Perihelion Passage Research Articles

Have stellar and supernovae radiations altered the pristine nature of comets?

Effect of stellar and supernova radiations on cometary nuclei in the Oort cloud is investigated. Radiation dose received by a comet is calculated and compared with the one which Halley's comet receives by one perihelion passage. Stellar radiation provides 10 to 50% of Halley unit over 4 billion years. Inclusion of sublimation of volatile molecules such as CO or N2 does not allow the temperature to rise to 30 K by irradiation of bright OB stars, contrary to the claim of Stern and Shull. A chance encounter with a SN provides radiation dose which is just sufficient to raise to 30 K the surface layer which is 1 m thick on the assumption that the radiation is wholly communicated to the interior. Thus, the comets remain pristine under the effect of stellar and SN radiations.

Using comet light-curve asymmetries to predict comet returns

THE gravitational attractions of the Sun and planets do not account completely for the orbital motions of short-period comets, which tend to return to perihelia with some delay or advance as compared with the gravitational solution. Debate over the physical cause of these 'nongravitational effects' has continued from the time they were discovered, in the early nineteenth century. Uncertainty over their origin has made prediction of these effects for any particular comet impossible. To clarify the roles of the radial and transverse components of the nongravitational force, and thus to arrive at an accurate picture with predictive power, we have used observational data on gas production rates from comets as a diagnostic of the force. The shapes of the production curves, based mostly on visual light curves, correlate very well with the nongravitational delays or advances of a number of comets, and we have used this correlation to predict a substantial advance of the recent perihelion passage of comet P/Brorsen–Metcalf, as verified by observations.

The Strange Periodic Comet Machholz

The recently discovered periodic comet Machholz 1986 VIII (1986e) travels closer to the sun than any known planet and any known comet with an orbital period of less than 150 years, thus providing astronomers with a unique object for studying cometary evolution. The comet is spiraling steadily closer to the sun, from perihelion distance q [unknown] 0.9 astronomical unit at about A.D. 700 to q [unknown] 0.13 at present (orbital period, 5.25 years), to an expected q [unknown] 0.03 by about 2450; should the comet survive such increasingly close perihelion passages, q will begin steadily to increase shortly thereafter. A review of observations made since discovery is presented, together with a discussion of numerical investigations of the comet's orbit over 4000 years and prospects for observing the upcoming return to perihelion in 1991.

Comet orbits from observations during 1984 and orbital elements for the next perihelion passage

AbstractCorrected orbits for 21 comets from observations published in the Minor Planets Circulars have been calculated. Orbital elements for the next perihelion passages of short period comets are also given.

A Postencounter View of Comets

Ground-based and space observations of Comet Halley during its 1986 perihelion passage are reviewed, with an emphasis on their implications for theoretical models. Consideration is given to the shape, surface morphology, and composition of the comet nucleus; the shape, dynamics, and composition of the dust tail; neutral and ionic gas species in the head and plasma tail; and the comet/solar-wind interaction. Extensive diagrams, graphs, and sample images are provided, and the potential value of the new kinds of data to be obtained with the NASA Comet-Rendezvous/Asteroid-Flyby spacecraft is discussed.

Sulfides and oxides in comets

view Abstract Citations (8) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Sulides and Oxides in Comets Rietmeijer, Frans J. M. Abstract Metal abundances associated with Sun-grazing P/comet Ikeya-Seki 1965f, the mineralogy of chondritic interplanetary dust particles and cosmochemical affinities of Co, V, Cr, and Ni in extraterrestrial materials and probable vaporization data for nonsilicate minerals are used to evaluate the putative dearth of nonsilicates in short-period comets. It is concluded that sulfides and oxides are common, albeit minor, constituents of these comets. Sulfides and oxides can form in situ during perihelion passage in the nucleus of active short-period comets by sulfidation of Mg, Fe-silicates. Publication: The Astrophysical Journal Pub Date: August 1988 DOI: 10.1086/185253 Bibcode: 1988ApJ...331L.137R Keywords: Chemical Composition; Comets; Cosmochemistry; Interplanetary Dust; Oxides; Sulfides; Chondrites; Iron Compounds; Magnesium Compounds; Mineralogy; Astrophysics; COMETS; INTERPLANETARY MEDIUM full text sources ADS |

Precession of triaxial cometary nuclei

Comet nuclei are modeled as rotating, homogeneous, triaxial ellipsoids that precess during perihelion passage due to a subsolar dust and gas jet induced by solar heating. We develop a phase-averaged system of equations in which the torque is averaged over a rotation about the spin axis. The inertial space precession of the rotation axis is computed numerically using the phase-averaged system. The result is checked against independent calculations using the exact Euler-Poisson system of equations.

