The rotation of Ganymede and Callisto

<p>The rotation rates of Ganymede and Callisto, the two largest satellites of Jupiter, are on average equal to their orbital mean motion but cannot be constant as a result of the varying gravitational torque exerted by Jupiter on the satellites. For a Keplerian orbit, the period of the torque and of the rotation variations is equal to the orbital period. Gravitational interaction with the other Galilean satellites and the Sun induces deviations from a purely Keplerian orbital motion, leading to changes in the gravitational torque of Jupiter on the satellites with respect to the mean Keplerian orbital motion and therefore to additional rotation variations. Here we discuss small variations from the average rotation on different time scales and assess the potential of using rotation as a probe of the interior structure.</p> <p>The ESA JUICE (JUpiter ICy moons Explorer) mission will measure the rotation and tides of Ganymede and Callisto in the early 30s, and will in particular very accurately determine those quantities for Ganymede during the orbital phase of the spacecraft around that satellite starting in 2032. We report on different theoretical aspects of the rotation for realistic models of the interior of the satellites, include tidal deformations and take into account the low-degree gravity field and topography of Ganymede and Callisto. We assess the advantages of a joint use of rotation and tides to constrain the satellite's interior structure, in particular its ice shell and ocean.</p>

Hibernation as a Tool for Radiation Protection in Space Exploration

With new and advanced technology, human exploration has reached outside of the Earth’s boundaries. There are plans for reaching Mars and the satellites of Jupiter and Saturn, and even to build a permanent base on the Moon. However, human beings have evolved on Earth with levels of gravity and radiation that are very different from those that we have to face in space. These issues seem to pose a significant limitation on exploration. Although there are plausible solutions for problems related to the lack of gravity, it is still unclear how to address the radiation problem. Several solutions have been proposed, such as passive or active shielding or the use of specific drugs that could reduce the effects of radiation. Recently, a method that reproduces a mechanism similar to hibernation or torpor, known as synthetic torpor, has started to become possible. Several studies show that hibernators are resistant to acute high-dose-rate radiation exposure. However, the underlying mechanism of how this occurs remains unclear, and further investigation is needed. Whether synthetic hibernation will also protect from the deleterious effects of chronic low-dose-rate radiation exposure is currently unknown. Hibernators can modulate their neuronal firing, adjust their cardiovascular function, regulate their body temperature, preserve their muscles during prolonged inactivity, regulate their immune system, and most importantly, increase their radioresistance during the inactive period. According to recent studies, synthetic hibernation, just like natural hibernation, could mitigate radiation-induced toxicity. In this review, we see what artificial hibernation is and how it could help the next generation of astronauts in future interplanetary missions.

Investigation of nonlinearity in inverse problems of satellite dynamics

The report presents the results of a study of the total and intrinsic nonlinearities as applied to the outer satellites of Jupiter observed on a short arc. The relationship between the nonlinearities and the conditions of satellite observations is revealed. In particular, it is shown that the total nonlinearity is very strong when the observation period is less than 0.1 of the orbital period, while the intrinsic nonlinearity is weak enough for all satellites, which indicates the possibility of using nonlinear methods for adequate modeling of their orbital uncertainty.

Astronomical observatory of Taras Shevchenko National University of Kyiv in 2020

In 2020, the Astronomical Observatory had 58 employees, of which 48 full-time and 10 part-time, scientists – 34 (6 doctors of sciences and 17 candidates of sciences). During the year, 4 budget and 3 contractual topics were implemented. The main scientific results. It is shown that the observed gamma radiation of the TeV range around the SGR 1900 + 14 magnetar – a neutron star with a superpowerful magnetic field – can be generated by two related sources: the undiscovered Hypernova residue, which generated SGR 1900 + 14 and / or the magnetar-wind nebula generated by this magnetar. As part of the implementation of joint international research projects, observations were made at the 6th BTA (SAO RAS), the 2nd HST (India), the 2.6th ZTSh (KrAO), the 2nd (Terskol), the 1.3th (Slovakia) and others telescopes, which resulted in a large array of photometric, polarimetric and spectral data of 9 comets, 6 asteroids, 8 satellites of Jupiter and Saturn and 1 centaur. According to the research results, 2 monographs, 76 scientific articles were published, 72 reports were made at scientific conferences.

Formation of moon systems around giant planets

The four major satellites of Jupiter, known as the Galilean moons, and Saturn’s most massive satellite, Titan, are believed to have formed in a predominantly gaseous circum-planetary disk during the last stages of formation of their parent planet. Pebbles from the protoplanetary disk are blocked from flowing into the circumplanetary disk by the positive pressure gradient at the outer edge of the planetary gap, so the gas drag assisted capture of planetesimals should be the main contributor to the delivery of solids onto circum-planetary disks. However, a consistent framework for the subsequent accretion of the moons remains to be built. Here, we use numerical integrations to show that most planetesimals that are captured within a circum-planetary disk are strongly ablated due to the frictional heating they experience, thus supplying the disk with small dust grains, whereas only a small fraction “survives” their capture. We then constructed a simple model of a circum-planetary disk supplied by ablation, where the flux of solids through the disk is at equilibrium with the ablation supply rate, and we investigate the formation of moons in such disks. We show that the growth of satellites is mainly driven by accretion of the pebbles that coagulate from the ablated material. The pebble-accreting protosatellites rapidly migrate inward and pile up in resonant chains at the inner edge of the circum-planetary disk. We propose that dynamical instabilities in these resonant chains are at the origin of the different architectures of Jupiter’s and Saturn’s moon systems. The assembly of moon systems through pebble accretion can therefore be seen as a down-scaled manifestation of the same process that forms systems of super-Earths and terrestrial-mass planets around solar-type stars and M-dwarfs.

Characterization of the precision premium in astrometry

ABSTRACT The precision premium, a concept in astrometry that was first presented by Pascu in 1994, initially means that the relative positional measurement of the Galilean satellites of Jupiter will be more accurate when their separations are small. Correspondingly, many observations have been obtained of these Galilean satellites since it was introduced. However, the exact range of separations at which the precision premium takes effect is not clear yet, nor the variation of the precision with separation. In this article, observations of open cluster M35 are used to study the precision premium and the newest star catalogue Gaia DR2 is used in data reduction. Our results show that the precision premium applies at less than 100 arcsec for two specific objects and the relative positional precision can be well fitted by a sigmoidal function. Observations of Uranian satellites are also reduced as an example of the precision premium.