Vol 52, No 5 (2018)

The data obtained in the recent Rosetta space mission to comet 67P/Churyumov–Gerasimenko have had a profound impact on the understanding of the nature of comets. In addition to revising the notions on the physical properties and structure of comets, this addresses dynamical aspects of the formation of the observed cometary populations (short- and long-period comets, Centaurs, trans-Neptunian objects, and Oort-cloud objects). In the review, we discuss new problems that have appeared in the theory of dynamical evolution and origin of comets due to the Rosetta mission.

Embryos of the Moon and the Earth may have formed as a result of contraction of a common parental rarefied condensation. The required angular momentum of this condensation could largely be acquired in a collision of two rarefied condensations producing the parental condensation. With the subsequent growth of embryos of the Moon and the Earth taken into account, the total mass of as-formed embryos needed to reach the current angular momentum of the Earth–Moon system could be below 0.01 of the Earth mass. For the low lunar iron abundance to be reproduced with the growth of originally iron-depleted embryos of the Moon and the Earth just by the accretion of planetesimals, the mass of the lunar embryo should have increased by a factor of 1.3 at the most. The maximum increase in the mass of the Earth embryo due to the accumulation of planetesimals in a gas-free medium is then threefold, and the current terrestrial iron abundance is not attained. If the embryos are assumed to have grown just by accumulating solid planetesimals (without the ejection of matter from the embryos), it is hard to reproduce the current lunar and terrestrial iron abundances at any initial abundance in the embryos. For the current lunar iron abundance to be reproduced, the amount of matter ejected from the Earth embryo and infalling onto the Moon embryo should have been an order of magnitude larger than the sum of the overall mass of planetesimals infalling directly on the Moon embryo and the initial mass of the Moon embryo, which had formed from the parental condensation, if the original embryo had the same iron abundance as the planetesimals. The greater part of matter incorporated into the Moon embryo could be ejected from the Earth in its multiple collisions with planetesimals (and smaller bodies).

The interaction between a large meteoroid and the atmosphere is modeled as its destruction into a cloud of fragments and vapors moving with a common shock wave. Under the action of aerodynamic forces the shape of this cloud is deformed—it is expanded in the direction transverse to the motion and compressed in the longitudinal direction. With allowance for the pressure distribution over the surface of a body varying its shape (it is assumed that the sphere is transformed into a flattened spheroid), the relation for the rate of increase in the midsection radius of a fragmented meteoroid has been obtained. This rate significantly depends on the degree of the meteoroid flattening which leads to a significantly smaller increase in the transverse size of the meteoroid along the trajectory as compared to similar models used in the literature where the influence of the body shape was not considered. The proposed model also takes into account the change in the density of the cloud of fragments due to an increase in gaps between them. An approximate analytical solution of equations of the physical theory of meteors with drag and heat transfer coefficients varying along the trajectory has been obtained for a fragmented meteoroid. The interaction of the Chelyabinsk meteoroid with the atmosphere is modeled and the solution obtained for the energy release curve is compared with the observational data.

The paper presents the results of a study of the dynamic structure of the orbital space of the navigation systems GLONASS and GPS. It is shown that the dynamic structure of the GLONASS region is determined by the action of one stable Lidov–Kozai secular resonance. The motion of almost all the retired objects of the GLONASS system is stable throughout the 100-year study period. In the GPS region, there is an orbital resonance and a large number of secular resonances. Their combined influence leads to a rapid increase in the eccentricity of the orbits of the retired objects of the system. Features of the dynamic structure of the orbital space are used to find the graveyard (parking) orbits of the retired objects of navigation systems.