The evolution of topography on a comet

We have developed a model to investigate how topographical features on a comet evolve through the focusing of thermal energy due to the concave nature of the topography. The energy balance equation has been solved for an infinitely long trench whose cross section is a segment of a circle. The model includes incident sunlight, shadowing, sticking of molecules as they sublime from one wall and strike the other, and radiative heating of one wall by the other. It is found that the trench has a lens effect: the bottom of the trench will always lose more mass to sublimation than the edge of the trench, and all points in the trench lose more mass per day than a flat surface. Over the course of a large number of perihelion passages, the net effect will be for a concave topographical feature to accentuate rather than diminish in magnitude. This process may have implications for the net sublimation rate of a comet surface, for the splitting of comets, and for the formation of jets coming from a comet surface.

Observations of comet Giacobini-Zinner at the 1.667 GHz OH line

view Abstract Citations (5) References (3) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Observations of Comet Giacobini-Zinner at the 1.667 GHz OH Line Galt, J. Abstract Comet Giacobini-Zinner was observed during an 18 day interval just prior to its perihelion passage. The 1.667 GHz OH line appeared in absorption, as expected, but showed large intensity fluctuations from day to day. Comparisons are made with subsequent observations of comet Halley. Publication: The Astronomical Journal Pub Date: July 1987 DOI: 10.1086/114461 Bibcode: 1987AJ.....94..174G Keywords: Absorption Spectra; Astronomical Spectroscopy; Giacobini-Zinner Comet; Hydroxyl Emission; Line Spectra; Halley'S Comet; Periodic Variations; Radiant Flux Density; COMETS; GIACOBINI-ZINNER; OBSERVATIONS; ABSORPTION; INTENSITY; SPECTRA; COMPARISONS; EARTH-BASED OBSERVATIONS; WAVELENGTHS; RADIO WAVES; DATA; Astronomy; Comets; COMETS full text sources ADS |

The Giotto mission to Comet Halley

Giotto encountered Comet Halley on 14 March 1986, about four weeks after the comet's perihelion passage when it was most active. Giotto passed the comet nucleus at a distance of 600 km on the sunward side. Its scientific payload-comprising a narrow-angle camera, three mass spectrometers for the compositional and isotopic analysis of the neutral gas, ions and dust particles in the cometary coma, various dust impact detectors, a photopolarimeter and a set of plasma instruments-worked flawlessly and returned a wealth of data. The mission's scientific objectives, the spacecraft and the mission from launch to encounter are described. After the encounter Giotto was retargeted to return to the Earth. An Earth swing-by manoeuvre on 2 July 1990 will allow Giotto to encounter Comet Grigg-Skjellerup in July/August 1992.

Periodic photmmetric variations in the near-nucleus zone of P/Giacobini-Zinner

We observed photometrically P/Giacobini-Zinner before its 1985 perihelion passage with the IHW standard filter set. We found a periodicity of 9.5 ± .2hr in the lightcurve of a color index of the nuclear region of the comet, which we interpret as being produced by albedo variations and also possibly by a nonspherical geometry of the spinning nucleus. A statistical analysis of our result indicates that the period we found is significant within at least a 97% confidence level. The same analysis, when applied to published data for P/d'Arrest, indicates that any periodicity there is significant to within no more than an 80% confidence level. The nucleus of P/Giacobini-Zinner is probably not smaller than 0.6 km.

The Discovery of Dust Trails in the Orbits of Periodic Comets

Analysis of data from the Infrared Astronomical Satellite has yielded evidence for narrow trails of dust coincident with the orbits of periodic comets Tempel 2, Encke, and Gunn. Dust was found both ahead of and behind the orbital positions of these comets. This dust was produced by the low-velocity ejection of large particles during perihelion passage. More than 100 additional dust trails are suggested by the data, almost all near the detection limits of the satellite. Many of these dust trails may be derived from previously unobserved comets.

A model of cometary gas and dust production and nongravitational forces with application to P/Halley

Abstract A program that computes gas and dust production rates and idealized nongravitational force components has been developed and applied to the case of Comet Halley. We use a modified form of our earlier comet model (F.P. Fanale and J.R. Salvali[(1984) Icarus 60 , 476–511] to which coma effects and a section on nongravitational forces have been added. The possibility of grain cohesion is also included. These models are used together with observations from 1910 and semiempirically derived data to investigate the effects of obliquity and thermal conductivity of the near thermal conductivity of the nucleus on gas and dust production. The results indicate that the thermal conductivity of the nucleus is of the order of 10 5 ergs/cm-s-°K, which implies that the ice near the surface is in the crystalline form. A general method is presented for calculating the radii of cometary nuclei using theoretically derived and semiempirically derived nongravitational force components. This method is used to calculate possible radii for Comet Halley that depend on the model variation chosen. The method used and the results presented herein should have greater significance and value when the observational data from Halley's current perihelion passage become available.

