.
Methods. High-resolution spectra of the comets were taken with the 0.84 m telescope at the Schiaparelli Observatory at rh = 0.36 and rh = 0.15 au, respectively, the observations closest to the Sun since C/1965 S1. To model the data, we assumed that alkali phenoxides are present in the aromatic fraction of organic dust at the nucleus surface where they react with carbon dioxide ejecting alkali atoms. Results. NaI and KI were detected in emission lines of exceptional intensity in both comets, with no evidence of LiI emission. The NaI/KI ratios were determined: 31 ± 5 and 26 ± 8 in comets C/2020 F3 and C/2024 G3, respectively, whereas solar Na/K ≈ 15. This excess and its observed trend with the heliocentric distance are consistent with chemistry between CO2 and alkali phenoxides at the nucleus surface. The Li upper limit for comet C/2020 F3 is very stringent at Na/Li > 3.4 × 104, a factor of 34 greater than the solar value. This Li depletion is consistent with the reaction rate of lithium phenoxides, which is a factor of 104 slower than sodium phenoxides.
Conclusions. The widespread chemistry of carbon dioxide with organic dust may provide a significant energy and mass sink of carbon dioxide in all comets also at rh > 1 au, reconciling recent models of cometary activity with Rosetta CO2 measurements. At rh < 0.5 au potassium was observed in all comets, so that we predict the formation of a KI tail spatially resolved from the NaI tail
IUE observations of faint comets
No full textUltraviolet spectra of seven comets taken with the same instrument are presented. Comets P/Encke (1980), P/Tuttle (1980 h), P/Stephan-Oterma (1980 g), and Meier (1980 q) were observed during November-December 1980 with the International Ultraviolet Explorer (IUE) satellite observatory, while comets P/Borrelly (1980 i) and Panther (1980 u) were observed with IUE on 6 March 1981. The spectra of these comets are compared with those of comet Bradfield (1979 X), studied extensively earlier in 1980 with IUE, as well as with each other. In order to simplify the interpretation of the data and to minimize the dependence upon a specific model, the spectra are compared at approximately the same value of heliocentric distance whenever possible. Effects due to heliocentric velocity, geocentric distance, and optical depth are also discussed. All of the cometary spectra are remarkably similar, which suggests that these comets may have a common composition and origin.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/24271/1/0000537.pd
Astrometric observations of comets and minor planets
No full textComets and planet crossing asteroids are observed so that accurate positions can be determined. The observations are made with the Palomar 1.5 m telescope equipped with a CCD array. The combination of telescope and detector is quite effective at recording faint comets and minor planets. This proves useful for early acquisition of comets and asteroids returning for a new opposition. The resulting positions permit accurate orbits to be determined and allow the properties of the comets and asteroids to be measured by other observers using a variety of techniques. Recoveries and other notable observations of comets and planet crossing asteroids observed during the past years are discussed
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