Jupiter is the largest planet in our Solar System. With a mass equivalent to one thousandth that of the Sun or 318 times that of Earth and a composition that partly resembles that of the Sun, it almost might have become a sun itself. Jupiter is the second brightest planet in the night sky after Venus. Even a small telescope reveals its typical colored cloud bands, the Great Red Spot that has been known since Galileo‘s time and the movements of the four Galilean moons that are named after their discoverer.

Jupiter takes somewhat less than twelve years to orbit the Sun, traveling at an average speed of 13 kilometers per second. Through its enormous mass, the giant planet influences the paths of all the other bodies in the Solar System. Thus, for example, Jupiter‘s gravitational influence causes gaps in the asteroid belt, diverts comets from their original flight path and even captures some of them, and interferes with the orbits of the other planets – an effect that must always be figured in when orbits are calculated for prolonged periods. Furthermore, we use its powerful gravitational field to accelerate space probes and change their course noticeably (Voyager, Ulysses, Cassini- Huygens). Jupiter‘s radius (RJ) measures 71,500 kilometers, and its rotation period falls just short of ten hours. Together with Jupiter‘ s low mean density of 1.33 g/cm3, this high speed is the reason why Jupiter is visibly flatter at the poles. Read more...

In 1610, Galileo Galilei‘s discovery of the Galilean moons Io, Europa, Ganymede and Callisto established the existence of satellites that orbit a larger body, confirming the theory that it is not Earth which stands at the center of the universe – or at least of the Solar System as it was then known – and that the Sun forms the center of a planetary system within which the Earth orbits it as one of several planets. Io is the innermost of the Galilean moons. It resembles the Moon both in volume and density. Io orbits Jupiter on a nearly circular path on which it meets Europa at intervals of three and a half days. Together with Jupiter‘s enormous attraction, this resonance causes tides which, many times more powerful than the influence of the Moon on Earth, generate heat in the interior of Io. This, in turn, causes intense volcanic activity. More than a dozen active volcanoes and more than one hundred volcanic ejection centers were registered during the Voyager flybys and the observations of Galileo. Reaching a height of up to 11,000 meters, Io‘s mountains are probably also of volcanic origin, as are its stratified structures which are up to 1,700 meters high. Io is a body whose surface is constantly being reshaped by volcanic activity. Erupting volcanoes eject material to a height of several hundred kilometers, from which it distributes itself over large areas on the surface. The movements of the moon caused by Jupiter‘s magnetic field induce powerful electric currents and ionize large parts of the volcanic ejecta, which are then lost to space. Read more...