The comet Halley dust and gas environment

Quantitative descriptions of environments near the nucleus of comet P/Halley have been developed to support spacecraft and mission design for the flyby encounters in March, 1986. To summarize these models as they exist just before the encounters, we review the relevant data from prior Halley apparitions and from recent cometary research. Orbital elements, visual magnitudes, and parameter values and analysis for the nucleus, gas and dust are combined to predict Halley's position, production rates, gas and dust distributions, and electromagnetic radiation field for the current perihelion passage. The predicted numerical results have been useful for estimating likely spacecraft effects, such as impact damage and attitude perturbation. Sample applications are cited, including design of a dust shield for spacecraft structure, and threshold and dynamic range selection for flight experiments. We expect that the comet's activity may be more irregular than these smoothly varying models predict, and that comparison with the flyby data will be instructive.

The size, mass, mass loss and age of Halley's comet

Halley's comet (1910 II, 1982i) has a D2pυ value of 5.90±0.33 km2, where D km is the diameter of the nucleus and pυ is the geometric albedo. The mass of the nucleus is |$7.5\times10^{15}\enspace \varrho p_\upsilon^{-1.5}\enspace \text g$|⁠, where ϱ is the density. Reasonable assumptions as to pυ and ϱ yield diameter and mass values of 9.4 km and 2.2×1017 g. In the 1910 apparition the comet lost a mass of 2.8×1014 g which is equivalent to an absolute magnitude change of 9×10−4 per apparition. The mass of the meteor stream, produced by the decay of P/Halley, is consistent with the statement that the comet has had 2300 previous close passages of the Sun. The comet will probably disappear altogether alter another 2300 close perihelion passages.

The problem of the 1/a-distribution and cometary fading

AbstractThe background to the problem of explaining the frequency distribution of cometary 1/a-values is briefly reviewed and it is emphasised that the explanation in terms of the Oort Cloud model relies on an ad hoc fitting function - the fading/disruption probability per revolution. Assuming an underlying steady-state Oort Cloud and the integral equation formalism developed by Oort and Yabushita to predict the 1/a-distribution for an arbitrary fading probability, we have been able to constrain the unknown fading function by comparison with observations. In agreement with previous work we find that the tendency for fading or disruption should be strong at small 1/a-values and weak at large 1/a-values. The mean fading probability per revolution, k(x), is found to lie within a factor roughly of order 2 about k(x) ≈ 0.3(1+(x/4 × 10−3)2)−3/2, where × is 1/a in units AU−1. A physical model for fading which might qualitatively account for this behaviour is tentatively proposed. This depends on the thermal shock experienced by a long-period comet nucleus around perihelion passage. It is emphasised that until a viable model for fading has been found, the validity of the steady-state primordial hypothesis remains unresolved.

An idealized short-period comet model: Surface insolation, H2O flux, dust flux, and mantle evolution

