scholarly journals Structure of Comets and the Possible Origin of Faint Asteroids

1971 ◽  
Vol 12 ◽  
pp. 465-469
Author(s):  
V. Vanýsek
Keyword(s):  
Final Stage ◽  
Heliocentric Distance ◽  
Outer Boundary ◽  
Very Old ◽  
Cometary Nuclei ◽  
Mean Motion ◽  
Short Period ◽  
Asteroidal Belt ◽  
The Mean

It is shown that very old and still active icy-conglomerate or “clathrate” cometary nuclei may exist at the outer boundary of the asteroidal belt and belong to the group of relatively stable “outer” short-period comets of which only a small fraction have been discovered or recognized as cometary objects.hich is much more convincing, it explains their brightness dependence on the heliocentric distance.The search for these comets among very faint asteroids with the mean motion μ ≤ 600”, and the study of their behavior could be very important to our knowledge about the final stage of cometary nuclei.

the effects of resonance with Jupiter in the system of short-period comets

1974 ◽  
Vol 22 ◽  
pp. 193-203
Author(s):  
L̆ubor Kresák
Keyword(s):  
Structural Effects ◽  
Order Resonance ◽  
Mean Motion ◽  
Short Period ◽  
The Mean ◽  
Compound Effect ◽  
Low Order

AbstractStructural effects of the resonance with the mean motion of Jupiter on the system of short-period comets are discussed. The distribution of mean motions, determined from sets of consecutive perihelion passages of all known periodic comets, reveals a number of gaps associated with low-order resonance; most pronounced are those corresponding to the simplest commensurabilities of 5/2, 2/1, 5/3, 3/2, 1/1 and 1/2. The formation of the gaps is explained by a compound effect of five possible types of behaviour of the comets set into an approximate resonance, ranging from quick passages through the gap to temporary librations avoiding closer approaches to Jupiter. In addition to the comets of almost asteroidal appearance, librating with small amplitudes around the lower resonance ratios (Marsden, 1970b), there is an interesting group of faint diffuse comets librating in characteristic periods of about 200 years, with large amplitudes of about±8% in μ and almost±180° in σ, around the 2/1 resonance gap. This transient type of motion appears to be nearly as frequent as a circulating motion with period of revolution of less than one half that of Jupiter. The temporary members of this group are characteristic not only by their appearance but also by rather peculiar discovery conditions.

scholarly journals Mean Motion Resonances in The Asteroid Belt

1999 ◽  
Vol 172 ◽  
pp. 381-382
Author(s):  
D. Nesvorný ◽  
A. Morbidelli
Keyword(s):  
Asteroid Belt ◽  
Mean Motion Resonances ◽  
Mean Motion ◽  
Lyapunov Time ◽  
Asteroidal Belt ◽  
Kirkwood Gaps ◽  
The Mean ◽  
Three Body

The Kirkwood gaps in the main asteroidal belt (2 – 3.5 AU) coincide with the mean motion resonances with Jupiter (4/1, 3/1, 5/2, 7/3, 2/1). Similarly, several narrower gaps are observed in the outer asteroid belt (3.5 – 4 AU) at places of 11/6, 9/5, 7/4 and 5/3 Jovian resonances (Holman and Murray 1996). As it is now generally accepted, the formation and preservation of these gaps is due to the chaos of the resonant space and efficient ejection of the primordial and collisionaly injected bodies towards high eccentricities and planet-crossing orbits.The Jovian mean motion resonances are not the most important in what concerns the chaos of the observed (i.e. remaining) asteroid population. It was estimated by Šidlichovský and Nesvorný (1998) that about 40% of known objects have the Lyapunov time less than 105 years. It was later found (Nesvorný and Morbidelli 1998, 1999; Morbidelli and Nesvorný 1999) that the resonances responsible for this chaos are, in decreasing order of importance: 1) three-body resonances with Jupiter and Saturn, 2) exterior resonances with Mars, 3) moderate order Jovian resonances, and 4) three-body resonances with Mars and Jupiter.

scholarly journals A Comparison of Averaged and Full Models to Study the Third-Body Perturbation

