Jupiter, Io, and Callisto 10 Dec 2011

Jupiter, the fifth planet from the Sun and the largest planet within the Solar System, is classified as a gas giant. It has a mass one-thousandth that of the Sun, but is two and a half times the combined mass of all the other planets in our Solar System. Saturn, Uranus, and Neptune are also classified as gas giants and together with Jupiter are sometimes referred to as the Jovian or outer planets. Making it the third brightest object in the night sky after the Moon and Venus, Jupiter can reach an apparent magnitude of -2.94.

On the lower right of Jupiter's surface, in this image, is the Great Red Spot (GRS), a giant storm that is known to have existed since at least the 17th century when it was first seen by telescope.

Jupiter has at least 64 moons, four of which were discovered by Galileo Galilei in 1610. These four moons, called the Galilean moons, are Ganymeade, Europa, Io, and Callisto. Io and Callisto can be seen in this image to the right of Jupiter (Io is far right). Callisto is the second-largest moon in the Jovian system, but third-largest within the Solar System. Io is the fourth-largest moon in the Solar System and With over 400 active volcanoes, it the most geologically active object as well.