A model of cometary activity is developed which integrates the feedback processes involving heat, gas, and dust transport, and dust mantle development. The model includes the effects of latitude, rotation, and spin axis orientation. Results are obtained for various grain size distributions, dust-to-ice ratios, and spin axis orientations. Attention is focused on the development, change of structure and distribution of dust mantles and their mutual interaction with ice surface temperature and gas and dust production. In this model the dust mantle controls the mechanism of gas transport not onlu by its effect on the temperature but, more importantly, by its own dynamic stability. Results suggest that an initially homogeneous short-period comet with a “cosmic” dust-to-water ice ratio, typical orbit, rotation rate, and grain size distribution would develop at most only a thin (<1 mm) cyclic mantle at all points on the nucleus. Such a fully developed temporary mantle would exist throughout the diurnal cycle only beyond ∼4AU. Thus, cyclic behavior would be expected for such an idealized comet, at least for most of its lifetime. Long-term irreversible mantle development on comets with typical rotation rates was not found except regionally on Encke and also on objects with perihelia ≳1.5 AU. Even in these cases, free silicate exists, after a few cycles, only as relatively rare large grains and agglomerates with radii ∼1 cm scattered over exposed ice. Full mantle development would require hundreds to thousands of cycles. In the case of an initially homogeneous comet Encke, this slow incipient mantle development is shown to be the direct result of its peculiar axial orientation. High obliquity appears required for long-term mantle development for typical rotation rates and perihelia ≲1.5 AU. Heat conduction into the nucleus for an incompletely mantled or bald comet has been found to be very important in maintaining relatively higher ice surface temperatures, and hence fluxes, during those portions of the diurnal and orbital cycles which would otherwise be cooler. It is also shown to be at least one cause of post perihelion brightness asymmetries, especially in lower obliquity comets. Maximum heliocentric distances at which 1-μm dust, sand, pebbles, cobbles, and boulders can be permanently ejected from the subsolar point by H2O (CO2) are (in AU): 6.9 (16.8), 5.2 (11.5), 1.8 (3.0), 0.21 (0.34) and 0.07 (0.11), respectively. A detailed anatomy of temperature, gas and dust fluxes vs latitude and longitude for a homogeneous rotating comet with fixed axis is given for comparison with future observations. Most H2O flux histories deduced from brightness data are found to be in reasonable agreement with the model, allowing for uncertainty in radius and albedo. A clear exception is Encke. It is shown that the large discrepancy between Encke's observed and model predicted fluxes, based on radar cross section, can be used to evaluate the extent of exposed ice (<10%). The model is then used to place an active area so as to explain a reported sharp drop in flux on approach to the Sun at 0.78 AU. An active area or areas, <10% of the comet's surface, centered near 65°N latitude appears indicated. Although cyclic mantles are generally indicated for the set of parameters we used, our results show that a global mantle only 1 to 3 cm thick (depending on the orbit) consisting of a full range of grain sizes can cause irresversible evolution to a noncometary body. We investigated the long-term evolution of such a postulated initially thinly mantled cometary object. It was found that after the first few passes and until the end of its dynamic lifetime the object averaged <3 × 10−12 g cm−1 sec−1 H2O flux. Therefore, if cometary objects evolve into Apollo asteroids, ice should always be accessible within 10 m of the surface despite numerous close perihelion passages. The possible impact of factors not included in the model, such as initial inhomogeneities, coma scattering of radiation, and global redistribution of ejected silicate around the nucleus, are discussed.

How stable is an astronomical clock that can trigger mass extinctions on Earth?

The periodicity in mass extinctions observed in the fossil record1,2 may be driven by an astronomical clock consisting of a companion star to the Sun3,4. Each perihelion passage of the companion star would result in an enhanced rate of arrival of comets in the inner planetary system, some of which could collide with the Earth and perturb the atmosphere strongly enough to cause a catastrophic extinction of many, if not most, species5,6. A similar periodicity has been observed in the cratering rate on the Earth7,8. On Earth, the dating of both mass extinctions and crater formation is subject to observational uncertainties. In the heavens, too, we should not expect the ‘double star clock’ to be perfect, because of the influence of the galactic tidal force field and the perturbations of passing stars. On the basis of orbit calculations reported below, I expect the irregularity in the period of revolution over the past 250 Myr to be at least 10%, and more likely around 20%. This suggests that even completely accurate dating of mass extinctions and crater impacts should not be expected to yield perfectly sharp peaks in the power spectrum of a Fourier analysis.

Orbital stability of the unseen solar companion linked to periodic extinction events

Recent suggestions of astronomical causes for the closely periodic extinction events in the geological record1 and the possible periodicities in the cratering record2 have involved two basic ideas. First, it has been suggested3–5 that the oscillations of the Solar System through the galactic plane with semi-periods of the order of 30 Myr make possible close encounters with giant molecular clouds, or sub-units thereof, which are expected to scatter Oort cloud comets into the terrestrial zone. Second, it has been suggested6,7 that a low-mass, and as yet unseen, companion to the Sun, tentatively named Nemesis, is in an eccentric orbit (e ∼0.7), such that perihelion passages bring it through the postulated dense inner Oort cloud8. Perturbations of cometary bodies there result in scattering of some 109 comets into the terrestrial zone. Those that impact the Earth are assumed to loft dust into the atmosphere, causing climatic chaos5,9. To achieve the period that matches the best estimates for the intervals between extinction events, semi-major axes in the range 9 × 104−1 × 105 AU are required. Whether such orbits are indeed stable over the lifetime of the Solar System is not known. We present here evidence from three-dimensional numerical modelling that only orbits with a limited range in inclination with respect to the galactic plane are formally stable for the required length of time. Perturbations bypassing stars and molecular clouds may make even these orbits unstable.

Statistical nature of perihelion passages of long-period comets

The distributions of annual flux and time interval of perihelion passages of long-period comets are examined statistically. The former is found to be well represented by the Poisson distribution, while the latter by the exponential one. This means that the perihelion passages of long-period comets are an almost random phenomenon, at least within the time scale of a few hundred years, even if the samples are affected by observational selection and/or the effect of physical disruption.