2013 ◽  
Vol 2013 ◽  
pp. 1-16 ◽  
Author(s):  
Carlos Renato Huaura Solórzano ◽  
Antonio Fernando Bertachini de Almeida Prado
Keyword(s):  
Initial Conditions ◽  
Artificial Satellite ◽  
Main Body ◽  
Long Term Effects ◽  
Third Body ◽  
Mean Motion ◽  
Short Period ◽  
The Mean ◽  
Averaged Models ◽  
Averaged Model

The effects of a third-body travelling in a circular orbit around a main body on a massless satellite that is orbiting the same main body are studied under two averaged models, single and double, where expansions of the disturbing function are made, and the full restricted circular three-body problem. The goal is to compare the behavior of these two averaged models against the full problem for long-term effects, in order to have some knowledge of their differences. The single averaged model eliminates the terms due to the short period of the spacecraft. The double average is taken over the mean motion of the satellite and the mean motion of the disturbing body, so removing both short period terms. As an example of the methods, an artificial satellite around the Earth perturbed by the Moon is used. A detailed study of the effects of different initial conditions in the orbit of the spacecraft is made.

Models of comets with strongly variable nongravitational effects

1999 ◽  
Vol 173 ◽  
pp. 381-387
Author(s):  
M. Królikowska ◽  
G. Sitarski ◽  
S. Szutowicz
Keyword(s):  
Cometary Nucleus ◽  
Spin Axis ◽  
Reactive Force ◽  
Orbital Elements ◽  
Cometary Nuclei ◽  
Short Period ◽  
Basic Parameters ◽  
Polar Radius ◽  
Cometary Activity ◽  
Nongravitational Effects

AbstractThe nongravitational motion of five “erratic” short-period comets is studied on the basis of published astrometric observations. We present the precession models which successfully link all the observed apparitions of the comets: 21P/Giacobini-Zinner, 31P/Schwassmann-Wachmann 2, 32P/Comas Solá, 37P/Forbes, and 43P/Wolf-Harrington. We used the Sekanina's forced precession model of the rotating cometary nucleus to include the nongravitational terms into equations of the comet's motion. Values of six basic parameters (four connected with the rotating comet nucleus and two describing the precession of spin-axis of the nucleus) have been determined along the orbital elements from positional observations of the comets. The solutions were derived with additional assumptions which introduce instantaneous changes of modulus of reactive force,Aand of maximum of cometary activity with respect to perihelion time. The present precession models impose some contraints on sizes and rotational periods of cometary nuclei. According to our solutions the nucleus of 21P/Giacobini-Zinner with oblateness along the spin-axis of about 0.32 (equatorial to polar radius of 1.46) is the most oblate among five investigated comets.

Covering of cometary nucleus by refractory crust and its evolution into asteroid-like body

1999 ◽  
Vol 173 ◽  
pp. 365-370
Author(s):  
Kh.I. Ibadinov
Keyword(s):  
Laboratory Experiments ◽  
Cometary Nucleus ◽  
Sublimation Rate ◽  
Cometary Nuclei ◽  
Short Period Comet ◽  
Short Period ◽  
Brightness Decrease ◽  
Crust Thickness ◽  
Ice Sublimation ◽  
Period Comet

AbstractFrom the established dependence of the brightness decrease of a short-period comet dependence on the perihelion distance of its orbit it follows that part of the surface of these cometary nuclei gradually covers by a refractory crust. The results of cometary nucleus simulation show that at constant insolation energy the crust thickness is proportional to the square root of the insolation time and the ice sublimation rate is inversely proportional to the crust thickness. From laboratory experiments resulted the thermal regime, the gas productivity of the nucleus, covering of the nucleus by the crust, and the tempo of evolution of a short-period comet into the asteroid-like body studied.

scholarly journals The lunar and solar diurnal variations of water-level in a well at Kew observatory, Richmond

1918 ◽  
Vol 94 (663) ◽  
pp. 476-478
Keyword(s):  
Water Level ◽  
Royal Society ◽  
Correlation Coefficients ◽  
Barometric Pressure ◽  
Diurnal Variations ◽  
Range Of Movement ◽  
Nearest Point ◽  
Similar Relation ◽  
Short Period ◽  
The Mean

