by Nadezhda YEVDOKIMOVA, Cand. Sc. (Phys. & Math.), Space Research Institute (IKI), Russian Academy of Sciences, Moscow

The RAS Institute of Space Research held an international conference in March of 2013 discussing the planned interplanetary voyage to Ganymede, Jupiter's largest satellite, and the research objectives and experiments of a descent module there. Scheduled for 2023, this mission will be a breakthrough in space studies. It is realized within the framework of the JUICE project (JUpiter ICy moon Explorer) of studying Jupiter's ice moons. The European Space Agency that initiated this project intends to send an orbital station to Ganymede a year before the start of the Russian Jovian probe. The conference and discussions held there were supported by COSPAR (Committee on Space Research) and the Russian Foundation of Basic Research. Taking part were as many as 50 planetary scientists of Europe, Russia and the United States involved with Jupiter and its satellites.

The flight toward Jupiter will take about eight years, and proceed in a combination ballistic system including four gravitational maneuvers at Venus and Earth at the heliocentrical stage of the mission; it will end in a cascade of maneuvers near the Jovian moons. The descent module to land on Ganymede will be another acid test for the mission.

In this country the complex of related problems is the responsibility of the RAS Institute of Space Research, the Keldysh Institute of Applied Mathematics, and the Lavochkin Research and Production Association. Meeting at the conference, delegates of these bodies reviewed problems of the initial stage of the project (such as a kit of research instruments on orbital and

land modules, interaction with the JUICE probe and ground stations) together with results of the work done over these last years in studying Ganymede and Jupiter.

Ganymede, the largest of the Jovian satellites (as many as fifty have been detected so far) and the largest moon in the solar system (larger than the planet Mercury) has long attracted the interest of planetary scientists. Its extremely rarefied atmosphere is found to contain oxygen (in the form of O₂, O₂, O₃), which is not at all proof positive of life as we know it. But scientists have discovered magnetic field anomalies of this heavenly body. This evidence, according to expert opinion, is one of the signs of the presence of a salt water layer in its core, a potential source of life. Exploration of Ganymede and its possible habitability is one of the objectives of the Russian and European space missions.

* Jupiter, the largest planet in the solar system. Its orbit lies between those of Mars and Saturn. Its mean distance from the sun is about 484 mln mi (774 mln km), and its year is over 11 earth years. The solar system has nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto, in order of their distance from the sun.

The planets are divided into inferior (inner) and superior (outer) planets--those inside the earth's orbit and without. They are also categorized as terrestrial planets, which include Mercury. Venus, Earth, and Mars; and Jovian planets, which include Jupiter (after which the group is named), Saturn, Uranus, Neptune, and Pluto. The terrestrial planets are small, and composed largely of heavier elements like silicates, iron, carbon and oxygen. The Jovian planets are extremely large and are composed mainly of lighter elements like hydrogen, helium, and methane.--Ed.

Studies of this remarkable celestial body go back to the 17th century as Galileo discovered four Jovian satellites in order of their distance from the planet (1610): Io, Europa, Ganymede, and Callisto. (Yet there is evidence they were discovered a year earlier by Simon Marius of Germany, but he did not report his discovery.) For centuries Ganymede as well as Io, Europa and Callisto--known also as the Galilean satellites--was no more than a tiny dot to astronomers, for they depended on primitive ground telescopes for their observations, and data they collected were much in error. A group of scientists watching stellar eclipses at an Indonesia-based observatory detected in 1972 a thin atmosphere on Ganymede, and its pressure was estimated at 0.1 Pa. But this information was disproved later on.

There came a leap in the study of Ganymede and other Jovian moons with the onset of the space era and the launchings of the American probes Pioneer (1973-1974), Voyager (1979), Galileo (1989-2003), and New Horizons (2006). The Galileo Jovian mission proved to be most productive in what concerned Ganymede. Research data thus culled on this heavenly body moved into a new level. Important results were collected by the Hubble orbital telescope (the joint project of NASA and the European Space Agency launched in 1990) as it became possible to trace a weak ultraviolet luminescence of monatomic oxygen in Ganymede's surface layer.

By now a good deal of knowledge has been collected on Ganymede largely through remote observations. Its structure, composition, geological record and other things--quite different from what we are having here on earth--might be of interest to a wide range of experts, from a planetary geologist and chemist to an astrobiologist.

The surface of this Jovian moon keeps many traces of the active geological past--tectonic and volcanic processes and what we call impact events (collisions with other bodies). Ganymede is coated with water (H₂O) ice for the most part, its concentration, depending on a particular region, from 50 to 90 percent. Present in the upper stratum of this putative planet is a large amount of hydrated minerals, such as sodium and magnesium sulfates. Planetary scientists (like Hans Draget of the United States and colleagues) suppose such minerals might have entered there from intraglacial cavities. More than that, some astronomers are certain about an

ocean within Ganymede's core similar to one found in Europa, another satellite traveling around Jupiter. Its exact dimensions and the composition of the liquid intraglacial cavity are still being disputed. Apart from the water ice, Ganymede's surface is found to contain scattered sinks of CO₂ (carbon dioxide) ice as well as trace amounts of ammonia and sulfur oxide.

The chemical composition of Ganymede's surface was determined through remote (orbital) spectroscopy, though such data could be collected only from a surface 100 urn layer. Besides the clearly identified spectral lines some others could not be assigned to any of the known substances, that is in the absorption bands of 3.7, 3.88, and 4.05 μm. We do not know yet the exact composition of a dark substance present everywhere--whether it carries admixtures of organic matter, stratified silicates or any other substance.

