1,720,965 research outputs found
Dynamical effects of the Galaxy on the Oort's Cloud
No full textThe radial effects due to the Galaxy tide on some cometary nuclei of the Oort's Cloud are taken into account. We model the galactic structure by only considering the three components which are able to exert the main dynamic effects: the bulge, the disk and the dark halo. First we prove the stability role of the dark matter on a solar-type stellar orbit, either by numerical simulations or from the theoretical point of view. Then we simulate the orbital motions of some of the cometary nuclei which lie on the galactic plane at different longitude. The aim is to study the perturbations on the Keplerian orbits of these nuclei as soon as the whole galactic potential is switched. By assuming the solar distance from the Galaxy center as a parameter, a drastic reduction of the perihelions appears for some of them when the solar distance decreases. Finally the connection with the development of the Life inside the solar system is considered
Cenni storici sul decentramento del sito della Vita
No full textViene fatta una carrellata storica lungo i tre secoli trascorsi dalle prime osservazioni di Galileo al Grande Dibattito fra Shapley e Curtis
del 1920, ultimo atto della rivoluzione copernicano e data d'inizi della moderna cosmologia
Origin and evolution of the Vesta asteroid family
No full textWe model the formation and subsequent collisional evolution of the Vesta asteroid family. The outcomes of the cratering event(s) which generated the family are predicted from current cratering physics, whereas the subsequent erosion of the family members due to collisions with background asteroids is simulated according to the model of Marzari et al. (1995Icar..113..168M). Comparing the size and orbital distribution of the model Vesta families with the observed family, we estimate the number and size of the projectile(s) which have impacted Vesta. The observed morphology of the family suggests two possible scenarios: (1) The family is the outcome of a major cratering event, resulting from the impact of an asteroid =~40km in diameter on the surface of Vesta about 1Gyr ago, and followed by another more recent lower-energy impact (by a projectile =~20km in diameter), producing the family's subgrouping close to the 3:1 mean motion Jovian resonance. (2) A single impact occurred =~1 Gyr ago and formed the whole family at the same time. In this case we have to assume that the fragments were ejected isotropically over a hemispheric region of Vesta, instead of being concentrated near the surface of a 90deg aperture cone, as suggested by laboratory impact experiments with planar targets. This different ejection geometry yields a more scattered distribution of the orbital elements, resulting into a better agreement with the observed family. In both scenarios the cratering event(s) which formed the family is/are likely to have injected a significant number of km-sized and smaller fragments into the 3:1 resonance, thus generating V-type near-Earth asteroids and HED meteorites. However, it appears likely that the current influx of HED meteorites cannot be directly traced back to the family-forming event(s), but results from more recent, smaller impacts on Vesta (or other family members)
Trojan collision probability: a statistical approach
No full textWe study the long term evolution of the collision probability and of the impact velocity vimp in the two Trojan asteroid swarms. The new mathematical formalism by Dell'Oro and Paolicchi (1998) has been used since, in the calculation of the collision probability, it allows to account for the dynamical links among the Trojans and Jupiter orbital angles, due to the 1:1 resonance. This statistical method permits to compute both and vimp over a long timespan (we considered 1 Myr) without making use of heavy numerical integrations. Moreover, it allows to easily update the values of and vimp anytime more complete samples of Trojan orbits are available. The values of and vimp over a short timescale have been compared to those of Marzari et al. (1997) and a good agreement has been observed. Over a long timescale the influence of the secular frequency g5-g6 is clearly visible in . The large oscillations due to the secular frequency are wider for L4 than for L5. We have considered two different initial samples of orbits. The first is the same sample used by Marzari et al. (1997) and includes the orbits of 114 Trojans. The second, more complete, includes 223 objects. We observe an increase of in both the swarms when the more complete sample of Trojan orbits (223) is used. The vimp, instead, slightly decreases compared to the vimp found by Marzari et al. (1997) from the sample of 114 Trojans
Orbital evolution of meteoroids from short period comets
No full textWe perform an accurate modelling of orbital evolution of dust grains taking into account both the ejection parameters derived from the analysis of the dust tail of each considered parent comet (Fulle, 1989A&A...217..283F), and the integration of the Newton equations in the context of a nine-body problem (Sun, seven major planets and the dust particle) plus solar radiation and wind forces. Among Short Period Comets (SPC) we have selected P/Schwassmann-Wachmann 1 (P/SW1) and P/Griegg-Skjellerup (P/GS), which represent two significantly different objects from a dynamical point of view. Dust from P/SW1 is dominated by Jupiter perturbations: after 2x10^4^-years, about 7% of the grains are ejected in hyperbolic orbits, 80% of the grains have the perihelion out of 4AU from the Sun, and only 1% of them reaches the Sun distance of 1AU, thus contributing to the inner zodiacal cloud. Dust from P/GS is dominated by the P-R drag, although large grains, due to their longer collapse lifetime, are sensitive to Jupiter perturbations. Therefore the Tisserand criterion represents a useful tool both to estimate the orbital evolution of grains larger than 100μm (i.e. the most likely canditates to replenish the zodiacal dust cloud, Gruen et al. 1985Icar...62..244G), and in distinguishing the parent sources of meteoroids collected with near Earth space experiments able to measure the impact velocity vectors. Jupiter perturbations oppose to the P-R drag forces and reduce significantly the contribution of SPC to the inner zodiacal dust: the simple sum of the dust mass contribution from each SPC may be an overestimate of their actual supply