In a paper communicated to the Royal Meteorological Society, it was shown that the experimental well at Kew Observatory responded to the lunar fortnightly oscillation of mean level in the River Thames, which is 300 yards from the Observatory at its nearest point. The sensitiveness of the water-level to barometric pressure has also been investigated, and the results have been given in a paper recently read before the Royal Society. The present paper deals with the effects of the short-period tides in the solar and lunar series, S 1 , S 2 , S 3 , S 4 , and M 1 , M 2 , M 3 , M 4 . Two-hourly measurements, both in lunar and solar time, were made on the traces obtained during the first two years, August, 1914-August, 1916, omitting days of very irregular movement. Monthly mean inequalities were then computed. Well marked solar and lunar diurnal variations were found in each month, taking the form of double oscillations with two maxima and two minima during the 24 hours. The range of movement was in each case found to be highly associated with the mean height of the water in the well, the correlation coefficients being 0·89 (lunar) and 0·90 (solar). A similar relation had been previously found to exist in the case of barometric pressure.

The density variation of the earth's central core*

1942 ◽  
Vol 32 (1) ◽  
pp. 19-29
Author(s):  
K. E. Bullen
Keyword(s):  
Pure Iron ◽  
Outer Boundary ◽  
Density Variation ◽  
Central Core ◽  
Good Precision ◽  
Wide Margin ◽  
The Core ◽  
The Earth ◽  
The Mean ◽  
Published Paper

ABSTRACT A detailed analysis of the problem of the earth's density variation has been extended to the earth's central core. It is shown that in the region between the outer boundary of the core and a distance of about 1400 km. from the earth's center the density ranges from 9.4 gm/cm.3 to 11.5 gm/cm.3 within an uncertainty which, if certain general assumptions are true, does not exceed 3 per cent. The density and pressure figures are, moreover, compatible with the existence of fairly pure iron in this part of the earth. The result for the earth's outer mantle as given in a previously published paper, together with those in the present paper, are found to give with good precision the density distribution in a region occupying 99 per cent of the earth's volume. Values of the density within 1400 km. of the earth's center are subject, however, to a wide margin of uncertainty, and there appears to be no means of resolving this uncertainty for the present. The most that can be said is that the mean density in the latter region is greater than 12.3 gm/cm.3 and may quite possibly be several gm/cm.3 in excess of this figure. In the present paper figures are also included for the variation of gravity and the distribution of pressure within the central core. The gravity results are shown to be subject to an appreciable uncertainty except within about 1000 km. of the outer boundary of the core, but the pressure results are expected to be closely accurate at all depths.

scholarly journals Dynamics of Comets

1992 ◽  
Vol 152 ◽  
pp. 255-268 ◽  
Author(s):  
A. Carusi ◽  
G.B. Valsecchi
Keyword(s):  
Molecular Clouds ◽  
Vertical Component ◽  
The Sun ◽  
Giant Molecular Clouds ◽  
Mean Motion Resonances ◽  
Galactic Field ◽  
Mean Motion ◽  
Short Period ◽  
Gravitational Processes

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.

482Direct oral anticoagulant rivaroxaban in very old and frail patients: A one-year prospective follow-up of a large-scale cohort (SAFIR-AC)

2019 ◽  
Vol 40 (Supplement_1) ◽  
Author(s):  
O Hanon ◽  
J Vidal ◽  
E Chaussade ◽  
J P David ◽  
N Boulloche ◽  
...  
Keyword(s):  
Ischemic Stroke ◽  
Major Bleeding ◽  
Large Scale ◽  
Bleeding Risk ◽  
Geriatric Population ◽  
Very Old ◽  
The Mean ◽  
One Year ◽  
The One

