Vol 52, No 4 (2018)

Europa and Callisto are two “extreme members” in a sequence of the Galilean ice satellites formed at different distances from Jupiter. The difference in their mean density probably reflects the material density gradient that appeared even in the subplanetary disk of Jupiter. At the same time, general peculiarities in the composition of the surfaces of Europa and Callisto apparently characterize the accumulated effect of all subsequent evolutionary processes, including current volcanic activity on the satellite Io and its ionized material transfer in Jovian magnetosphere, as well as chemical reactions taking place under low-temperature (within ~90–130 K) and irradiation conditions. In 2016–2017, we observed the leading and trailing hemispheres of Europa and Callisto in the spectral range of 1.0–2.5 μm at 2-m telescope of Caucasian Mountain Observatory (CMO) of Sternberg Astronomical Institute (SAI) of Moscow State University (MSU). We found that, on a global scale, Europa and Callisto exhibit similar spectral characteristics and, particularly, the maxima in the distributions of sulfuric acid hydrate in the trailing hemispheres of the both moons, which agrees with the data of previous measurements. This can be considered as evidence for general ion implantation on these and other moons in the radiation belts of Jupiter. Moreover, our spectral data suggest that water ice and hydrates (clathrates) of other compounds are dominant or abundant in the leading hemispheres of Europa and Callisto. Specifically, we detected a weak absorption band of CH₄ clathrate centered at ~1.67 μm in the reflectance spectra of the leading (the band is more intense) and trailing (the band is less intense) hemispheres of Europa. Weak signs of the same absorption band are also in the reflectance spectra of Callisto measured at its different orientations.

The description of an optical complex at the Assy-Turgen Observatory (Kazakhstan) is given. This complex is suited for astrometric and photometric observations of active and passive objects (small-size space debris included) in geostationary orbits. It may be used to reveal, identify, and track potentially hazardous objects (asteroids and comets). The results of test observations are presented.

The possibilities of deflecting an asteroid from its collision course with the Earth by changing its velocity with an impact are considered. Using the asteroid Apophis as an example, the time dependence of the positions and sizes of the keyholes leading to collision is studied. It has been found that the possibility of deflecting this asteroid usually exists, and the impact can be accomplished in principle, given the capabilities of modern space technology. A change in the velocity should be performed before the encounter of 2029 in order to use the gravitational maneuver effect. The possible accuracy of determining Apophis’ orbit and the keyholes that lead to collision and are associated with the resonance returns are considered.

The discontinuous behavior of coordinates of planets and the Moon and their derivatives, which are determined from their modern ephemerides, at the boundaries of adjacent interpolation intervals is illustrated using the example of the DE436 ephemerides. The numerical integration of the equations of motion of two asteroids demonstrates that the integration accuracy increases by several orders of magnitude if the step of numerical integration is matched to the boundaries of ephemeris interpolation intervals. In addition, an algorithm for ephemeris smoothing at the boundaries of interpolation intervals is developed and applied in order to eliminate the jumps of coordinates and their first-order derivatives emerging in extended- and quadprecision calculations. This algorithm allows one to remove the jumps of coordinates and their derivatives up to any given order. It is demonstrated that the use of ephemerides smoothed to the first-order derivatives in quad-precision calculations increases the accuracy of numerical integration by ~10 orders of magnitude.