Trojan collision probability: a statistical approach
No full textWe study the long term evolution of the collision probability and of the impact velocity vimp in the two Trojan asteroid swarms. The new mathematical formalism by Dell'Oro and Paolicchi (1998) has been used since, in the calculation of the collision probability, it allows to account for the dynamical links among the Trojans and Jupiter orbital angles, due to the 1:1 resonance. This statistical method permits to compute both and vimp over a long timespan (we considered 1 Myr) without making use of heavy numerical integrations. Moreover, it allows to easily update the values of and vimp anytime more complete samples of Trojan orbits are available. The values of and vimp over a short timescale have been compared to those of Marzari et al. (1997) and a good agreement has been observed. Over a long timescale the influence of the secular frequency g5-g6 is clearly visible in . The large oscillations due to the secular frequency are wider for L4 than for L5. We have considered two different initial samples of orbits. The first is the same sample used by Marzari et al. (1997) and includes the orbits of 114 Trojans. The second, more complete, includes 223 objects. We observe an increase of in both the swarms when the more complete sample of Trojan orbits (223) is used. The vimp, instead, slightly decreases compared to the vimp found by Marzari et al. (1997) from the sample of 114 Trojans
Gas drag effects on planetesimals in the 2:1 resonance with proto-Jupiter
No full textThe combined effects are studied of gas drag and gravitational perturbations by a proto-Jupiter on the orbital evolution of a swarm of planetesimals in the primordial asteroid belt in the 2:1 mean motion resonance region. The gas drag in the primordial nebula causes planetesimals to spiral towards the Sun and, therefore, to cross mean motion resonances with proto-Jupiter. The dynamics of planetesimals are numerically investigated while passing through an inner resonance in a planar model. It is found that eccentricities are drastically increased and the maximum value reached by each planetesimal depends only on the resonance argument ψ at the resonance entry. The higher average eccentricity of the swarm within the resonance borders induces a faster spiralling rate of planetesimals and a consequent decrease of their number density, in particular at the 2:1, the most relevant resonance in the asteroid belt. This phenomenon causes the formation of a gap in the swarm at the resonance location. By integrating a large number of planetesimal orbits, the gap formation process is analysed; it is found that the planetesimal number density near the resonance centre is reduced to 10-40% of its average value, depending on the free eccentricity assumed for the proto-Jupiter. Relative velocities between planetesimals are increased by a factor of four by resonant perturbations, favouring fragmentation at impacts; higher impact velocities and the reduced planetesimal density slow down the planetesimal accretion process and inhibit the formation of big bodies in the resonance region
ROSETTA Mission: satellite orbits around a cometary nucleus
No full textThis paper discusses the problem of orbiting a comet nucleus from a perspective of orbital stability. The main forces perturbing the motion of the spacecraft around the comet : shape and rotation rate of the nucleus, comet outgassing, solar radiation pressure; are derived and quantified for the nominal case of the ROSETTA spacecraft at the comet Wirtanen. Their effects on the stability of the spacecraft orbit are analyzed in detail and orbital stability criteria are developed analytically. These criteria have been tested numerically, for select cases of interest, integrating the spacecraft orbit about a Wirtanen model constructed from mascons (mass concentrations at a point). This numerical model allows very irregular nucleus shapes to be modelled accurately if a large number of point masses is used. The stability criteria derived in this paper denote stability of the spacecraft against crashing onto the comet surface or escaping from the comet on a hyperbolic orbit. They are developed and applied only over the relatively short time scales that are of interest to a spacecraft mission. The stability criteria are expressed in terms of minimum periapsis radii for stability against the non-spherical gravitational field, in terms of maximum semi-major axis for stability against escape due to the solar radiation pressure, and in terms of preferred planes and orbit elements of a spacecraft orbit for stability against the combined non-gravitational forces of comet outgassing and solar radiation pressure. For orbits close to an irregular body the gravity perturbations are minimized, and orbital stability achieved, if the periapsis radius is above five mean comet radii if the inclination is close to 0° and above three mean comet radii if inclinations are between 90 and 180° with respect to the comet rotation pole. When considering the combined solar radiation pressure and comet outgassing forces, stable orbits can be found which ``freeze'' the orbit geometry with respect to the rotating reference frame defined along the comet-sun line. The dependence of these stability criteria on the comet model parameters is discussed. The analysis in this paper is general enough to be applicable to a wide range of orbital cases, including spacecraft orbits about asteroids and natural satellites about comets and asteroids
Modulation of the cosmogenic nuclide production expected from the galactovertical motion of the solar system.
No full textA variable-rate cosmogenic production model is proposed which takes account of the changes in Galactic cosmic-ray irradiation conditions arising from the vertical oscillation of the solar system about the Galactic plane. The model incorporates and substantiates several ideas and suggestions previously put forward in the literature. The time distribution of the Be-10 concentration predicted by the model appears to be consistent with the data of the deep-sea sediment core RC 12-65. A comparison of the present model with the conventional constant-production-rate model is carried out, especially with reference to time-integrated concentrations in meteorites
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