Abstract Background/Introduction Age is one of the strongest predictors/risk factors for ischemic stroke in subjects with atrial fibrillation (AF). Direct oral anticoagulants (DOACs) have been shown to be effective in the prevention of this condition; however, clinical evidence on bleeding risk with this therapeutic strategy in very old and frail geriatric patients is poor. Purpose To assess bleeding risk in French geriatric patients aged ≥80 years and diagnosed with AF newly treated with rivaroxaban. Methods Subjects, presenting to one of 33 geriatric centers, with non-valvular AF and recent initiation of a treatment with rivaroxaban were enrolled in the study and followed-up every 3 months for 12 months. Clinical and routine laboratory data and evaluation scores, such as HAS-BLED, HEMORR2HAGES, ATRIA, and CHA2DS2-VASc, as well as comprehensive geriatric evaluation were reported. Major bleeding, as defined in ROCKET AF study, was reported at each visit, and this primary outcome was adjudicated by an independent committee. Results of this cohort were compared with findings from a similar cohort treated with vitamin K antagonists (VKAs) from the same centers (n=924). Results A total of 1045 subjects were enrolled in the study of whom 995 (95%) had a one-year follow-up (analyzed population). The mean (standard deviation (SD)) age was 86.0 (4.3) years, with the majority of patients being female (61%), 23% aged 90 years or older, and 48% having an estimated glomerular filtration rate (eGFR) <50 mL/min. The main comorbidities were hypertension in 77% of subjects, malnutrition 49%, anemia 43%, dementia 39%, heart failure 36%, and falls 27%. The mean (SD) score for CHA2DS2-VASc was 4.8 (1.4), HAS-BLED 2.4 (0.9), Mini-Mental State Examination (MMSE) 21.5 (6.9), Activities of Daily Living (ADL) 4.4 (1.9), and Charlson Comorbidity Index 6.7 (2.0). The one-year rate of major bleeding events was 6.4% of which 0.8% were fatal and 1.1% intracranial hemorrhages (ICH), whereas the one-year rate of ischemic stroke was 1.4% and all-cause mortality 17.9%. Computed with VKA cohort findings and adjusted for age, gender, eGFR and Charlson score, this would result in a hazard ratio of 0.54 (95% confidence interval [CI], 0.38 to 0.78) for major bleeding, 0.36 (0.17 to 0.76) for ICH, 0.62 (0.29 to 1.33) for ischemic stroke, and 0.82 (0.65 to 1.02) for all-cause mortality, in favor of rivaroxaban. Conclusions This is the first large-scale prospective study in geriatric population in AF subjects treated with DOAC (rivaroxaban) Major bleeding risk appeared higher in very old than younger population, however major bleeding and ICH rates were significantly lower with rivaroxaban than with VKAs when used in the same geriatric population. This study indicates that Rivaroxaban can be used in very old and frail patients for the treatment of non-valvular AF. Acknowledgement/Funding Unrestricted grant from Bayer

scholarly journals The Polar Fields and Time Fluctuations of the General Magnetic Field of the Sun

1971 ◽  
Vol 43 ◽  
pp. 675-695 ◽  
Author(s):  
A. B. Severny
Keyword(s):  
Magnetic Field ◽  
Close Agreement ◽  
Present Knowledge ◽  
Short Time Scale ◽  
The Sun ◽  
Statistical Nature ◽  
General Magnetic Field ◽  
Short Period ◽  
The Mean ◽  
Short Time

In an attempt to summarize the present knowledge on the general magnetic field (gmf) of the Sun we pointed out the fine structure and the statistical nature of the gmf as one of its most important properties. The dipole-like behaviour of the mean polar field strengths is combined sometimes (since 1964) with a bias of the S-polarity flux for both poles. Highly uneven distribution of gmf with latitude and longitude, the disappearance of gmf at the South pole for months, and short period, almost synchronous at both poles, variations in the sign of gmf are pointed out. The fluctuations with time of the mean magnetic field of the Sun seen as a star (as well as mf at different latitudes) shows periodicity connected with the rotation of the Sun and very close agreement with the fluctuations of the interplanetary field (sector structure). The effect of faster rotation of N-polarities as compared with S-polarities as well as the bias of mean solar as well as interplanetary S-polarity fields are also pointed out. The possibility of short time-scale (hours) intrinsic changes in the local pattern of gmf is demonstrated.

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