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.

scholarly journals 3. The Constitution of Cometary Nuclei

1977 ◽  
Vol 39 ◽  
pp. 25-35
Author(s):  
F. L. Whipple
Keyword(s):  
Unit Area ◽  
Cosmic Ray ◽  
Cometary Nucleus ◽  
Radial Force ◽  
Oort Cloud ◽  
Cometary Nuclei ◽  
Basic Assumptions ◽  
Short Period ◽  
Total Energy Input ◽  
Cometary Activity

For many comets the accelerations radially from the Sun have been determined by deviations from gravitational orbits. The radial force per unit area can be calculated on the basis of certain assumptions concerning the nature of the vaporizing material, the physical circumstances, and the geometry of the cometary nucleus. The observed accelerations combined with the calculated forces yield numerical values for (1-A)/R where A is the albedo, and R the radius of the nucleus. At extreme solar distances photometry provides the well-known quantity area-times-albedo or RA1/2. Solutions are then possible for R and A for the nuclei of the comets. The derived quantity, (1-A)A1/2, provides a limiting check on the basic assumptions and, therefore, on the basic physical properties of the nuclei. For ten short-period comets with q < 1.5 AU, the observations are satisfied by H2O ice. About half show “spotty” surfaces.For comets of a single apparition H2O ice is generally not volatile enough to produce the observed radial accelerations. The resulting problems are discussed including the possibility that in some cases displacements of the photometric from the gravitational nucleus may produce spurious non-gravitational accelerations.The physical characteristics of comets vary with their orbits and with their age. New comets on their first near solar passage from the Oort cloud are extremely active. The activity falls statistically with increasing age. This sequence must represent a corresponding sequence or layering of structure from the surface of a new comet inwards and is described qualitatively in this paper.The excessive activity of new comets is ascribed to cumulative cosmic-ray damage that activates the outer few hundred gm cm-2 from the surface. The total energy input in 4.6 x 109 yr reaches 50,000 cal gm-1 near the surface so that both crystalline structures and molecules are severely damaged if not completely destroyed. Annealing at T - 10 K must be very small. Hence significant exothermic energy in the form of defects, vacancies and radicals is added to produce the extraordinary activity observed in new comets.Other aspects and problems of cometary activity are discussed.

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 On Nongravitational Effects in two Classes of Models for Cometary Nuclei

1972 ◽  
Vol 45 ◽  
pp. 287-293
Author(s):  
O. V. Dobrovol'skij ◽  
M. Z. Markovich
Keyword(s):  
Heliocentric Distance ◽  
Orbital Plane ◽  
Secular Change ◽  
Reactive Force ◽  
Cometary Nuclei ◽  
Ice Surface ◽  
First Case ◽  
Equation Of Heat Conduction ◽  
Change In Mean ◽  
Nongravitational Effects

Two types of cometary nuclei are considered: a homogeneous icy nucleus and an icy nucleus covered with a dispersive surface layer, 1 cm thick and having poor heat conductivity. The temperature of the evaporating ice surface of a nucleus rotating in a period of 6 hours about an axis perpendicular to the orbital plane was determined by numerical integration of the equation of heat conduction in the first case and of the Stefan problem in the second. For a nucleus of radius 105 cm and mass of 3.8 × 1015 g the reactive force was found to be about 107 dyn in either case, and the secular change in mean motion is of the order of the observed values. For the homogeneous nucleus the dependence of the reactive force on heliocentric distance is obtained.

scholarly journals Are the main belt comets, comets?

2009 ◽  
Vol 5 (S263) ◽  
pp. 215-217
Author(s):  
Javier Licandro ◽  
Humberto Campins
Keyword(s):  
Visible Spectrum ◽  
Oort Cloud ◽  
Orbital Elements ◽  
Main Belt ◽  
Cometary Nuclei ◽  
Earth Asteroid ◽  
William Herschel ◽  
Cometary Origin ◽  
Strong Contrast ◽  
Cometary Activity