According to its outward features Ganymede shows "light" and "dark" regions. The former are younger, not as much craterized, and cover the greater part of it. The latter ("dark" regions) studded with craters are older and could be 4 bln years old. They may contain stratified silicates (clays), hydrated minerals and organic substances.

Among its companion satellites Ganymede is a standout, both in size and in a high differentiation of its interior (J. (G.) Anderson of the United States and colleagues, relying on gravitational evidence of the Galileo probe, pointed to this phenomenon back in 1996). This Jovian moon has a well-defined core, mantle and an outer shell, the crust. According to updated evidence, the core of this putative planet is composed predominantly of Fe and FeS and is, by different estimates, from 400 to 1,300 km across. It is thought to be hot and, at least in part, molten, which fact is confirmed by the presence of a sufficiently strong magnetic field.

Ganymede has a thick silicate mantle enriched, by some estimates, with magnesium, and enclosed in a solid icy shield. This shield containing admixtures of salts and some other compound is 800-1,000 km thick; this fact is of immense interest to science. We cannot tell yet what kinds of ice are formed under extreme conditions there. This may be hexagonal ice known to us here on earth together with amorphous ice and its other modifications not found on our planet.

Both the liquid core and the inner ocean may be indicative of a cooling process still on in the wake of a catastrophic event that took place during the last billion years and that changed Ganymede's orbit, causing an immense heating of the planetoid through a burst of tidal energy.

The Galileo probe detected gravitational anomalies on Ganymede, but their cause is unknown yet. They may be due to accidental inhomogeneities formed in the internal structure in the course of geological evolution and also to major collisions and impacts of the past.

The atmosphere--or, rather, exosphere--of this Jovian moon discovered in 1995 by the Hubble telescope is all too weak. It is dominated by oxygen molecules (O₂), their concentration ranging, by different estimates, between 1.2∙10⁸ and 7∙10⁸ cm^-3. One probable source of O₂ is the reaction of H₂O decomposition at which hydrogen escapes faster than does oxygen. Consequently we may postulate: the atmosphere of Ganymede may contain H₂ as well as H and O present in negligible amounts compared with O₂.

Ganymede's magnetosphere is fairly complex, and it is likewise of great interest to planetary scientists. It was discovered by the Galileo space probe. The Jovian satellite possesses a strong magnetic field of its own (around 720 nT1 at the equator). It is thought to be generated by

the dynamo mechanism through convection in the liquid metal core. Ganymede keeps traveling in Jupiter's external magnetic field; it is weaker near the satellite's orbit and approaches 120 nT1. This "tertiary planet" gains another induced magnetic field equal to about 60 nT1, engendered by mobile charges in subsurface conducting liquid salt solutions. It was registered by a Galileo-borne magnetometer. Polar lights were observed at high latitudes of the satellite surrounded by a corona of hot oxygen atoms.

If successful, the descent module landing will help to get a closer look into some of the hidden nooks of this distant and enigmatic heavenly body.

Our scientists will send two research probes to Ganymede--an orbital one (chiefly for communication sessions) and a descent module. The voyage to Jupiter will take about six years because of complex gravitational maneuvers required for putting the space vehicle into a design trajectory around Ganymede at a lower velocity. Its movement should proceed at a safe distance from giant Jupiter so as to minimize the action of energized particles of its radiation belts on sensitive instruments. Experts of the Lavochkin R&D Association, the Skobeltsyn Institute of Nuclear Physics, the Keldysh Institute of Applied Mathematics as well as those of the Space Research Institute (IKI) are working to determine optimal ballistic scenarios for the Jovian mission.

Speaking at the international conference, Dr. Grushevsky of the Keldysh Institute reported the rated flight model of the Jupiter mission. He cited a set of type variants, namely with respect to flight schedules, maneuvers within Jupiter's system calculated for different fly-in plans, fuel consumption limits and maximum limits of radiation exposure.

The time of the voyage about the Galilean satellites, ranging from eighteen months to three years, will include dozens of maneuvers at Ganymede and Callisto, the fourth and remotest Jovian moon. Depending on specific objectives, it would be possible to use different vantage points during the flyby.

The simplest flight models make use of the method of conic sections joining (with ellipses joined for the most part). That is to say, the space vehicle will be moving from one conic section orbit to another by means of gravitational maneuvers and jet propulsion. Such movements will thus join together. This is but a skeleton of the real movement.

The Keldysh Institute chose a more elaborate model. Each chain in the cascade of gravitational maneuvers is like gun shooting: the space probe is like a bullet targeted by a precise shot. Hitting one target, it should hit another one on the rebound. And so forth... Otherwise fuel consumption will increase manifold.

Approaching Ganymede, the descent module will travel around in orbit as an artificial satellite, and then start making a descent.

Dr. Alexei Golikov, senior research fellow of the Keldysh Institute, told about possible scenarios of this stretch of the flight. For one, the module will circle Ganymede in a polar orbit about 100 km above the surface (this orbit being sufficiently stable) and then, upon several corrections, change into an intermediate orbit 15 to 100 km high. Next, several other flight corrections prior to the descent.

These ballistic scenarios are not final--a good deal depends on the body of the station, the launching time and many other things. The phases of the expedition will be calculated for both probes.

Cooperation with the European Space Agency is of great importance for making the Jupiter/Ganymede mission a complete success. The Russian project is planned in partnership with the JUICE mission scheduled for 2022. The JUICE space station will be a gobetween in radio communication sessions with our probe. JUICE photographs of Ganymede's surface may be used for choosing the landing site of the Russian descent module. These and other significant technical matters related to the two cooperative projects were reviewed at the international conference.

Photos by courtesy of the National Aeronautics and Space Administration (NASA)