AbstractWe present the visible spectrum of asteroid-comet transition object 133P/Elst-Pizarro (7968), the first member of the new population of objects called Main Belt Comets (Hsieh & Jewitt 2006). The spectrum was obtained with the 4.2m William Herschel Telescope at the “Roque de los Muchachos” observatory. The orbital elements of 133P place it within the Themis collisional family, but the observed cometary activity during it last 3 perihelion passages also suggest a possible origin in the trans-Neptunian belt or the Oort Cloud, the known sources of comets. We found a clear similarity between our spectrum of 133P and those of other members of the Themis family such as 62 Erato, and a strong contrast with those of cometary nuclei, such as 162P/Siding-Spring. This spectral comparison leads us to conclude that 133P is unlikely to have a cometary origin. This conclusion is strengthened by spectral similarities with activated near-Earth asteroid 3200 Phaethon, and suggest that there are activated asteroids in the near-Earth asteroid and main belt populations with similar surface properties.

scholarly journals Constraining the orientation of the spin axes of extrasolar minor bodies 1I/2017 U1 (‘Oumuamua) and 2I/Borisov

2020 ◽  
Vol 643 ◽  
pp. A18
Author(s):  
C. de la Fuente Marcos ◽  
R. de la Fuente Marcos
Keyword(s):  
Monte Carlo Simulation ◽  
Monte Carlo ◽  
Random Search ◽  
Cometary Nucleus ◽  
Spin Axis ◽  
The Sun ◽  
Axis Orientation ◽  
Prolate Shape ◽  
Probable Spin ◽  
Cometary Activity

Context. The orientation of the spin axis of a comet is defined by the values of its equatorial obliquity and its cometocentric longitude of the Sun at perihelion. These parameters can be computed from the components of the nongravitational force caused by outgassing if the cometary activity is well characterized. The trajectories of known interstellar bodies passing through the Solar System show nongravitational accelerations. Aims. The spin-axis orientation of 1I/2017 U1 (‘Oumuamua) remains to be determined; for 2I/Borisov, the already released results are mutually exclusive. In both cases, the values of the components of the nongravitational force are relatively well constrained. Here, we investigate – within the framework of the forced precession model of a nonspherical cometary nucleus – the orientation of the spin axes of ‘Oumuamua and 2I/Borisov using public orbit determinations that consider outgassing. Methods. We applied a Monte Carlo simulation using the covariance matrix method together with Monte Carlo random search techniques to compute the distributions of equatorial obliquities and cometocentric longitudes of the Sun at perihelion of ‘Oumuamua and 2I/Borisov from the values of the nongravitational parameters. Results. We find that the equatorial obliquity of ‘Oumuamua could be about 93°, if it has a very prolate (fusiform) shape, or close to 16°, if it is very oblate (disk-like). Different orbit determinations of 2I/Borisov gave obliquity values of 59° and 90°. The distributions of cometocentric longitudes were in general multimodal. Conclusions. Our calculations suggest that the most probable spin-axis direction of ‘Oumuamua in equatorial coordinates is (280°, +46°) if very prolate or (312°, −50°) if very oblate. Our analysis favors a prolate shape. For the orbit determinations of 2I/Borisov used here, we find most probable poles pointing near (275°, +65°) and (231°, +30°), respectively. Although our analysis favors an oblate shape for 2I/Borisov, a prolate one cannot be ruled out.

scholarly journals On the 2PN Pericentre Precession in the General Theory of Relativity and the Recently Discovered Fast-Orbiting S-Stars in Sgr A*

Universe ◽  
2021 ◽  
Vol 7 (2) ◽  
pp. 37
Author(s):  
Lorenzo Iorio
Keyword(s):  
General Theory ◽  
Present Author ◽  
Theory Of Relativity ◽  
General Theory Of Relativity ◽  
Orbital Elements ◽  
Analytical Expression ◽  
Potential Interest ◽  
Short Period ◽  
The Future ◽  
Sgr A

Recently, the secular pericentre precession was analytically computed to the second post-Newtonian (2PN) order by the present author with the Gauss equations in terms of the osculating Keplerian orbital elements in order to obtain closer contact with the observations in astronomical and astrophysical scenarios of potential interest. A discrepancy in previous results from other authors was found. Moreover, some of such findings by the same authors were deemed as mutually inconsistent. In this paper, it is demonstrated that, in fact, some calculation errors plagued the most recent calculations by the present author. They are explicitly disclosed and corrected. As a result, all of the examined approaches mutually agree, yielding the same analytical expression for the total 2PN pericentre precession once the appropriate conversions from the adopted parameterisations are made. It is also shown that, in the future, it may become measurable, at least in principle, for some of the recently discovered short-period S-stars in Sgr A*, such as S62 and S4714.

scholarly journals Radar Observations of Comet Nuclei and Comae

2005 ◽  
Vol 13 ◽  
pp. 763-763
Author(s):  
Donald B. Campbell ◽  
John K. Harmon ◽  
Micael C. Nolan ◽  
Steven J. Ostro
Keyword(s):  
Cross Sections ◽  
Cometary Nucleus ◽  
Size Distributions ◽  
Radar Systems ◽  
The Earth ◽  
Radar Cross Sections ◽  
Short Period ◽  
Grain Size Distributions ◽  
Resolution Imaging

Nine comets have been detected with either the Arecibo (12.6 cm wavelength) or Goldstone (3.5 cm) radar systems. Included are six nucleus detections and five detections of echoes from coma grains. The radar backscatter cross sections measured for the nuclei correlate well with independent estimates of their sizes and are indicative of surface densities in the range of 0.5 to 1.0 g cm-3. Like most asteroids, comets appear to have surfaces that are very rough at scales much larger than the radar wavelength. Coma echo models can explain the radar cross sections using grain size distributions that include a substantial population of cm-sized grains. A long term goal of the cometary radar program has been the high resolution imaging of a cometary nucleus. Eleven short period comets are potentially detectable over the next two decades a few of which may be suitable for imaging. We are always waiting for the arrival of a new comet with an orbit that brings it within 0.1 AU of the earth.

scholarly journals Surface Morphology of Cometary Nuclei

1991 ◽  
Vol 116 (2) ◽  
pp. 761-767
Author(s):  
D. Möhlmann ◽  
E. Kührt
Keyword(s):  
Imaging Techniques ◽  
Cometary Nucleus ◽  
Complex Data ◽  
Cometary Nuclei ◽  
Precise Knowledge ◽  
New Information ◽  
Consistent Model ◽  
Topological Surface ◽  
In Situ Observations

Abstract.The first in situ observations of a cometary nucleus by imaging techniques on board the VEGA and Giotto spacecraft in 1986 opened the possibility of directly studying the properties and structures of a cometary surface. This new information, combined with results from other experiments made by these spacecraft and with results from laboratory experiments and Earth-bound radar observations, gave a better basis for coming from speculation to more precise knowledge in describing properties of cometary surfaces and related activity. It is the aim of this paper to discuss the different views developed to understand properties of cometary surfaces and to propose a consistent model for understanding topological surface properties and related cometary activity. We accomplish this by extending former hypotheses and approaches and making them more precise in the light of the new and more complex data.

scholarly journals Wolf-Rayet binaries: old friends and new acquaintances

1999 ◽  
Vol 193 ◽  
pp. 26-37 ◽  
Author(s):  
Virpi S. Niemela ◽  
Roberto Gamen ◽  
Nidia I. Morrell ◽  
Sixto Giménez Benítez
Keyword(s):  
Radial Velocity ◽  
Binary Systems ◽  
Magellanic Cloud ◽  
Elliptic Orbit ◽  
Spectral Lines ◽  
Small Magellanic Cloud ◽  
Orbital Elements ◽  
Spectroscopic Observations ◽  
Short Period ◽  
Old Friends

Observations of WR stars in binary systems are discussed, emphasizing constraints on our knowledge of the binary frequency of WR stars, and of WR stars as a distinctive class of objects. Radial velocity orbits of newly discovered binaries, e.g., WR 29, a short period WN7+OB binary in our Galaxy, and SMC/AB 7, a massive WN+O7 binary in the Small Magellanic Cloud, are presented. New spectroscopic observations of binary systems with previously known orbits are also reported, showing in the case of WR 21 evidence of change of the orbital elements as derived from different spectral lines. An elliptic orbit for CV Ser is also illustrated.

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